Prevalence of SARS-CoV-2 in Household Members and Other Close Contacts of COVID-19 Cases: A Serologic Study in Canton of Vaud, Switzerland

Abstract

Background: Research on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission within households and other close settings using serological testing is scarce.

Methods: We invited coronavirus disease 2019 (COVID-19) cases diagnosed between February 27 and April 1, 2020, in Canton of Vaud, Switzerland, to participate, along with household members and other close contacts. Anti-SARS-CoV-2 immunoglobulin G antibodies were measured using a Luminex immunoassay. We estimated factors associated with serological status using generalized estimating equations.

Results: Overall, 219 cases, 302 household members, and 69 other close contacts participated between May 4 and June 27, 2020. More than half of household members (57.2%; 95% CI, 49.7%-64.3%) had developed a serologic response to SARS-CoV-2, while 19.0% (95% CI, 10.0%-33.2%) of other close contacts were seropositive. After adjusting for individual and household characteristics, infection risk was higher in household members aged ≥65 years than in younger adults (adjusted odds ratio [aOR], 3.63; 95% CI, 1.05-12.60) and in those not strictly adhering to simple hygiene rules like hand washing (aOR, 1.80; 95% CI, 1.02-3.17). The risk was lower when more than 5 people outside home were met during semiconfinement, compared with none (aOR, 0.35; 95% CI, 0.16-0.74). Individual risk of household members to be seropositive was lower in large households (22% less per each additional person).

Conclusions: During semiconfinement, household members of a COVID-19 case were at very high risk of getting infected, 3 times more than close contacts outside home. This highlights the need to provide clear messages on protective measures applicable at home. For elderly couples, who were especially at risk, providing external support for daily basic activities is essential.

Keywords: COVID-19; SARS-CoV-2; household; serology; transmission.

Figure 1.

Percentage of participants with a positive serology test result, by type of participant. Index cases: crude proportion, calculation of 95% confidence interval using the Clopper-Pearson method. For household members and close contacts outside the household, proportion and corresponding 95% confidence interval were estimated using GEEs (exchangeable correlation structure). Abbreviation: GEEs, generalized estimating equations.

Figure 2.

Unadjusted association between characteristics of household members of index cases and seropositivity (bivariable analysis). For calculation of odds ratio, the correlation between household members of the same index case was taken into account using GEEs (exchangeable correlation structure, logit link function). Error bars represent the limits of the 95% confidence interval for the odds ratio. ^aAdult participants only. ^bChildren aged <12 considered nonsmokers. Abbreviation: GEEs, generalized estimating equations.

Figure 3.

Percentage of household members reporting specific symptoms, according to serology test result (household members not reporting prior nasal or throat swabbing to detect severe acute respiratory syndrome coronavirus 2). The correlation between household members of the same index case was taken into account using GEEs (exchangeable correlation structure). ^aDifference not statistically significant at the .05 level. Abbreviation: GEEs, generalized estimating equations.