Aim The aim of this study was to estimate the household transmissibility of SARSCoV-2 for lineage B.1.1.7 compared with other lineages, by age and viral load . Furthermore, we wanted to estimate whether there is a multiplicative or additive effect of the increased transmissibility of B.1.1.7 compared with other lineages . Background New lineages of SARS-CoV-2 are of potential concern due to higher transmissibility, risk of severe outcomes, and/or escape from neutralizing antibodies . Lineage B.1.1.7 has been estimated to be more transmissible than other previously known lineages, but the association between transmissibility and risk factors, such as age of primary case and viral load is still unknown . Methods We used comprehensive administrative data from Denmark, comprising the full population, all SARS-CoV-2 RT-PCR tests, and all WGS lineage data (January 11 to February 7 , 2021), to estimate household transmissibility stratified by lineage B.1.1.7 and other lineages . Results We included 5,241 households with primary cases; 808 were infected with SARS-CoV-2 lineage B.1.1.7 and 4,433 were infected with other lineages . The attack rate was 38% in households with a primary case infected with B.1.1.7 and 27% in households with a primary case infected with other lineages . Primary cases infected with B.1.1.7 had an increased transmissibility of 1.5-1.7 times that of primary cases infected with other lineages . The increased transmissibility of B.1.1.7 was multiplicative across age and viral load . Conclusions The results found in this study add new knowledge that can be used to mitigate the further spread of SARS-CoV-2 lineage B.1.1.7, which is becoming increasingly widespread in numerous countries . Our results clarify that the transmissibility of B.1.1.7 should be included as a multiplicative effect in mathematical models used as a tool for decision makers . The results may have important public health implications, as household transmission may serve as a bridge between otherwise separate transmission domains, such as schools and physical workplaces, despite implemented non-pharmaceutical interventions.