Household transmission of the SARS-CoV-2 Omicron variant in Denmark
Frederik Plesner Lyngse, Laust Hvas Mortensen, Matthew Denwood, Lasse Engbo Christiansen, Camilla Holten Møller, Robert Skov, Katja Spieß, Anders Fomsgaard, Ria Lassaunière, Morten Rasmussen, Marc Stegger, Claus Henrik Nielsen, Raphael N. Sieber, Arieh S. Cohen, Frederik Trier Møller, Maria Overvad, Kåre Mølbak, Tyra Grove Krause, Carsten Kirkeby
Abstract
In late 2021, the Omicron SARS-CoV-2 variant overtook the previously dominant Delta variant, but the extent to which this transition was driven by immune evasion or a change in the inherent transmissibility is currently unclear. We estimate SARS-CoV-2 transmission within Danish households during December 2021. Among 26,675 households (8,568 with the Omicron VOC), we identified 14,140 secondary infections within a 1-7-day follow-up period. The secondary attack rate was 29% and 21% in households infected with Omicron and Delta, respectively. For Omicron, the odds of infection were 1.10 (95%-CI: 1.00-1.21) times higher for unvaccinated, 2.38 (95%-CI: 2.23-2.54) times higher for fully vaccinated and 3.20 (95%-CI: 2.67-3.83) times higher for booster-vaccinated contacts compared to Delta. We conclude that the transition from Delta to Omicron VOC was primarily driven by immune evasiveness and to a lesser extent an inherent increase in the basic transmissibility of the Omicron variant.