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https://doi.org/10.1038/s41467-020-19802-w

OPEN

Post-lockdown SARS-CoV-2 nucleic acid screening
in nearly ten million residents of Wuhan, China

1234567890():,;

Shiyi Cao1,11, Yong Gan1,11, Chao Wang1,11, Max Bachmann2, Shanbo Wei3, Jie Gong4, Yuchai Huang1,
Tiantian Wang1, Liqing Li5, Kai Lu6, Heng Jiang7,8, Yanhong Gong1, Hongbin Xu1, Xin Shen1, Qingfeng Tian9,
Chuanzhu Lv10 ✉, Fujian Song 2 ✉, Xiaoxv Yin1 ✉ & Zuxun Lu 1 ✉Stringent COVID-19 control measures were imposed in Wuhan between January 23 and April
8, 2020. Estimates of the prevalence of infection following the release of restrictions could
inform post-lockdown pandemic management. Here, we describe a city-wide SARS-CoV-2
nucleic acid screening programme between May 14 and June 1, 2020 in Wuhan. All city
residents aged six years or older were eligible and 9,899,828 (92.9%) participated. No new
symptomatic cases and 300 asymptomatic cases (detection rate 0.303/10,000, 95% CI
0.270–0.339/10,000) were identiﬁed. There were no positive tests amongst 1,174 close
contacts of asymptomatic cases. 107 of 34,424 previously recovered COVID-19 patients
tested positive again (re-positive rate 0.31%, 95% CI 0.423–0.574%). The prevalence of
SARS-CoV-2 infection in Wuhan was therefore very low ﬁve to eight weeks after the end of
lockdown.

1 Department of Social Medicine and Health Management, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology,

Wuhan, Hubei, China. 2 Norwich Medical School, Faculty of Medicine and Health Science, University of East Anglia, Norwich, UK. 3 Wuhan Municipal HealthCommission, Wuhan, Hubei, China. 4 Wuhan Centre for Clinical Laboratory, Wuhan, Hubei, China. 5 Department of Management Science and Engineering,
School of Economics and Management, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi, China. 6 Tongji Hospital, Huazhong Universityof Science and Technology, Wuhan, Hubei, China. 7 Centre for Alcohol Policy Research, School of Psychology and Public Health, La Trobe University,
Melbourne, VIC, Australia. 8 Melbourne School of Population and Global Health, University of Melbourne, Melbourne, VIC, Australia. 9 School of Public
Health, Zhengzhou University, Zhengzhou, Henan, China. 10 Department of Emergency, Hainan Clinical Research Centre for Acute and Critical Diseases, The
Second Afﬁliated Hospital of Hainan Medical University, Haikou, Hainan, China. 11These authors contributed equally: Shiyi Cao, Yong Gan, Chao Wang.
✉email: lyuchuanzhu@hainmc.edu.cn; Fujian.song@uea.ac.uk; yxx@hust.edu.cn; zuxunlu@yahoo.com
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T

he Coronavirus Disease 2019 (COVID-19) was ﬁrst
reported in December 2019, and was classiﬁed as a pandemic by the World Health Organization on March 11,20201. Following strict lockdown measures, the COVID-19 epidemic was generally under control in China, and the wholecountry has progressed into a post-lockdown phase. In this phase,countries face new problems and challenges, including how toaccurately assess the post-lockdown risk of the COVID-19 epidemic, how to avoid new waves of COVID-19 outbreaks, andhow to facilitate the resumption of economy and normal social
life. As the city most severely affected by COVID-19 in China,
Wuhan had been under lockdown measures from January 23
until April 8, 2020. During the ﬁrst 2 months after city’s
reopening, there were only a few sporadic COVID-19 cases in
Wuhan (six newly conﬁrmed cases from April 8 to May 10,20202). However, there was still concern about the risk of
COVID-19 in Wuhan, which seriously affected the resumption ofindustrial production and social services, and hampered thenormal lives of residents. In order to ascertain the current statusof the COVID-19 epidemic, the city government of Wuhan carried out a comprehensive citywide nucleic acid screening of
SARS-CoV-2 infection from May 14, 2020 to June 1, 2020.
The citywide screening of SARS-CoV-2 infection in Wuhan isa mass screening programme in post-lockdown settings, andprovided invaluable experiences or lessons with international
relevance as more countries and cities around the world entering
the post-lockdown phase. In this study, we report the organisation process, detailed technical methods used, and results of thiscitywide nucleic acid screening.
Results
There were 10,652,513 eligible people aged ≥6 years in Wuhan(94.1% of the total population). The nucleic acid screening wascompleted in 19 days (from May 14, 2020 to Jun 1, 2020), andtested a total of 9,899,828 persons from the 10,652,513 eligible
people (participation rate, 92.9%). Of the 9899,828 participants,
9,865,404 had no previous diagnosis of COVID-19, and 34,424
were recovered COVID-19 patients.
The screening of the 9,865,404 participants without a history of
COVID-19 found no newly conﬁrmed COVID-19 cases, andidentiﬁed 300 asymptomatic positive cases with a detection rate of
0.303 (95% CI 0.270–0.339)/10,000. The median age-stratiﬁed Ctvalues of the asymptomatic cases were shown in Supplementary
Table 1. Of the 300 asymptomatic positive cases, two cases camefrom one family and another two were from another family.
There were no previously conﬁrmed COVID-19 patients in these
two families. A total of 1174 close contacts of the asymptomatic
positive cases were traced, and they all tested negative for the
COVID-19. There were 34,424 previously recovered COVID-19
cases who participated in the screening. Of the 34,424 participants with a history of COVID-19, 107 tested positive again,
giving a repositive rate of 0.310% (95% CI 0.423–0.574%).
Virus cultures were negative for all asymptomatic positive andrepositive cases, indicating no “viable virus” in positive cases
detected in this study.
All asymptomatic positive cases, repositive cases and theirclose contacts were isolated for at least 2 weeks until the
results of nucleic acid testing were negative. None of detected
positive cases or their close contacts became symptomatic or
newly conﬁrmed with COVID-19 during the isolation period.
In this screening programme, single and mixed testing was
performed, respectively, for 76.7% and 23.3% of the collected
samples. The asymptomatic positive rates were 0.321 (95% CI
0.282–0.364)/10,000 and 0.243 (95% CI 0.183–0.315)/10,000,
respectively.
2

The 300 asymptomatic positive persons aged from 10 to 89
years, included 132 males (0.256/10,000) and 168 females (0.355/
10,000). The asymptomatic positive rate was the lowest in children or adolescents aged 17 and below (0.124/10,000), and thehighest among the elderly aged 60 years and above (0.442/10,000)(Table 1). The asymptomatic positive rate in females (0.355/
10,000) was higher than that in males (0.256/10,000).
The asymptomatic positive cases were mainly domestic andunemployed residents (24.3%), retired older adults (21.3%), andpublic service workers (11.7%) (Fig. 1).
The asymptomatic positive rate in urban districts was on
average 0.456/10,000, ranging from 0.317/10,000 in Hongshan to
0.807/10,000 in Wuchang district. A lower rate of asymptomatic
positive cases was found in suburban districts (0.132/10,000),
ranging from 0.047/10,000 in Xinzhou to 0.237/10,000 in Jiangan
district (Fig. 2).
Among the 7280 residential communities in Wuhan, asymptomatic positive cases were identiﬁed in 265 (3.6%) communities(only one case detected in 246 communities), while no asymptomatic positive cases were found in other 96.4% communities.
Testing of antibody against SARS-CoV-2 virus was positive
IgG (+) in 190 of the 300 asymptomatic cases, indicating that
63.3% (95% CI 57.6–68.8%) of asymptomatic positive cases wereactually infected. The proportion of asymptomatic positive caseswith both IgM (−) and IgG (−) was 36.7% (95% CI: 31.2–42.4%;
n = 110), indicating the possibility of infection window or false
positive results of the nucleic acid testing (Table 2).
Higher detection rates of asymptomatic infected persons werein Wuchang, Qingshan and Qiaokou districts, and the prevalenceof previously conﬁrmed COVID-19 cases were 68.243/10,000,
53.767/10,000, and 100.047/10,000, respectively, in the three
districts. Figure 3 shows that districts with a high detection rate ofasymptomatic positive persons generally had a high prevalence of
conﬁrmed COVID-19 cases (rs = 0.729, P = 0.002).
Discussion
The citywide nucleic acid screening of SARS-CoV-2 infection in
Wuhan recruited nearly 10 million people, and found no newly
conﬁrmed cases with COVID-19. The detection rate of asymptomatic positive cases was very low, and there was no evidence oftransmission from asymptomatic positive persons to traced close
contacts. There were no asymptomatic positive cases in 96.4% of
the residential communities.
Previous studies have shown that asymptomatic individuals
infected with SARS-CoV-2 virus were infectious3, and might
subsequently become symptomatic4. Compared with symptomatic patients, asymptomatic infected persons generally have lowquantity of viral loads and a short duration of viral shedding,which decrease the transmission risk of SARS-CoV-25. In thepresent study, virus culture was carried out on samples fromasymptomatic positive cases, and found no viable SARS-CoV-2
virus. All close contacts of the asymptomatic positive cases tested
negative, indicating that the asymptomatic positive cases detectedin this study were unlikely to be infectious.
There was a low repositive rate in recovered COVID-19
patients in Wuhan. Results of virus culturing and contract tracing
found no evidence that repositive cases in recovered COVID-19
patients were infectious, which is consistent with evidence fromother sources. A study in Korea found no conﬁrmed COVID-19
cases by monitoring 790 contacts of 285 repositive cases6.
The ofﬁcial surveillance of recovered COVID-19 patients in
China also revealed no evidence on the infectiousness of repositive cases7. Considering the strong force of infection of COVID198–10, it is expected that the number of conﬁrmed cases is
associated with the risk of being infected in communities. We

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Table 1 Characteristics of asymptomatic positive individuals.

Total
Sex
Male
FemaleAge (years old)
≤17
18–44
45–59
≥60
Administrative Districts in Wuhan
Wuchang
QingshanQiaokou
Hanyang
JianghanHongshanEast Lake High-tech Development Area
JianganCaidianJiangxia
Huangpi
HannanDongxihu
Xinzhou
East Lake Scenic Area of Wuhan

Total (%)
9,899,828 (100.0)

Asymptomatic positive persons (%)
300 (100.0)

Detection rate per 10,000 (95% CI)
0.303 (0.270–0.339)

P value

5,162,960 (52.2)
4,736,868 (47.8)

132 (44.0)
168 (56.0)

0.256 (0.214–0.303)
0.355 (0.303–0.413

0.005

969,014 (9.8)
4,448,230 (44.9)
2,492,943 (25.2)
1,989,641 (20.1)

12 (4.0)104 (34.7)
96 (32.0)
88 (29.3)

0.124 (0.064–0.216)
0.234 (0.191–0.283)
0.385 (0.312–0.470)
0.442 (0.355–0.545)<0.001

904,636 (9.1)
414,312 (4.2)
583,440 (5.9)
717,429 (7.2)
524,224 (5.3)
1,103,079 (11.1)
782,987 (7.9)
800,440 (8.1)
503,595 (5.1)
671,248 (6.8)
979,920 (9.9)
417,022 (4.2)
777,204 (7.9)
634,408 (6.4)
85,884 (0.9)

73 (24.3)
23 (7.7)
32 (10.7)
29 (9.7)
19 (6.3)
35 (11.7)
19 (6.3)
19 (6.3)
11 (3.7)
14 (4.7)
14 (4.7)
4 (1.3)5 (1.7)3 (1.0)0 (0.0)0.807 (0.633–1.015)
0.555 (0.352–0.833)
0.548 (0.375–0.774)
0.404 (0.271–0.581)
0.362 (0.218–0.566)
0.317 (0.221–0.441)
0.243 (0.146–0.379)
0.237 (0.143–0.371)
0.218 (0.109–0.391)
0.209 (0.114–0.350)
0.143 (0.078–0.240)
0.096 (0.026–0.246)
0.064 (0.021–0.150)
0.047 (0.010–0.138)
0.000 (0.000–0.430)<0.001χ2 test was used to assess the association between the detection rate of asymptomatic cases increased and sex and age. Urban districts of Wuhan includes Wuchang, Qingshan, Qiaokou, Hanyang,
Jiangan, Jianghan, and Hongshan; Suburban districts of Wuhan includes Hannan, Caidian, Dongxihu, Xinzhou, Jiangxia, Huangpi, East Lake High-tech Development Area, and East Lake Scenic Area
of Wuhan.

Housework or unemployment
The emeritus and retirees

47, 15.7%

2, 0.7%4, 1.3%Service workers in public place73, 24.3%

6, 2.0%Industrial labourer
Cadres staff

13, 4.3%

Agricultural labourer
17, 5.7%
Student
19, 6.3%

64, 21.3%

Teacher
Medical staff

20, 6.7%

Transportation service personnel
35, 11.7%

OthersFig. 1 The occupation distribution of asymptomatic positive cases (%). Note: Others included the self-employed, military personnel, and so on. (Source
data are provided as s Source Data ﬁle.).

found that asymptomatic positive rates in different districts of
Wuhan were correlated with the prevalence of previously conﬁrmed cases. This is in line with the temporal and spatial evolution (especially the long-tailed characteristic) of infectious
diseases11.
Existing laboratory virus culture and genetic studies9,10 showedthat the virulence of SARS-CoV-2 virus may be weakening overtime, and the newly infected persons were more likely to be
asymptomatic and with a lower viral load than earlier infectedcases. With the centralized isolation and treatment of all COVID19 cases during the lockdown period in Wuhan, the risk ofresidents being infected in the community has been greatly
reduced. When susceptible residents are exposed to a low dose ofvirus, they may tend to be asymptomatic as a result of their own

immunity. Serological antibody testing in the current study foundthat at least 63% of asymptomatic positive cases were actuallyinfected with SARS-CoV-2 virus. Nonetheless, it is too early to becomplacent, because of the existence of asymptomatic positive
cases and high level of susceptibility in residents in Wuhan.
Public health measures for the prevention and control of COVID19 epidemic, including wearing masks, keeping safe social distancing in Wuhan should be sustained. Especially, vulnerable
populations with weakened immunity or co-morbidities, or both,should continue to be appropriately shielded.
Findings from this study show that COVID-19 was well controlled in Wuhan at the time of the screening programme. After
two months since the screening programme (by August 9, 2020),
there were no newly conﬁrmed COVID-19 cases in Wuhan.

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Huangpi
0.143

Xinzhou
0.047

Dongxihu
0.064
Qingshan
0.555
ScenicCaidian
0.218

Hongshan
0.317

Hannan0.096

Jiangxia
0.209

0

10 Km

Detection rate of asymptomaticpatients at District level (per 10,000)
0.000
0.001–0.100
0.101–0.200
0.201–0.300
0.301–0.400
0.401–0.600
0.601–0.750

Fig. 2 The geographic distribution of the detection rate of asymptomatic positive cases. Note: 1 represents Jianghan district; 2 represents Qiaokou
district. (Source data are provided as s Source Data ﬁle.).

Table 2 Results of the detection of antibody in 300
asymptomatic positive persons.
IgM

IgG

Asymptomatic positive persons% (95% CI)

+
−
+
−

161
110
29
0

53.7 (47.8–59.4)
36.7 (31.2–42.4)
9.7 (6.6–13.6)
0.0 (0.0–1.2)

Results−−++“−” indicates negative; “+” indicates positive.

4

Further testing of SARS-CoV-2 in samples collected from market
environment settings in Wuhan were conducted, and found no
positive results after checking a total of 52,312 samples from 1795
market setting during June 13 to July 2, 202012.
This study has several limitations that need to be discussed.
First, this was a cross-sectional screening programme, and we areunable to assess the changes over time in asymptomatic positiveand reoperative results. Second, although a positive result of
nucleic acid testing reveals the existence of the viral RNAs, somefalse negative results were likely to have occurred, in particular
due to the relatively low level of virus loads in asymptomatic
infected individuals, inadequate collection of samples, and limited
accuracy of the testing technology13. Although the screening
programme provided no direct evidence on the sensitivity andspeciﬁcity of the testing method used, a meta-analysis reported a

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NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19802-wa120.000

Prevalence of previously confirmed
patients (per 10,000)

100.047
100.000
77.58280.00068.24360.00068.20165.12453.76746.06438.93740.00031.58030.57126.02125.05015.99520.00011.097

10.591

b

C
ai
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0.000

0.900
0.807

Detection rate of asymptomatic
infection (per 10,000)

0.800
0.700
0.600

0.555

0.548

0.500
0.404
0.362

0.400

0.317
0.300

0.243

0.237

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0.209

0.200

0.143
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Fig. 3 The prevalence of previously conﬁrmed patients and the detection rate of asymptomatic positive cases of COVID-19 in each district in Wuhan.
a The prevalence of previously conﬁrmed patients of COVID-19 in each district in Wuhan. b The detection rate of asymptomatic positive cases of COVID19 in each district in Wuhan. (Source data are provided as s Source Data ﬁle.).

pooled sensitivity of 73% (95% CI 68–78%) for nasopharayngeal
and throat swab testing of COVID-1914. Testing kits used in thescreening programme were publicly purchased by the government and these kits have been widely used in China and othercountries. Multiple measures were taken to possibly minimise
false negative results in the screening programme. For example,
standard training was provided to health works for sample collection to ensure the sample quality. The experiment procedures,
including specimen collection, extraction, PCR, were according toofﬁcial guidelines (Supplementary Note 1). For the real-time RTPCR assay, two target genes were simultaneously tested. Even so,
false negative results remained possible, particularly in any mass
screening programmes. However, even if test sensitivity was as
low as 50%, then the actual prevalence would be twice as high asreported in this study, but would still be very low. Around 7.1% of
eligible residents did not participate in the citywide nucleic acidscreening and the screening programme did not collect detailed
data on reasons for nonparticipation, which is a limitation of thisNATURE COMMUNICATIONS | (2020)11:5917 | https://doi.org/10.1038/s41467-020-19802-w | www.nature.com/naturecommunications

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study. Although there were no ofﬁcial statistics, a large number of
migrant workers and university students left Wuhan before thelockdown, joining their families in other cities or provinces for
traditional Chinese New Year. Therefore, it is likely that most
nonparticipants were not in Wuhan at the time of the screening.
The main objective of the screening programme was to assess therisk of COVID-19 epidemic in residents who were actually livingin the post-lockdown Wuhan. Therefore, the estimated positive
rates are unlikely to be materially inﬂuenced by nonparticipationof residents who were not in Wuhan or some residents who did
not participate in the screening for other reasons. Moreover,people who left Wuhan were the target population for monitoringin other provinces and cities and were required to take nucleic
acid testing. Although there was no ofﬁcial statistics showing thepositive rate of nucleic acid testing in this population, there wasno report that shown a higher positive rate of nucleic acid testing
than our ﬁndings.
In summary, the detection rate of asymptomatic positive casesin the post-lockdown Wuhan was very low (0.303/10,000), andthere was no evidence that the identiﬁed asymptomatic positive
cases were infectious. These ﬁndings enabled decision makers toadjust prevention and control strategies in the post-lockdown
period. Further studies are required to fully evaluate the impacts
and cost-effectiveness of the citywide screening of SARS-CoV-2
infections on population’s health, health behaviours, economy,
and society.
Methods
Study population and ethical approvals. Wuhan has about 11 million residents
in total, with seven urban and eight suburban districts. Residents are living in 7280
residential communities (or residential enclosures, “xiao-qu” in Chinese), and eachresidential community could be physically isolated from other communities forpreventing transmission of COVID-19.
The screening programme recruited residents (including recovered COVID-19
patients) currently living in Wuhan who were aged ≥6 years (5,162,960 males, 52.2%).
All participants provided written or verbal informed consent after reading a statementthat explained the purpose of the testing. For participants who aged 6–17 years old,
consent was obtained from their parents or guardians. The study protocol for an
evaluation of the programme based on anonymized screening data was approved by the
Ethics Committee of the Tongji Medical College Institutional Review Board, Huazhong
University of Science and Technology, Wuhan, China (No. IROG0003571).
Organizational guarantee and community mobilization. A citywide nucleic acidscreening group was formed, with specialized task teams contributing to comprehensive coordination, technical guidance, quality control, participation invitation, information management, communication, and supervision of the screening.
The city government invested 900 million yuan (RMB) in the testing programme.
From 14 May to 1 June 2020, in the peak time, up to 2907 sample collection sites
were functioning at the same time in Wuhan. Each sample collection site had an
assigned sample collection group, including several health professionals (staffedaccording to the number of communities’ residents), 2–4 community managers,
1–2 police ofﬁcers, and 1–2 inspectors. The sampling sites were set up based on thenumber and accessibility of local residents. Local community workers were
responsible for a safe and orderly sampling process to minimise the waiting time.
In addition, mobile sampling teams were formed by primary health care professionals and volunteers to conduct door-to-door sampling for residents who had
physical difﬁculties or were unable to walk.
About 50,000 health professionals (mainly doctors and nurses from community
health centers) and more than 280,000 person-times of community workers andvolunteers contributed to sample collection, transport of equipment and samples
collected, arrangement of participation process, and maintaining order of samplingsites. Public information communication and participant invitation were
implemented through mass media, mobile messages, WeChat groups, andresidential community broadcasts, so as to increase residents’ awareness and theparticipation.
Acquisition, preservation, and transport of samples. All sampling personnel
received standard training for the collection of oropharyngeal swab samples. To
minimise the risk of cross-infection, the sampling process strictly followed a disinfection process and environmental ventilation were ensured. The collected
samples were stored in a virus preservation solution or immersed in isotonic saline,
tissue culture solution, or phosphate buffer (Supplementary note 1). Then, allsamples were sent to testing institutions within 4 h using delivery boxes for6

biological samples refrigerated with dry ice to guarantee the stability of nucleic acidsamples.
Technical methods for laboratory testing of collected samples. A total of 63
nucleic acid testing laboratories, 1451 laboratory workers and 701 testing equipment were involved in the nucleic acid testing. Received samples were stored at
4 °C and tested within 24 h of collection. Any samples that could not be tested
within 24 h were stored at −70 °C or below (Supplementary note 1). In addition to“single testing” (i.e., separate testing of a single sample), “mixed testing” was also
performed for 23% of the collected samples to increase efﬁciency, in which ﬁvesamples were mixed in equal amounts, and tested in the same test tube. If a mixed
testing was positive for COVID-19, all individual samples were separately retested
within 24 h15.
Details regarding technical methods for sequencing and virus culture were
provided in Supplementary note 1. Real-time reverse transcriptase-polymerase
chain reaction (RT-PCR) assay method was used for the nucleic acid testing. We
simultaneously ampliﬁed and tested the two target genes: open reading frame 1ab(ORF1ab) and nucleocapsid protein (N) (Supplementary Note 1). A cycle thresholdvalue (Ct-value) less than 37 was deﬁned as a positive result, and no Ct-value or a
Ct-value of 40 or more was deﬁned as a negative result. For Ct-values ranging from37 to 40, the sample was retested. If the retest result remained less than 40 and theampliﬁcation curve had obvious peak, the sample was classiﬁed as positive;
otherwise, it was reported as being negative. These diagnostic criteria were basedon China’s ofﬁcial recommendations16.
For asymptomatic positive cases, virus culture was carried out in biosafety level3 laboratories. The colloidal gold antibody test was also performed forasymptomatic positive cases (Supplementary note 1). All testing results weredouble entered into a speciﬁcally designed database, and managed by the Big Dataand Investigation Group of the COVID-19 Prevention and Control Centre in
Wuhan, which was established to collect and manage data relevant to the COVID19 epidemic.
Participant data collection and management. Before sample collection, residents
electronically (using a speciﬁcally designed smartphone application) self-uploaded
their personal information, including ID number, name, sex, age, and place ofresidence. Then, the electronic machine system generated a unique personal barcode and stuck it on the sample tube to ensure the match between the sample andthe participant. Then trained staff interviewed each individual regarding the historyof COVID-19 and previous nucleic acid testing. There was a database of conﬁrmed
COVID-19 cases in Wuhan, which can be used to validate the self-reported previous COVID-19 infection. All information was entered into a central database.
The testing results were continually uploaded to the central database by testing
institutions. Contact tracing investigations were conducted on participants who
tested positive for SARS-CoV-2, to track and manage their close contacts. The preexisting unique identiﬁcation code for each resident was used as the programme’s
identiﬁcation number, to ensure information accuracy during the whole process of
screening, from sampling, nucleic acid testing, result reporting, the isolation of
detected positive cases, and tracing of close contacts of positive cases. All screening
information was kept strictly conﬁdential and was not allowed to be disclosed orused for other purposes other than clinical and public health management. Personal information of asymptomatic positive cases was only disclosed to designatedmedical institutions and community health centres for the purpose of medical
isolation and identiﬁcation of close contacts. Researcher was blind to the study
hypothesis during data collection.
Biological security guarantee. Nucleic acid testing was performed in biosafetylevel-2 (BSL-2) laboratories, and virus culture was conducted in biosafety level-3
laboratories. Sampling and testing personnel adopted the personal protective
measures according to the standard of biosafety level-3 laboratories. Participating
laboratories implemented control measures to guarantee biological safety inaccordance with relevant regulations17.
Result query and feedback. Two to three days after sample collection, participants could inquire about their test results using WeChat or Alipay application by
their unique ID numbers. The results included text descriptions of nucleic acidtesting and coloured health codes. A green coloured health code refers to a negative
result, and a red coloured health code indicates a positive result.
Deﬁnition and management of identiﬁed conﬁrmed cases and close contacts.
In this study, all conﬁrmed COVID-19 cases were diagnosed by designated medical
institutions according to National Guidelines for the Prevention and Control of
COVID-19 (Supplementary Note 2). Asymptomatic positive cases referred toindividuals who had a positive result during screening, and they had neither ahistory of COVID-19 diagnosis, nor any clinical symptoms at the time of thenucleic acid testing. Close contacts were individuals who closely contacted with anasymptomatic positive person since 2 days before the nucleic acid sampling16.
Repositive cases refer to individuals who recovered from previously conﬁrmed
COVID-19 disease and had a positive testing again in the screening programme.
All repositive cases, asymptomatic positive persons, and their close contacts were

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NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19802-w

isolated for at least 2 weeks in designated hotels managed by primary health care
professionals, and they were released from isolation only if two consecutive nucleic
acid tests were negative.
Statistical analysis. Detection rate of asymptomatic positive or repositive cases
was calculated by dividing the number of individuals with a positive result of
nucleic acid testing by the number of participants tested. Because of extremely low
detection rates, we calculated 95% conﬁdence intervals of estimated proportions
using Pearson–Klopper exact method, implemented through R package “binom”
version 1.1-118. SPSS version 22.0 was used for other statistical analyses. We
analyzed the distribution of asymptomatic positive cases and assessed the Spearman correlation between the asymptomatic positive rate and the prevalence ofpreviously conﬁrmed COVID-19 cases in different districts of Wuhan. Differencesin asymptomatic positive rates by sex and age groups were assessed using the χ2
test. ArcGIS 10.0 was used to draw a geographic distribution map of asymptomatic
positive cases. A value of P < 0.05 (two-tailed) was considered statisticallysigniﬁcant.
Reporting summary. Further information on research design is available in the Nature
Research Reporting Summary linked to this article.

Data availability
Detailed data directly used to generate each ﬁgure or table of this study are available
within the article, Supplementary Information and source data are provided withthis paper.

Received: 18 August 2020; Accepted: 27 October 2020;

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Acknowledgements
We would like to thank all institutions and all citizens in Wuhan for their support forcitywide nucleic acid screening work. We also would like to thank the Wuhan city
government for this citywide nucleic acid testing, sampling and management, and thankthe big data and investigation group of COVID-19 prevention and control institution in
Wuhan (the data and investigation group of Wuhan Municipal Health Commission) for
their efforts in the data collection. In addition, we would like to thank the National Social
Science Foundation of China (Grant No. 18ZDA085) for supporting the fund.

Author contributions
S.Y.C., C.W., X.X.Y., and Z.X.L. conceived the study. C.W., Y.C.H., T.T.W., K.L., H.B.X.,
and X.S. participated in the acquisition of data. S.B.W. and J.G. were responsible for theon-site specimen collection, laboratory testing quality evaluation, and control. Y.C.H.,
T.T.W., and L.Q.L. analyzed the data. H.J., Y.H.G., and F.J.S. gave advice on methodology. Q.F.T. and C.Z.L. investigated the responses to the citywide nucleic acid testing
among residents lived in outside of Wuhan city. S.Y.C., Y.G., C.W., and X.X.Y. drafted
the manuscript, Y.G., M.B., and F.J.S. revised the manuscript, and M.B., C.Z.L., and F.J.S.
critically commented and edited the manuscript. All authors read and approved the ﬁnal
manuscript. Z.X.L. is the guarantor of this study.

Competing interests
The authors declare no competing interests.

Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41467020-19802-w.
Correspondence and requests for materials should be addressed to C.L., F.S., X.Y. or Z.L.
Peer review information Nature Communications thanks Junxiong Vincent Pang andthe other, anonymous reviewer(s) for their contribution to the peer review of this work.
Peer review reports are available.
Reprints and permission information is available at http://www.nature.com/reprints
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