ABSTRACT
1The SARS-CoV-2 Delta variant of concern (VOC), which has shown increased transmission compared with previous variants, emerged rapidly globally during the first half of 2021, and became one of the most widespread SARS-CoV-2 variants worldwide. We utilized total population data from 24,693 Danish households with 53,584 potential secondary cases to estimate household transmission of the Delta VOC in relation to vaccination status. We found that the vaccine effectiveness against susceptibility (VES) was 61% (95%-CI: 59-63) and that the vaccine effectiveness against transmissibility (VET) was 42% (95%-CI: 39-45). We also found that unvaccinated individuals with an infection exhibited a higher viral load (one third of a standard deviation) compared to fully vaccinated individuals with a breakthrough infection. Our results imply that vaccinations reduce susceptibility as well as transmissibility. The results are important for policy makers to select strategies for reducing transmission of SARS-CoV-2.