Household transmission of SARS-CoV-2 in Norway; a prospective, longitudinal study showing increased transmissibility of the Alpha variant (B.1.1.7) compared with other variants

ABSTRACT Background We studied the secondary attack rate (SAR), risk factors, and precautionary practices of household transmission in a prospective longitudinal study. Moreover, we compared household transmission between the Alpha (B.1.1.7) variant and non-variants of concern (non-VOCs). Methods We recruited households of confirmed COVID-19 cases from May 2020 to May 2021. Households received 8 home visits over 6 weeks. Biological samples and questionnaire data were collected. Results We recruited 70 confirmed COVID-19 cases and 146 household contacts. Transmission occurred in 60% of the households; the overall SAR for household contacts was 49.6%. The SAR was significantly higher for the Alpha variant (77.8%) compared with non-VOC variants (42.5%) and was associated with a higher viral load. SAR was higher in household contacts aged [≥]40 years (64%) than in younger contacts (40-47%), and for contacts of cases with loss of taste/smell. Close contact prior to confirmation of infection tended to give a higher SAR. A significantly lower SAR was found for sleeping separately from the primary case after confirmation of infection. Conclusion We found substantial household transmission, particularly for the Alpha variant. Precautionary practices seem to reduce SAR; however, prevention of transmission within households may become difficult with more transmissible variants.