Household studies provide key insights on the transmission of, and susceptibility to, SARS-CoV-2
Authors: Virginia E Pitzer, Ted Cohen
Journal: The Lancet Infectious Diseases (2020)

Studies of household members and close contacts of individuals infected with a communicable disease such as COVID-19 are a key source of information for infectious disease epidemiologists. Exposure to an infectious individual is the most important risk factor for communicable diseases. In household studies, only individuals who have been exposed are included, allowing for careful examination of other individual-level risk factors and quantification of transmission probabilities. The study by Qin-Long Jing and colleagues1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar published in The Lancet Infectious Diseases provides important insights into factors affecting transmission from COVID-19 primary cases and susceptibility of their close contacts. The considerable contact tracing effort undertaken in Guangzhou, China, by the Guangzhou Center for Disease Control and Prevention, enabled the comprehensive analysis by Jing and colleagues. Surveillance has shown that older individuals (aged ≥60 years) are disproportionately represented among diagnosed COVID-19 cases.2Bi Q Wu Y Mei S et al.Epidemiology and transmission of COVID-19 in 391 cases and 1286 of their close contacts in Shenzhen, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online April 27.)https://doi.org/10.1016/S1473-3099(20)30287-5Summary Full Text Full Text PDF PubMed Scopus (1082) Google Scholar However, this observation might reflect the fact that older individuals are more likely to be infected or that they have greater severity of symptoms than younger individuals, making these individuals more likely to be diagnosed. In this retrospective cohort study, the close contacts of primary cases (all ages) were identified and quarantined, with nasal swabs collected on days 1 and 14 for RT-PCR testing.1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar Thus, typical biases associated with which individuals are exposed and which are tested were minimised. Compared with the oldest age group (≥60 years), the risk of household infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was lower among younger age groups (odds ratio [OR] 0·23 [95% CI 0·11–0·46] among individuals aged younger than 20 years; OR 0·64 [0·43–0·97] among individuals aged 20–59 years) and only 5% of contacts aged younger than 20 years were infected, which suggests that older age is associated with increased risk of infection conditional on exposure.1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar Although household studies provide a unique opportunity to study transmission in a clearly identified cohort of close contacts, it is still possible that transmission events might have been missed. For example, individuals with an asymptomatic infection that resolved during the 2 weeks between follow-up tests would remain undetected. The proportion of identified infections among contacts that were asymptomatic was lower in this study (5%)1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar than estimates from other studies of SARS-CoV-2 (13–18%).3Mizumoto K Kagaya K Zarebski A Chowell G Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020.Euro Surveill. 2020; 252000180Crossref Scopus (1572) Google Scholar, 4Kimball A Hatfield KM Arons M et al.Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility–King County, Washington, March 2020.MMWR Morb Mortal Wkly Rep. 2020; 69: 377-381Crossref PubMed Google Scholar Unidentified asymptomatic infections might affect the interpretation of the estimated secondary attack rate and could lead to underestimation of the secondary attack rate. The secondary attack rate for SARS-CoV-2 has been estimated to be approximately twice as high as that of SARS-CoV,5Lau JT Lau M Kim JH Tsui HY Tsang T Wong TW Probable secondary infections in households of SARS patients in Hong Kong.Emerg Infect Dis. 2004; 10: 235-243Crossref PubMed Scopus (76) Google Scholar, 6Pitzer VE Leung GM Lipsitch M Estimating variability in the transmission of severe acute respiratory syndrome to household contacts in Hong Kong, China.Am J Epidemiol. 2007; 166: 355-363Crossref PubMed Scopus (36) Google Scholar and comparable to that of influenza,7Lau MS Cowling BJ Cook AR Riley S Inferring influenza dynamics and control in households.Proc Natl Acad Sci USA. 2015; 112: 9094-9099Crossref PubMed Scopus (32) Google Scholar although the estimated daily probability of transmission was similar to SARS-CoV. The key difference between SARS-CoV-2 and SARS-CoV is that the probability of transmission is substantially higher during the presymptomatic incubation period for SARS-CoV-2,1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar, 8He X Lau EHY Wu P et al.Temporal dynamics in viral shedding and transmissibility of COVID-19.Nat Med. 2020; 26: 672-675Crossref PubMed Scopus (2883) Google Scholar whereas little to no transmission occurred before symptom onset for SARS-CoV.6Pitzer VE Leung GM Lipsitch M Estimating variability in the transmission of severe acute respiratory syndrome to household contacts in Hong Kong, China.Am J Epidemiol. 2007; 166: 355-363Crossref PubMed Scopus (36) Google Scholar Thus, SARS-CoV was much easier to control through case isolation and quarantine of contacts.9Fraser C Riley S Anderson RM Ferguson NM Factors that make an infectious disease outbreak controllable.Proc Natl Acad Sci USA. 2004; 101: 6146-6151Crossref PubMed Scopus (789) Google Scholar Jing and colleagues estimate that prompt case isolation only prevented 20–50% of secondary cases of COVID-19 in Guangzhou.1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar Stringent control measures were implemented in Guangzhou on Jan 23, 2020, soon after the epidemic was found to have spread from Wuhan. To determine how control measures affected the transmission of SARS-CoV-2 at the population level, Jing and colleagues estimated the effective reproductive number (Rt) as the mean number of contacts per case multiplied by the probability of transmission to those contacts on each day of the epidemic.1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar This approach differs from statistical methods typically used to estimate Rt from population-level data.10Wallinga J Teunis P Different epidemic curves for severe acute respiratory syndrome reveal similar impacts of control measures.Am J Epidemiol. 2004; 160: 509-516Crossref PubMed Scopus (804) Google Scholar The approach used by Jing and coauthors might underestimate Rt, even for the most permissive scenario (represented by the upper bounds of Rt), since they assume that all contacts have been identified and reported, and that all asymptomatic infections have been identified. These assumption seem unlikely to have been be fully met, considering that more than 50% of all identified cases in Guangzhou were considered primary infections (ie, with no known source of exposure, or were assumed to have been infected outside of Guangzhou) and the low proportion of asymptomatic infections.1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar Furthermore, this approach does not account for local primary cases (ie, primary cases who had not recently travelled to or resided in Hubei province), which have no identified source of exposure and accounted for 16% of all identified cases in Guangzhou.1Jing Q-L Liu M-J Yuan J et al.Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort study.Lancet Infect Dis. 2020; (published online June 17.)https://doi.org/10.1016/S1473-3099(20)30471-0Summary Full Text Full Text PDF PubMed Scopus (301) Google Scholar Thus, these estimates might underestimate Rt and overstate the effectiveness of control measures imposed in Guangzhou. Understanding of the factors that affect SARS-CoV-2 transmission has been rapidly evolving over the past few months. This study demonstrates the value of carefully collected contact tracing data to understand risk factors for transmission and susceptibility. The findings also confirm the relative importance of presymptomatic transmission, and the association between older age and susceptibility, key insights which should inform the design of intervention strategies. VEP reports reimbursement for travel expenses from Merck and Pfizer, outside the submitted work; and is a member of the WHO Immunization and Vaccine-related Implementation Research Advisory Committee. TC declares no competing interests. Household secondary attack rate of COVID-19 and associated determinants in Guangzhou, China: a retrospective cohort studySARS-CoV-2 is more transmissible in households than SARS-CoV and Middle East respiratory syndrome coronavirus. Older individuals (aged ≥60 years) are the most susceptible to household transmission of SARS-CoV-2. In addition to case finding and isolation, timely tracing and quarantine of close contacts should be implemented to prevent onward transmission during the viral incubation period. Full-Text PDF