Secondary attack rates of COVID-19 in Norwegian families: A nation-wide register-based study

Telle, K.; Jorgensen, S. B.; Hart, R. K.; Greve-Isdahl, M.; Kacelnik, O.

Background: Reported transmission rates of SARS-CoV-2 within families vary widely, and there are few reports on transmission from children to other family members. More knowledge is needed to guide infection control measures. Objective: To characterize the family index case for detected SARS-CoV-2 and describe testing and secondary attack rates in the family. Design: Register-based cohort study. Setting: Individual-level administrative data of all families and all PCR tests for SARS-CoV-2 in Norway in 2020. Participants: All families with at least one parent and one child below the age of 20, who lived at the same address (N=662 582), where at least one member tested positive for SARS-CoV-2 in 2020. Main outcome measures: Secondary attack rates (SAR7) were defined as the share of non-index family members with a positive PCR test within seven days of the index case. SARs were calculated separately for parent- and child-index cases, and for parent- and child-secondary cases. Results: We identified 7548 index cases, comprising 26 991 individuals, of which 12184 were parents and 14808 children. The index was a parent in 66% of the cases. Among the children, 42% of the index cases were in the age group 17-20 and only 8% 0-6 years. When the index was a parent, SAR7 was 24% (95%CI 24 to 25), whilst SAR7 was 14% (95%CI 13 to 15) when the index was a child. However, SAR7 was 24% (95%CI 20 to 28) when the index was a child aged 0-6 years and declined steeply with increasing age of the index child. SAR7 from index parent to other parents was 35% (95%CI 33 to 36), and from index child to other children 12% (95%CI 11 to 13). SAR7 from index child aged 0-6 to parents was 27% (95%CI 22 to 33). The percent of non-index family members tested within 7 days after the index case, increased from about 20% in April to 80% in December, however, SAR7 stabilized at about 20% from May. Conclusion: Parents and older children are most often index cases for SARS-CoV-2 in families in Norway, while parents and young children more often transmit the virus within the families. This study suggests that whilst the absolute infection numbers are low for young children because of their low introduction rate, when infected, young children and parents transmit the virus to the same extent within the family.