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

Our take —

At the start of the pandemic, 112 households in Bergen, Norway, with 112 index cases of SARS-CoV-2 and 291 total members were enrolled in a study to understand household transmission. The study used serological testing to understand attack rates, and found an overall attack rate of 45%. There were no significant differences by age or gender, and notably attack rates among young children were not lower than among adults. The results from serological testing proved to be better at capturing all those that had the virus (more sensitive) than PCR testing alone, suggesting a valuable place for serological testing to understand household transmission dynamics of SARS-CoV-2.

Study design

Case Series

Study population and setting

Between February 28 and April 4, 2020, 112 households in Begen, Norway, with a total of 291 participants, were enrolled into a study to understand household transmission of SARS-CoV-2. Households were selected based on index cases (positive RT-PCR test) identified at the Bergen Municipality Emergency Clinic. Household members included all those who lived in the same household as the index patient. Households where the index patient lived alone were excluded. SARS-CoV-2 antibodies were measured (SARS-CoV-2-spike- specific IgG) six to eight weeks after the index case tested positive in order to determine household attack rate. RT-PCR testing for household members was completed among those who reported COVID-19 related symptoms.

Summary of Main Findings

The secondary household attack rate was 45% (95% CI: 28-53%) based on serological testing (positive antibodies). The attack rate increased to 47% when adding those who tested negative for antibodies, but who were symptomatic and had positive RT-PCR tests. A total of 70 individuals received RT-PCR testing, and 32 tested positive. Using the standard RT-PCR testing algorithm which would have tested only symptomatic individuals, the attack rate would have been 17.9% (32/179). The risk of transmission was increased if the index patient had a fever (adjusted [a]OR: 3.31, 95%CI: 1.52-7.24), or difficulty breathing (aOR: 2.25, 95% CI: 1.80-4.62). There were no significant differences in attack rate by household member age, though those at risk for infection who were 60 years and older had an attack rate of 73% (95%CI 48-89%), and those 0-10 years old had an attack rate of 48% (95%CI: 29-67%).

Study Strengths

Testing of household members using antibody or serological tests provides a more sensitive, and likely more robust, estimate of household attack rate than RT-PCR alone.


This study occurred at the beginning of the pandemic when fewer public health mitigation strategies had been put into place, which may make it difficult to generalize these results to other settings. It is possible that some selection bias occurred in which index cases and households agreed to participate in the study (228 initially eligible: 84/228 excluded, and 32/228 redefined as household members), though many were excluded because they resided in a single-person household (51/84).

Value added

This study shows the added value of utilizing serological tests for understanding transmission dynamics, particularly among household members.

This review was posted on: 28 May 2021