Seroconversion in household members of COVID-19 outpatients.

We read with interest the Article by Qifang Bi and colleagues, in which they reported a household secondary attack rate, as detected by repeated RT-PCR tests, of approximately 11%. We have found substantially higher attacks rates in western Norway through detection of antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

The first case of COVID-19 in Norway was identified in Bergen on Feb 28, 2020, before the outbreak was declared a pandemic, allowing rigorous testing of suspected cases before and during the rise in confirmed cases. All suspected COVID-19 cases in the peak period between Feb 28 and April 4 were referred to the Bergen municipality emergency room for centralised evaluation and testing according to a strict exposure likelihood algorithm, allowing an overview of the early virus spread in the population. If a family was exposed, only the index case was tested for SARS-CoV-2 infection. Both cases and household members were tested for specific antibodies to the receptor binding domain of SARS-CoV-2, as described by Stadlbauer and colleagues, at 6 weeks after the index patient tested positive by RT-PCR.

Of 158 cases, 125 (79%) tested positive for antibodies and 12 (8%) were defined as borderline. In 77 household members, 24 (31%) tested positive and two (3%) were borderline. Our results show that detection of seroconversion might provide a more accurate picture of attack rates in households than intermittent RT-PCR testing.