Estimating relative generation times and relative reproduction numbers of Omicron BA.1 and BA.2 with respect to Delta in Denmark

The Omicron variant is the most transmissible variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) we had so far. The BA.1 and BA.2 sublineages of Omicron are circulating in Europe and it is urgent to evaluate the transmissibility of these sub-lineages. Using a mathematical model describing trajectories of variant frequencies that assumes a constant ratio in generation times and a constant ratio in effective reproduction numbers among variants, trajectories of variant frequencies in Denmark from November 22, 2021 to February 26, 2022 were analyzed. We found that the generation times of Omicron BA.1 and BA.2 are 0.60 (95%CI: 0.59-0.62) and 0.51 (95%CI: 0.50-0.52) of the length of that of Delta, respectively. We also found that the effective reproduction number of Omicron BA.1 is 1.99 (95% CI: 1.98-2.02) times and that of Omicron BA.2 is 2.51 (95% CI: 2.48-2.55) times larger than the effective reproduction number of Delta. The generation times of Omicron BA.2 is 0.85 (95% CI:0.84-0.86) the length of that of BA.1 and that the effective reproduction number of Omicron BA.2 is 1.26 (95% CI:1.25-1.26) times larger than that of Omicron BA.1. These estimates on the ratio of generation times and the ratio of effective reproduction numbers has epidemiologically important implications. The duration of quarantine for people who contacted with an Omicron BA.1 and BA.2 patient can be reduced to 60% and 51% of that for Delta, respectively. The control measures against Omicron BA.1 and BA.2 need to reduce contacts between infectious and susceptible people respectively by 50% (95% CI: 49-50%) and 60% (95% CI: 60-61%) compared to that against Delta to achieve the same effect on their control.