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: Understanding community-based SARS-CoV-2 transmission is crucial to inform public health decisions. Research on SARS-CoV-2 transmission within households and other close settings using serological testing is scarce.Methods: We invited 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 IgG antibodies were measured using a Luminex immunoassay. We estimated factors associated with serological status using generalized estimating equations.Findings: Overall, 219 COVID-19 index 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 or more than in younger adults (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 the semi-confinement, compared to none (aOR 0·35, 95%CI 0·16-0·74). The individual risk of household members to be seropositive was lower in large households (22% less per each additional person).Interpretation: We find that, during semi-confinement, 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 specific protective measures applicable at home. For elderly couples, who were especially at risk, providing them external support for daily basic activities is essential.Funding: Center for Primary Care and Public Health (Unisanté), Canton of Vaud, Leenaards Foundation, Fondation pour l'Université de Lausanne. SerocoViD is part of Corona Immunitas coordinated by SSPH+.Declaration of Interests: We declare no competing interests.Ethics Approval Statement: The Cantonal Ethics Committee of Vaud, Switzerland (ID 2020-00887) approved the protocol.