Attack rates amongst household members of outpatients with confirmed COVID-19 in Bergen, Norway: A case-ascertained study

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).

Household transmission of SARS-CoV-2; a prospective longitudinal study showing higher viral load and transmissibility of the Alpha variant compared to previous strains

“…Our SAR-estimate of 42.5% for non-VOC viruses is higher than the estimates of around 17 % found in other early reviews (2, 14), but is in accordance with another Norwegian household study from the first wave of the pandemic, which estimated a SAR of 47% based on RT-qPCR and seroconversion (15). Other studies performed in the UK, the Netherlands, and the US in the beginning/middle of 2020 also found similar SARs of 37%-53% (16-18).…”

“…However, our estimate is in accordance with the 37% found in a prospective study performed in the UK in the beginning of 2020 [12], even though they also included probable secondary cases not confirmed by RT-PCR. A household study from Norway in the spring of 2020 reported a SAR of 47% based on RT-PCR and seroconversion [13]. Similar high SARs were found in the US (53%) and the Netherlands (51% in adults, 35% in children) [14,15].…”

Long COVID in a prospective cohort of home-isolated patients

“…We collected demographic and clinical data as well as blood samples. Household members of patients who tested positive were included to ensure completeness of the cohort, and their infection was diagnosed by SARS-CoV-2-specific antibodies at 2 months 15 . At 6-month follow-up, the study population available for analysis comprised 312 patients, of whom 247 were home-isolated and 65 were hospitalized (Fig.…”

“…The study population included SARS-CoV-2 RT-PCR-positive patients diagnosed at Bergen Municipality Emergency Clinic and those admitted to the two neighboring city hospitals: Haukeland University Hospital and Haraldsplass Deaconess Hospital. Household contacts of identified patients were invited to participate in the study as secondary cases (seroconverters) or seronegative controls 15 . All registered patients with SARS-CoV-2 were invited to participate.…”

Prevalence of SARS-CoV-2 antibodies in Denmark: nationwide, population-based seroepidemiological study

“…This is in line with a previous meta-analysis which have estimated a secondary attack rate of 16.6% (95% CI, 14.0–19.3%) [ 38 ] and with two register-based studies carried out in Denmark where secondary attack rates between 10%-30% were found [ 11 , 39 ]. However, a Norwegian study found an overall attack rate of 45% in households by use of serology [ 40 ]. We were unable to disentangle the direction of transmission between generations and our design did not allow us to shed light on seroconversion of siblings or other household members who were not the legal parent of the child.…”