# Developing sero-diagnostic tests to facilitate Plasmodium vivax Serological Test-and-Treat approaches: modeling the balance between public health impact and overtreatment

Thomas Obadia, Narimane Nekkab, Leanne Robinson, Chris Drakeley, Ivo Mueller, Michael White

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Thomas Obadia, Narimane Nekkab, Leanne Robinson, Chris Drakeley, Ivo Mueller, et al. Developing sero-diagnostic tests to facilitate Plasmodium vivax Serological Test-and-Treat approaches: modeling the balance between public health impact and overtreatment. BMC Medicine, 2022, 20 (1), pp.98. 10.1186/s12916-022-02285-5. pasteur-03617088

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# Developing sero-diagnostic tests to facilitate *Plasmodium vivax* Serological Test-and-Treat approaches: modeling the balance between public health impact and overtreatment

Thomas Obadia¹,²,<fnref n="3" />*, Narimane Nekkab¹,²,⁴,⁵, Leanne J. Robinson⁶,⁷,⁸, Chris Drakeley⁹, Ivo Mueller²,⁶,⁷ and Michael T. White¹,²

**Abstract**

**Background:** Eliminating *Plasmodium vivax* will require targeting the hidden liver-stage reservoir of hypnozoites. This necessitates new interventions balancing the benefit of reducing *vivax* transmission against the risk of over-treating some individuals with drugs which may induce haemolysis. By measuring antibodies to a panel of *vivax* antigens, a strategy of serological-testing-and-treatment (*PvSeroTAT*) can identify individuals with recent blood-stage infections who are likely to carry hypnozoites and target them for radical cure. This provides a potential solution to selectively treat the *vivax* reservoir with 8-aminoquinolines.

**Methods:** *PvSeroTAT* can identify likely hypnozoite carriers with ~80% sensitivity and specificity. Diagnostic test sensitivities and specificities ranging 50–100% were incorporated into a mathematical model of *vivax* transmission to explore how they affect the risks and benefits of different *PvSeroTAT* strategies involving hypnozoiticidal regimens. Risk was measured as the rate of overtreatment and benefit as reduction of community-level *vivax* transmission.

**Results:** Across a wide range of combinations of diagnostic sensitivity and specificity, *PvSeroTAT* was substantially more effective than bloodstage mass screen and treat strategies and only marginally less effective than mass drug administration. The key test characteristic determining of the benefit of *PvSeroTAT* strategies is diagnostic sensitivity, with higher values leading to more hypnozoite carriers effectively treated and greater reductions in *vivax* transmission. The key determinant of risk is diagnostic specificity: higher specificity ensures that a lower proportion of uninfected individuals are unnecessarily treated with primaquine. These relationships are maintained in both moderate and low transmission settings (qPCR prevalence 10% and 2%). Increased treatment efficacy and adherence can partially compensate for lower test performance. Multiple rounds of *PvSeroTAT* with a lower performing test may lead to similar or higher reductions in *vivax* transmission than fewer rounds with a higher performing test, albeit with higher rate of overtreatment.

<footnote>*Correspondence: tobadia@pasteur.fr</footnote>

<footnote n="3">³ Hub de Bioinformatique et Biostatistique, Département de Biologie
Computationnelle, Institut Pasteur, F-75015 Paris, France
Full list of author information is available at the end of the article</footnote>
**Conclusions:** At current performance, *PvSeroTAT* is predicted to be a safe and efficacious option for targeting the hypnozoite reservoir towards *vivax* elimination. *P. vivax* sero-diagnostic tests should aim for both high performance and ease of use in the field. The target product profiles informing such development should thus reflect the trade-offs between impact, overtreatment, and ease of programmatic implementation.

**Keywords:** *Plasmodium vivax* malaria, Test development, Serological test-and-treat, Modeling, Public health interventions, Overtreatment

## Background

*Plasmodium vivax* remains a major cause of malaria [1, 2], with its importance, relative to *P. falciparum*, increasing as case management and conventional vector-control measures such as long-lasting insecticidal bednets reduce transmission to low levels [3]. This difficulty in achieving pre-elimination levels of *P. vivax* transmission is largely attributed to the parasite's particular biology, with long-lasting liver-stage hypnozoites causing blood-stage relapses weeks to years following the initial infection. These relapses are thought to account for up to 80% of all detected *P. vivax* bloodstage infections [4, 5] and contribute to sustained malaria transmission. These hypnozoites form a hidden parasite reservoir that needs to be targeted in order to effectively reduce the *vivax* malaria burden.

The current mainstay of *P. vivax* control is treatment with 8-aminoquinolines such as primaquine or tafenoquine, the only drugs effective at clearing hypnozoites. However, their administration requires careful medical assessment due to the risk of potentially inducing severe hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency (G6PDd), an X-linked genetic disorder affecting ~5% of the population in malaria-endemic regions with geographical variations [6–8]. G6PDd may be diagnosed either by point-of-care rapid diagnostic tests or using quantitative laboratory assays. Primaquine efficacy is affected by dosage and patient adherence. The duration and dose of a primaquine regimen vary greatly in both duration of treatment (7 or 14 days) and total dose (3.5–7mg/kg) [9]. Poor adherence is regularly reported [10] which may substantially undermine the efficacy of radical cure of *vivax* malaria, resulting in the development of short-course high-dose primaquine regimens [11]. In recent clinical trials, tafenoquine has shown promising results with comparable efficacy to low-dose primaquine (3.5mg/kg total dose) in preventing relapses with only a single dose [12]. While a patient's G6PDd status affects eligibility for 8-aminoquinoline treatment, the efficacy of these drugs can also be altered by insufficient dosing or low CYP2D6 metabolization, for which treatment failures have been observed previously [13, 14].

For the treatment of individuals with symptomatic *P. vivax* infection, the risk of prescribing 8-aminoquinolines needs to be balanced against the benefit to the patient,

accounting for the high probability that they carry hypnozoites and will experience future relapses with associated chronic morbidity and sustain onward. For public health interventions where entire populations are targeted for treatment with 8-aminoquinolines, the balance between benefit and risk is different, given that a large proportion of individuals will not be hypnozoite carriers. One strategy is mass drug administration (MDA) where all individuals are offered treatment, subject only to some basic eligibility criteria (e.g., age, pregnancy status). MDA with 8-aminoquinolines has been predicted to be highly effective in aiding *P. vivax* control and elimination [4, 15]. However, given the high prevalence of G6PDd in most malaria endemic settings, MDA with 8-aminoquinolines without prior G6PD testing would expose many individuals (including those without hypnozoites) to potentially dangerous drugs and is thus considered unsafe in most remaining *P. vivax* endemic areas. As a consequence, WHO does not currently recommend the use of MDA with 8-aminoquinolines for *P. vivax* malaria [16]. An alternative to MDA is mass screen and treat (MSAT) where individuals are first tested for blood-stage parasites with a rapid diagnostic test (RDT) or light microscopy, and only individuals with detectable blood-stage parasites are treated. MSAT with blood-stage drugs and primaquine has been shown not to have an effect on *P. vivax* transmission [17]. This is thought to be because a high proportion of *P. vivax* blood-stage infections and all *P. vivax* liver-stage infections are not detectable by RDTs or light microscopy.

The existing options for population-level interventions that are based on treatment with 8-aminoquinolines either lack impact (MSAT) or expose populations to a high degree of risk (MDA). By measuring antibodies to a panel of *P. vivax* antigens, it is possible to identify individuals with recent blood-stage infections who are likely to carry hypnozoites and target them for treatment with 8-aminoquinolines [18]. *P. vivax* serological test and treat (*PvSeroTAT*), where people are screened by a serological test and, if classified as exposed, treated with a combination blood- and liver-stage treatment could thus provide a potentially safe and efficacious alternative for population-level treatment (see malaria serology use case 5 in Greenhouse et al. [19]).
Future programmatic implementation of *PvSeroTAT* will depend on having effective, field-deployable *P. vivax* sero-diagnostic tests (SDTs). As with any diagnostic test, a *P. vivax* SDT will have to trade off sensitivity and specificity, and thus, the ability to detect all true positives requiring treatment while limiting overtreatment. The current diagnostic performance of these serological markers is approximately 80% sensitivity and 80% specificity [18]. Here, we model a wider range of test performance values (50–100%) and explore the relationship between the *P. vivax* SDT performance and *PvSeroTAT* public health impact as well as overtreatment in order to inform *P. vivax* SDT target product profiles (TPPs). The mathematical model of *vivax* transmission from White *et al.* [20, 21] was adapted to implement *PvSeroTAT* campaigns at the population level. We explored the range of possible sensitivities and specificities of this serological diagnostic tool and evaluated the corresponding drop in *P. vivax* prevalence after evaluating different *PvSeroTAT* scenarios. These were compared to those potentially achievable in MDA or MSAT campaigns.

# Methods

## P. vivax transmission model

A previously developed *P. vivax* transmission model [21] was modified to simulate the impact of population-based testing and treatment strategies. Humans are represented in an individual-based, compartmental framework that models the particular biology of *P. vivax* infections. Mosquito population dynamics are modeled using a compartmental model. As the model was calibrated to replicate the transmission observed in Papua New-Guinea, its output represents a country with endemic *vivax* malaria where transmission is mostly peri-domestic.

Primaquine treatment and testing for G6PDd were simulated using a treatment pathway model accounting for primaquine eligibility, adherence, and overall effectiveness as per Nekkab *et al.* [22]. Briefly, any individual testing positive for recent *vivax* exposure undergoes a decision tree with respect to primaquine eligibility that depends on age, gender, pregnancy status, and G6PD activity. These treatment pathways applied both for routine case-management and public-health interventions.

## Demographics, G6PD deficiency, and CYP2D6 slow metabolizers

Population demographics were calibrated to reflect that of individuals found in *vivax* endemic areas such as PNG, with young populations (mean age 22.5 years) [21]. The prevalence of G6PDd was fixed at 4.6% as observed in a recent clinical trial conducted in the neighboring Solomon Islands (data not shown), consistent with reports from other studies of ~4.5% prevalence of G6PDd in

Melanesia [6]. To that aim, individual levels of G6PD activity scores were sampled from sex-specific distributions. Prevalence of low CYP2D6 metabolism was chosen at 3.4% of the population, as observed in the same clinical trial.

## Model simulations

We modeled sustained malaria transmission in a population of size 20,000 over a period of 30 years, with daily time increments where individuals get exposed to mosquitoes and may become infected with *P. vivax*. Transmission was assumed to be perennial and non-seasonal so that seasonal effects did not confound evaluation of the effectiveness of simulated interventions. Two transmission settings were investigated, a low transmission setting (qPCR prevalence ~2%) and a moderate one (qPCR prevalence ~10%).

Case-management at the start of the simulation assumed that primaquine was routinely administered for patients treated after a symptomatic *P. vivax* infection, so that blood-stage and liver-stage parasites could be cleared with efficacy depending on modeled scenario (see below). Single or multiple rounds of interventions were evaluated, with up to three intervention rounds each separated with a 6-month period.

## Intervention parameters and outcome measures

Three population-level treatment strategies were simulated: (i) MDA, (ii) MSAT with light microscopy detection of blood-stage parasitemia, and (iii) *PvSeroTAT*. All intervention campaigns assumed a base coverage of 80%. For *PvSeroTAT*, interventions were simulated under all combinations of *P. vivax* SDT sensitivity and specificity ranging 50–100% by steps of 2.5%. Our current serological assay is described in detail by Longley *et al.* [18] This serological assay includes 8 *P. vivax* proteins including RBP2b<sub>161-1454</sub> (PVX_094255) and MSP1-19 (PVX_099980). It is anticipated that optimization of the existing panel of proteins will lead to improved diagnostic sensitivity and specificity.

Three main scenarios were considered: (i) a “best-case” setting, where a hypothetical drug with no contra-indication achieved 100% efficacy in clearing liver-stages of the parasite regardless of metabolic disorders and where drug accessibility was never an issue; (ii) a high efficacy, high adherence setting where we modeled the effect of an 8-aminoquinoline comparable to the high dose, short-course primaquine course administered in the IMPROV study [11]; and (iii) a “real-life” setting corresponding to the situation in Brazil where primaquine is routinely prescribed, and where there is good quality data available on adherence to treatment, case-management and drug efficacy. For every combination of parameters, 100
stochastic simulations were conducted and results were averaged across replicates. A summary of relevant model parameters along with their values is provided in Table 1.

Public health impact, the outcome of interest in every simulation, was defined as the reduction of *P. vivax* prevalence measured by qPCR 6 months after the last round of Intervention and was calculated as *Prev. before 1st intervention*—*Prev. at assessment period*.

*Prev. before 1st intervention*

Overtreatment with a hypnooiticidal drug was defined as the administration of the drug in a participant that had not been infected with *P. vivax* in the previous 9 months [18].

# Results

## Intervention-driven drop in qPCR prevalence

In the absence of sustained vector-control, the prevalence of *vivax* malaria is expected to rebound to its initial level after the last round of public health intervention is enforced, if elimination is not achieved. As such,

<table><caption>**Table 1** Summary of drug accessibility, adherence, efficacy, and counter-indications across all three envisioned scenarios. The “best-case” scenario corresponds to a hypothetical drug that achieves 100% clearance of liver-stage parasites and does not pose any threat to health regardless of various deficiencies. The “High efficacy” scenario presents parameters corresponding to those observed in a clinical trial with administration of high-dose primaquine (7mg/kg total dose administered over 7 days) to clear liver-stages in eligible patients [11]. The “Real-life” scenario depicts the typical parameter values observed in Brazil where primaquine (3.5mg/kg total dose administered over 7 days) is routinely administered for *P. vivax*</caption><thead><tr><th></th><th>Best-case</th><th>High efficacy</th><th>Real-life</th></tr></thead><tbody><tr><td>G6PD deficiency</td><td>4.6%</td><td>4.6%</td><td>4.6%</td></tr><tr><td>CYP2D6 slow metabolizers</td><td>3.4%</td><td>3.4%</td><td>3.4%</td></tr><tr><td>Pregnancy</td><td>7.5%</td><td>7.5%</td><td>7.5%</td></tr><tr><td><strong>Coverage</strong></td><td></td><td></td><td></td></tr><tr><td>Case-management</td><td>100%</td><td>80%</td><td>80%</td></tr><tr><td>Intervention</td><td>80%</td><td>80%</td><td>80%</td></tr><tr><td><strong>Bloodstage drug</strong></td><td></td><td></td><td></td></tr><tr><td>Efficacy (alone)</td><td>100%</td><td>100%</td><td>89.9%</td></tr><tr><td>Efficacy (with anti-hpz)</td><td>100%</td><td>100%</td><td>94.6%</td></tr><tr><td>Prophylaxis duration</td><td>28 days</td><td>28 days</td><td>14 days</td></tr><tr><td><strong>Hypnooiticidal drug</strong></td><td></td><td></td><td></td></tr><tr><td>Efficacy</td><td>100%</td><td>80%</td><td>71.36%</td></tr><tr><td>Prophylaxis duration</td><td>28 days</td><td>8 days</td><td>8 days</td></tr><tr><td>Adherence</td><td>100%</td><td>80%</td><td>66.7%</td></tr><tr><td>Minimum age</td><td>0 days</td><td>180 days</td><td>180 days</td></tr><tr><td>Pregnancy</td><td>administer</td><td>don't administer</td><td>don't administer</td></tr><tr><td>G6PD deficiency</td><td>administer</td><td>don't administer</td><td>don't administer</td></tr><tr><td>CYP2D6 slow metabolizers</td><td>effective</td><td>not effective</td><td>not effective</td></tr></tbody></table>

the choice of an assessment window largely affects the measured impact, as can be seen in Fig. 1. In a moderate transmission setting, the prevalence is expected to rebound to its initial level within ~5 years, while in a low transmission setting, this duration is closer to ~8–10 years due to the slower rate of reinfection (Additional File 1: Figure S1). In all tested scenarios, MDA provided the highest impact as may be expected from a campaign enforcing radical cure of *P. vivax* population-wide, only excluding individuals because of 8-aminoquinole toxicity concerns. MSAT always resulted in the poorest impact, by systematically missing asymptomatic hypnozoite carriers at the time of screening. The reduction in *P. vivax* PCR prevalence caused by *PvSeroTAT* was substantially greater than the reduction caused by MSAT, but less than the reduction caused by MDA.

## Upper and lower limits of achievable impact are defined by MDA and MSAT

Under ideal conditions (i.e., perfect clearance of hypnozoites, no safety concerns), 1, 2, or 3 rounds of MDA would yield prevalence reductions of respectively 65.8%, 85.6%, and 92.4% in a low transmission (2% PCR prevalence, Table 2) setting and 59.5%, 76.4%, and 84.1% in a moderate setting (10% PCR prevalence, Additional File 2: Table S2), respectively. However, MDA also results in overtreating 67.5–73.7% of the population in low transmission settings and 46.0–61.8% in moderate transmission settings. MDA campaigns with the “high efficacy” primaquine regimen are predicted to reduce PCR prevalence by 33.6–65.0% in low transmission areas and by 31.7–56.7% in moderate, with an overtreatment rate of 65.7–68.0% and 46.9–53.5%, respectively. In both scenarios, MDA impact and overtreatment rates were further reduced in the “real-life” primaquine scenario (Table 2 & Additional File 2: Table S2).

MSAT with RDT had a substantially lower predicted impact on *P. vivax* transmission than MDA in every scenario as it failed to detect and cure the reservoir of asymptomatic, hypnozoite carriers (low transmission: “best-case” 7.1–15.9%, “high efficacy” 5.1–13.2%, and “real-life” 4.5–10.1%; moderate transmission: “best-case” 5.4–9.6%, “high efficacy” 3.3–7.1%, and “real-life” 2.9–6.5%). However, MSAT did not cause any overtreatment, as only individuals who had detectable blood-stage parasitemia were treated.

## PvSeroTAT as a safer and efficient alternative to MDA

Under the “best-case” implementation scenario, *PvSeroTAT* with a *P. vivax* SDT of 80% sensitivity and 80% specificity is predicted to cause 51.9–82.9% in a low transmission setting (Table 2) and 45.9–71.7% reduction in *P. vivax* prevalence in a moderate setting (Additional
<figure><img src="image_3.png" /></figure>

File 2: Table S2). This equates to 77.1–89.7% of what would be achieved by implementing the corresponding 1, 2, or 3 rounds of MDA campaigns (Fig. 1 and Additional File 1: Figure S1). *PvSeroTAT* similarly achieves 80.4–83.6% and 81.9–84.3% of the predicted MDA impact under the “high efficacy” and “real-life” scenarios (Tables 2 & Additional File 2: Table S2). These effects come with the benefit of a 5-fold reduction in overtreatment compared to MDA, only exposing 9.2–14.5% of the

target population to unnecessary drugs (Tables 2 & Additional File 2: Table S2), in the moderate and low transmission settings, respectively.

**Relationship between diagnostic performance and PvSeroTAT impact and overtreatment**

The predicted impact of *PvSeroTAT* on transmission was largely dependent on the sensitivity of the sero-diagnostic test, while the amount of overtreatment dispensed during
<table><caption>**Table 2** Public health impact and overtreatment modeled in a low transmission setting (qPCR prevalence ~2%). Impact was defined as the reduction in qPCR prevalence observed 6 months after the last round of intervention; overtreatment corresponded to the administration of a hypnooiticidal drug to a person whose last blood-stage infection occurred more than 9 months ago. The “Best-case,” “High efficacy,” and “Real-life” scenario correspond to those described in the main text, with decreasing adherence, efficacy, and eligibility</caption><thead><tr><th rowspan="2">Intervention</th><th rowspan="2">Rounds</th><th rowspan="2">Sensitivity</th><th rowspan="2">Specificity</th><th colspan="3">Impact (%)</th><th colspan="3">Overtreatment (%)</th></tr><tr><th>Best-case</th><th>High efficacy</th><th>Real-life</th><th>Best-case</th><th>High efficacy</th><th>Real-life</th></tr></thead><tbody><tr><td>MDA</td><td>1</td><td></td><td></td><td>65.8</td><td>33.6</td><td>27.1</td><td>67.5</td><td>65.7</td><td>66.3</td></tr><tr><td>MSAT</td><td>1</td><td></td><td></td><td>7.1</td><td>5.1</td><td>4.5</td><td>0</td><td>0</td><td>0</td></tr><tr><td rowspan="5">PvSeroTAT</td><td>1</td><td>0.700</td><td>0.700</td><td>45.5</td><td>24.8</td><td>19.9</td><td>20.2</td><td>19.7</td><td>19.9</td></tr><tr><td>1</td><td>0.800</td><td>0.800</td><td>51.9</td><td>27.8</td><td>21.4</td><td>13.5</td><td>13.2</td><td>13.3</td></tr><tr><td>1</td><td>0.900</td><td>0.900</td><td>56.6</td><td>30.2</td><td>22.9</td><td>6.7</td><td>6.5</td><td>6.6</td></tr><tr><td>1</td><td>1.000</td><td>1.000</td><td>61.9</td><td>32.3</td><td>25.7</td><td>0</td><td>0</td><td>0</td></tr><tr><td>1</td><td>0.650</td><td>0.950</td><td>41.9</td><td>23.6</td><td>17.2</td><td>3.4</td><td>3.3</td><td>3.3</td></tr><tr><td>1</td><td>0.950</td><td>0.650</td><td>60.6</td><td>31.1</td><td>23.9</td><td>23.7</td><td>23</td><td>23.2</td></tr><tr><td>MDA</td><td>2</td><td></td><td></td><td>85.6</td><td>52.9</td><td>43.5</td><td>69.2</td><td>66.1</td><td>66.7</td></tr><tr><td>MSAT</td><td>2</td><td></td><td></td><td>11.6</td><td>10</td><td>7.5</td><td>0</td><td>0</td><td>0</td></tr><tr><td rowspan="6">PvSeroTAT</td><td>2</td><td>0.700</td><td>0.700</td><td>67.5</td><td>40.4</td><td>33.6</td><td>20.5</td><td>19.8</td><td>20</td></tr><tr><td>2</td><td>0.800</td><td>0.800</td><td>73.1</td><td>44.6</td><td>36</td><td>13.7</td><td>13.2</td><td>13.3</td></tr><tr><td>2</td><td>0.900</td><td>0.900</td><td>77.5</td><td>48.2</td><td>38.9</td><td>6.8</td><td>6.6</td><td>6.7</td></tr><tr><td>2</td><td>1.000</td><td>1.000</td><td>81.9</td><td>51.6</td><td>41.2</td><td>0</td><td>0</td><td>0</td></tr><tr><td>2</td><td>0.650</td><td>0.950</td><td>63.1</td><td>38.5</td><td>30.8</td><td>3.4</td><td>3.3</td><td>3.3</td></tr><tr><td>2</td><td>0.950</td><td>0.650</td><td>81.8</td><td>49.4</td><td>40.6</td><td>24</td><td>23.2</td><td>23.3</td></tr><tr><td>MDA</td><td>3</td><td></td><td></td><td>92.4</td><td>65</td><td>54.7</td><td>73.7</td><td>68</td><td>68.3</td></tr><tr><td>MSAT</td><td>3</td><td></td><td></td><td>15.9</td><td>13.2</td><td>10.1</td><td>0</td><td>0</td><td>0</td></tr><tr><td rowspan="6">PvSeroTAT</td><td>3</td><td>0.700</td><td>0.700</td><td>78.5</td><td>52</td><td>43.4</td><td>21.6</td><td>20.3</td><td>20.3</td></tr><tr><td>3</td><td>0.800</td><td>0.800</td><td>82.9</td><td>55.8</td><td>46.8</td><td>14.5</td><td>13.5</td><td>13.5</td></tr><tr><td>3</td><td>0.900</td><td>0.900</td><td>86.4</td><td>59.4</td><td>49.5</td><td>7.2</td><td>6.8</td><td>6.8</td></tr><tr><td>3</td><td>1.000</td><td>1.000</td><td>89.8</td><td>62.9</td><td>52.6</td><td>0</td><td>0</td><td>0</td></tr><tr><td>3</td><td>0.650</td><td>0.950</td><td>73.7</td><td>48.6</td><td>39.5</td><td>3.6</td><td>3.3</td><td>3.4</td></tr><tr><td>3</td><td>0.950</td><td>0.650</td><td>89.6</td><td>61.3</td><td>52.5</td><td>25.6</td><td>23.8</td><td>23.9</td></tr></tbody></table>

rounds was directly proportional to the false positive rate (1 – specificity) of the sero-diagnostic test (see Additional Files 3, 4 and 5: Figures S3–S5). Under the “best-case” scenario the impact of a single-round *PvSeroTAT* impact increased by about 5% for each 10% increase in diagnostic sensitivity (Fig. 2). The impact of *PvSeroTAT* strategies varied substantially according to treatment scenarios (Fig. 3) but remained almost entirely linked to diagnostic sensitivity with impact rising from 24.8% and 19.9% for a 70% sensitivity & specificity test to 30.2% and 22.9% for 90% sensitivity & specificity test for a single round of *PvSeroTAT* under the “high efficacy” and “real-life” scenarios for the low transmission settings (Table 2). For all diagnostic performance levels, *PvSeroTAT* was predicted to have a greater effect size in the low transmission setting irrespective of the type of treatment or the number of *PvSeroTAT* rounds administered (Table 2 and Additional File 2: Table S2, Fig. 3 and Additional File 1: Figure S1).

Overtreatment with hypnooiticidal drugs is primarily the result of false positive diagnostics, that is, individuals who had higher antibody titers but did not have hypnozoites. The rate of overtreatment was not substantially affected by *PvSeroTAT* performance or the number of *PvSeroTAT* rounds administered and remained at approximately 5–7%, 10–14%, and 15–20% for 90%, 80%, and 70% diagnostic specificity tests in both transmission settings (Table 2 & Additional File 2: Table S2, Additional Files 3, 4 and 5: Figures S3–S5).

## Diagnostic performance and impact across multiple *PvSeroTAT* rounds

The impact of the *PvSeroTAT* interventions increased with the number of rounds delivered in both transmission settings and all three implementation scenarios. For three rounds of *PvSeroTAT* in the low transmission setting, impact rose to 78.5%, 82.9%, and 86.4% for 70/70, 80/80, and 90/90 sensitivity & specificity under the
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Exploration of the parameter space for the ROC curve of PvSeroTAT in a low transmission setting, with a single round of intervention using a hypothetical optimal hypnozoitcidal drug. A The red square on the full ROC curve represents the parameter space explored as part of the target product profiling. The three triangles correspond to a diagnostic test at 80% sensitivity and specificity (solid color; currently achieved by our antibody panel) as well as two alternatives at 65% and 90% (transparency; tradeoffs between sensitivity and specificity). B Association between impact (prevalence reduction after 6 months) and sensitivity. C Association between overtreatment and specificity. In panels B and C, dots are colored according to the diagnostic criteria not used on the x-axis</figcaption></figure>

“best-case” treatment scenario, 52.0%, 55.8%, and 59.4% under the “high efficacy” scenario, and 43.4%, 46.8%, and 49.5% under the “real-life” scenario (Table 2). The relative improvement of impact of 3 vs 1 round of PvSeroTAT was larger for “high efficacy” (80/80 +100%) and “real-life” (80/80 +119%) than the “best-case” scenario (80/80 +60%) and increased with decreasing assay performance (e.g., “high efficacy”: 70/70 +109%, 80/80: +100%, 90/90 +96%).

Even with increasing PvSeroTAT rounds, the pro-
portion of people overtreated remained constrained
below 15% under the low transmission setting, with very
low variations for the 80/80 test performance (relative
increase vs. a single round: “best-case” +7%, “high effi-
cacy” +3%, “real-life” +2%; Table 2). Similar trends, but
with slightly smaller improvements in impact and even
larger increases in overtreatment were predicted for the
high transmission settings (Additional File 2: Table S2).
However, the absolute number of people tested and to
a lesser degree those treated however increased linearly
with the number of PvSeroTAT rounds.

Discussion

Similar to previous observations from systematic reviews
of field studies, our models predict that MDA programs
have a strong but transient effect on *vivax* malaria trans-
mission [23] that is estimated to be higher in low trans-
mission settings [24, 25], but result in a large proportion
of people receiving unnecessary treatments. In contrast
with MDA, MSAT would result in poor impact in both
transmission settings, but no overtreatment. This con-
firms the finding from a clinical trial in Indonesia [17]

that treating only blood-stage *vivax* cases was not enough
to lower the overall transmission.

PvSeroTAT is a newly proposed public health inter-
vention where community members are screened using
an SDT and those classified as currently and previously
exposed to P. vivax infections treated with anti-blood-
and liver-stage therapy [18, 19]. PvSeroTAT can bridge
the gap between MDA and MSAT by achieving an impact
on transmission that is close to that of MDA while dra-
matically reducing the level of overtreatment. Using a test
with performance comparable to the currently available
research assay (i.e., 80% sensitivity and 80% specificity
[18]), PvSeroTAT is predicted to achieve approximately
80–85% of the impact of MDA while reducing the num-
ber of people over-treated by 80%. The modeling of dif-
ferent combinations of SDT diagnostic performance
showed that under both high and low transmission set-
ting and all implementation scenarios the impact of 1 to
3 rounds of PvSeroTAT impact is directly dependent on
the sensitivity of the sero-diagnostic test (Table 2 & Addi-
tional File 2: Table S2, Fig. 3). The degree of overtreat-
ment was however related entirely to test specificity.

This has important implications for *P. vivax* SDT devel-
opment. Trade-offs between sensitivity and specificity
during SDT development results in related trade-offs
between public health impact and the rate of overtreat-
ment. Yet, we found that multiple rounds of interven-
tions with a lower sensitivity test may achieve the same
impact as fewer rounds with a more sensitive one. For
example, a public health campaign consisting of three
rounds of *PvSeroTAT* 6 months apart, at 70% sensitivity
and 70% specificity would yield a public health impact
<figure><img src="image_5.png" /></figure>

comparable to that of two rounds at 90% sensitivity and 90% specificity (Tables 2 and 3). However, three rounds at 70/70 will result in more than four times as many people being unnecessarily treated compared to two rounds at 90/90. On the other hand, three rounds of *PvSeroTAT* at 80% sensitivity and 80% specificity diagnostic performance is predicted to result in a similar impact to that achieved with two MDA rounds but accompanied by a 3-fold reduction in overtreatment.

As with other public health interventions, the impact is predicted to decrease as the implementation scenario moves from “best-case” to “real-life”. While results from the ideal scenario provide a higher boundary of treatment efficacy (i.e., universal eligibility for hypnozoiticidal drug treatment), the differences in impact observed between the “high efficacy” and the “real-life” scenario are attributable to decreased efficacy and adherence to primaquine regimens. This highlights the importance of
strong locally led programmatic implementation and the need for effective communication and engagement with treated populations. Shorter treatment with primaquine at increased dosage, such as that used in the IMPROV study [11] or a single dose of tafenoquine may also provide increased adherence. Both the use of a *P. vivax* SDT with higher sensitivity or an increase in the number of *PvSeroTAT* rounds delivered can at least partially counteract the impact decays inherent in “real-life” programmatic implementation. In a recent Phase III clinical trial, tafenoquine demonstrated non-inferiority with respect to low-dose Primaquine regimen to prevent hypnozoite-caused relapses. Therefore, although we did not model explicitly Tafenoquine due to it not being readily available in many endemic regions, we expect its efficacy to be similar to what was observed in our “high efficacy” scenario.

A key output from our model was that *PvSeroTAT* alone was not enough to reach elimination either in low or moderate transmission settings. Rather than modeling elimination scenarios, we set out to estimate the expected magnitude of reduction in PCR prevalence upon implementing a population-level public health campaign. We propose *PvSeroTAT* to be used as a tool with high potential for temporarily shrinking the human parasite reservoir, jointly with tools such as vector control or other measures targeting residual pockets of transmission (e.g., reactive case-detection, RCD) and vector populations. In low-transmission settings, some countries have started to implement RCD to focus interventions around newly identified index cases. A notable example is the 1–3–7 surveillance and response strategy developed in China with the aim to report new cases within 1 day, investigate these within 3 days, and take action within 7 days [26–28]. As part of these actions, *PvSeroTAT* could be used to improve upon microscopy-based detection methods (i.e., MSAT) upon investigating possible asymptomatic infections linked to the index case. A limitation of our model lies in the absence of geographically-explicit enhanced actions to be taken in the vicinity of infections treated by the routine case-management health system.

In our modeling approach, overtreatment was defined as administering hypnozoiticidal drugs to an individual without a blood-stage infection in the previous 9 months. Most *P. vivax* relapses are expected to occur within 9 months of the primary infection [29]. Overtreatment is therefore the result of false positive diagnostics (i.e., blood-stage infection happened more than 9 months previously). A different definition of overtreatment, based on presence or absence of hypnozoites, would result in somewhat different estimates of overtreatment as the antibody levels may indicate a positive diagnostic signal

for individuals who had a blood-stage infection in the last 9 months but do not harbor hypnozoites anymore.

A limitation of our analysis is that we do not account for potential variation between geographic regions and across transmission intensities, factors which are known to affect diagnostic accuracy [30]. Another limitation of this analysis is that we did not account for the effects of targeting diagnostics and treatment strategies at high-risk populations. For example, in Cambodia, malaria transmission was found to be mostly associated with occupational activities such as forestry or living close to forested areas [31]. The effectiveness of *PvSeroTAT* interventions could be optimized by targeting people who are at increased risk of malaria infection, for example when transmission is mostly occupational. Furthermore, malaria transmission fluctuates seasonally and we did not set out to investigate the timing of interventions. Further studies will assess the impact of single or multiple rounds happening either during low or peak transmission season, so that public health campaigns can be enforced at the time where they will most reduce *P. vivax* transmission.

## Conclusion

Current antibody panels allow for detecting individuals who are at a high risk of *P. vivax* relapse with a diagnostic performance of approximately 80% sensitivity and 80% specificity. We predict that *PvSeroTAT* campaigns may achieve an impact similar to that of MDAs, with the benefit of massively reducing overexposure to primaquine, thus lowering the risks of G6PDd-induced haemolysis. To achieve effective *PvSeroTAT* implementation, the development of *P. vivax* SDTs should aim for both high performance and easy field-use thus facilitating deployment of multiple *PvSeroTAT* rounds. Further cost-impact modeling will be required to determine if a more expensive but higher performing test may have a better cost-effectiveness than a lower performing test that requires an additional *PvSeroTAT* round to achieve the same public health impact. We suggest that with a field-deployable test achieving a diagnostic performance of the current research assay (i.e., 80% sensitivity and 80% sensitivity [18]), *PvSeroTAT* would provide sufficient public-health impact for programmatic implementation in countries that have already achieved a reduction in transmission levels to low-to-moderate levels (2–10% *P. vivax* prevalence by qPCR), where highest *PvSeroTAT* impact is expected. The target product profiles informing *P. vivax* SDT development should reflect the trade-offs between impact, overtreatment, and ease of programmatic implementation that are identified here.
**Abbreviations**

MDA: Mass drug administration; MSAT: Mass screen-and-treat; PvSeroTAT: *Plasmodium vivax* serological test-and-treat; RDT: Rapid diagnostic test; G6PD: Glucose-6-phosphate dehydrogenase deficiency; CYP2D6: Cytochrome P450 2D6; TPP: Target product profile; SDT: Sero-diagnostic test; WHO: World Health Organization; qPCR: Quantitative polymerase chain reaction.

**Supplementary Information**

The online version contains supplementary material available at https://doi.org/10.1186/s12916-022-02285-5.

**Additional file 1: Figure S1.** Modeled *P. vivax* qPCR prevalence with various implementations of public health interventions in an endemic situation of moderate transmission. The columns correspond to the different scenarios and rows to the number of intervention rounds, with grey area presenting time of evaluation.

**Additional file 2: Table S2.** Public health impact and overtreatment modeled in a moderate transmission setting (qPCR prevalence ~10%). Impact was defined as the reduction in qPCR prevalence observed six months after the last round of intervention; overtreatment corresponded to the administration of a hypnozoiticidal drug to a person whose last blood-stage infection occurred more than 9 months ago. The "Best-case", "High efficacy" and "Real-life" scenario correspond to those described in the main text, with decreasing adherence, efficacy and eligibility.

**Additional file 3: Figure S3.** Heat maps for (A) impact and (B) overtreatment with an optimal hypnozoiticidal drug as defined by the "best-case" scenario and after 1, 2 or 3 rounds of *PvSeroTAT* and under two transmission pressures.

**Additional file 4: Figure S4.** ROC surfaces for (A) impact and (B) overtreatment with Primaquine administered in a high-efficacy scenario after 1, 2 or 3 rounds of *PvSeroTAT* and under two transmission pressures.

**Additional file 5: Figure S5.** Heat maps for (A) impact and (B) overtreatment with Primaquine in a real-life scenario after 1, 2 or 3 rounds of *PvSeroTAT* and under two transmission pressures.

**Authors' contributions**

IM and MTW conceived the study. TO, NN, LJR, CD, IM, and MTW wrote the manuscript. The authors read and approved the final manuscript.

**Funding**

This work is supported by the Bill and Melinda Gates Foundation grants (INV009103 & VISPA INV024368) and NHMRC grants (GNT1102297 & GNT1171795) and is part of the International Centers of Excellence for Malaria Research program "Understanding, tracking and eliminating malaria transmission in the Asia-Pacific Region," funded by the National Institutes of Health, MD, US (grant 1U19AI129392-01). IM is supported by an NHMRC Principal Research Fellowship (GNT1155075). MTW is supported by the Agence Nationale de la Recherche Investissement d'Avenir Programme, Laboratoire d'Excellence "Integrative Biology of Emerging Infectious Diseases" (ANR-10-LABX-62-IBEID) and INCEPTION program (ANR-16-CONV-0005). LJR is supported by an NHMRC Career Development Fellowship (GNT1161627).

**Availability of data and materials**

The model developments and C++ code are hosted on a GitLab repository at https://gitlab.pasteur.fr/mwhite/pv_mod. The model used to run all simulations corresponds to commit 85620fedb494b5f99351c4d876d6904665e579ce found in the master branch of that repository.

**Declarations**

**Ethics approval and consent to participate**

Not applicable

**Consent for publication**

All authors read and approved the final manuscript.

**Competing interests**

The authors declare that they have no competing interests.

**Author details**

¹G5 Épidémiologie et Analyse des Maladies Infectieuses, Département de Santé Globale, Institut Pasteur, F-75015 Paris, France. ²Unité Malaria: Parasites et Hôtes, Département Parasites et Insectes vecteurs, Institut Pasteur, F-75015 Paris, France. ³Hub de Bioinformatique et Biostatistique, Département de Biologie Computationnelle, Institut Pasteur, F-75015 Paris, France. ⁴Swiss Tropical and Public Health Institute, Basel, Switzerland. ⁵University of Basel, Basel, Switzerland. ⁶Population Health & Immunity Division, Walter and Eliza Hall Institute of Medical Research, Parkville, Australia. ⁷Department of Medical Biology, University of Melbourne, Melbourne, Australia. ⁸Burnet Institute, Melbourne, Victoria, Australia. ⁹Department of Immunology and Infection, London School of Hygiene and Tropical Medicine, London, UK.

Received: 26 October 2021 Accepted: 7 February 2022

Published online: 18 March 2022

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