Attack rates amongst household members of outpatients with confirmed COVID-19 in Bergen, Norway: A case-ascertained study
Kanika Kuwelker, Fan Zhou, Bjørn Blomberg, Sarah Lartey, Karl A. Brokstad, Mai-Chi Trieu, Amit Bansal, Anders Madsen, Florian Krammer, Kristin G. I. Mohn, Camilla Tøndel, Dagrunn Waag Linchausen, Rebecca Jane Cox, Nina Langeland, Annette Corydon, Francisco Gómez Real, Geir Bredholt, Hauke Bartsch, Helene Heitmann Sandnes, Juha Vahokoski, Kjerstin Jacobsen, Marianne Eidsheim, Marianne Sævik, Nina Urke Ertesvåg, Synnøve Ygre Hauge, Therese Bredholt Onyango
Abstract
Background Households studies reflect the natural spread of SARS-CoV-2 in immunologically naive populations with limited preventive measures to control transmission. We hypothesise that seropositivity provides more accurate household attack rates than RT-PCR. Here, we investigated the importance of age in household transmission dynamics. Methods We enroled 112 households (291 participants) in a case-ascertained study in Bergen, Norway from 28th February to 4th April 2020, collecting demographic and clinical data from index patients and household members. SARS-CoV-2-specific antibodies were measured in sera collected 6–8 weeks after index patient nasopharyngeal testing to define household attack rates. Findings The overall attack rate was 45% (95% CI 38–53) assessed by serology, and 47% when also including seronegative RT-PCR positives. Serology identified a higher number of infected household members than RT-PCR. Attack rates were equally high in children (48%) and young adults (42%). The attack rate was 16% in asymptomatic household members and 42% in RT-PCR negative contacts. Older adults had higher antibody titres than younger adults. The risk of household transmission was higher when the index patient had fever (aOR 3.31 [95% CI 1.52–7.24]; p = 0.003) or dyspnoea (aOR 2.25 [95% CI 1.80–4.62]; p = 0.027) during acute illness. Interpretation Serological assays provide more sensitive and robust estimates of household attack rates than RT-PCR. Children are equally susceptible to infection as young adults. Negative RT-PCR or lack of symptoms are not sufficient to rule out infection in household members. Funding Helse Vest (F-11628), Trond Mohn Foundation (TMS2020TMT05).