High Secondary Attack Rate and Persistence of SARS-CoV-2 Antibodies in Household Transmission Study Participants, Finland 2020

Abstract

Background: Household transmission studies offer the opportunity to assess both secondary attack rate and persistence of SARS-CoV-2 antibodies over time.Methods: We invited confirmed COVID-19 cases and their household members to attend up to four household visits with collection of nasopharyngeal and serum samples over 28 days after index case onset. We calculated secondary attack rates (SAR) based on the presence of SARS-CoV-2 nucleoprotein IgG antibodies (IgG Ab) and/or neutralizing antibodies (NAb) overall and per households. Three and six months later, we assessed the persistence of SARS-CoV-2 antibodies.Findings: We recruited 39 index cases and 90 household members. Among 87 household members evaluated, SAR was 48% (n=42), including 37 symptomatic secondary cases. In total, 80/129 (62%) participants developed both IgG Ab and NAb, while three participants only developed IgG Ab. Among participants who had both IgG Ab and NAb during the initial follow-up, 68/69 (99%) and 63/70 (90%) had IgG Ab and NAb at 3 months, while at 6 months, 59/75 (79%) and 63/75 (84%) had IgG Ab and NAb, respectively. Participants who required hospital care had initially 5-fold IgG Ab concentrations compared to cases with mild symptoms and 8-fold compared to asymptomatic cases.Interpretation: Following detection of a COVID-19 case in a household, other members had a high risk of becoming infected. Follow-up of participants showed strong persistence of antibodies in most cases.Funding Information: This study was supported by THL coordinated funding for COVID-19 research (Finnish Government’s supplementary budget) and by the Academy of Finland (Decision number 336431).Declaration of Interests: THL has received research funding for studies not related to COVID-19 from GlaxoSmithKline Vaccines (NE, CV, AAP and MM as investigators), Pfizer (AAP) and Sanofi Pasteur (AAP). Other authors have no competing interests.Ethics Approval Statement: The Finnish communicable diseases law and the law on the duties of the Finnish Institute for Health and Welfare allowed the implementation of the initial household transmission study and 3 months convalescent sample collection without seeking further institutional ethical review11,12. The protocol for the follow-up visits at 6 months was reviewed and approved by the HUS ethical committee and registered under the Development of seroprevalence in Finland during the new coronavirus (SARS-CoV-2) epidemic – serological population study protocol HUS/1137/2020. Informed consent was obtained from all cases and contacts or their parents or legal guardian before any procedure was performed.