Generation time of the Alpha and Delta SARS-CoV-2 variants

Summary Background In May 2021, the Delta SARS-CoV-2 variant became dominant in the UK. This variant is associated with increased transmissibility compared to the Alpha variant that was previously dominant. To understand ongoing transmission and interventions, a key question is whether the Delta variant generation time (the time between infections in infector- infectee pairs) is typically shorter–i.e., transmissions are happening more quickly–or whether infected individuals simply generate more infections. Methods We analysed transmission data from a UK Health Security Agency household study. By fitting a mathematical transmission model to the data, we estimated the generation times for the Alpha and Delta variants. Results The mean intrinsic generation time (the generation time if there had been a constant supply of susceptibles throughout infection) was shorter for the Delta variant (4·6 days, 95% CrI 4·0-5·4 days) than the Alpha variant (5·5 days, 95% CrI 4·6-6·4 days), although within uncertainty ranges. However, there was a larger difference in the realised mean household generation time between the Delta (3·2 days, 95% CrI 2·4-4·2 days) and Alpha (4·5 days, 95% CrI 3·7-5·4 days) variants. This is because higher transmissibility led to faster susceptible depletion in households, in addition to the reduced intrinsic generation time. Conclusions The Delta variant transmits more quickly than previously circulating variants. This has implications for interventions such as contact tracing, testing and isolation, which are less effective if the virus is transmitted quickly. Epidemiological models of interventions should be updated to include the shorter generation time of the Delta variant.