Intrinsic Generation Time of the SARS-CoV-2 Omicron Variant: An Observational Study of Household Transmission

Background: Starting from the final months of 2021, the SARS-CoV-2 Omicron variant expanded globally, swiftly replacing Delta, the variant that was dominant at the time. Many uncertainties remain about the epidemiology of Omicron, and it is yet to be established whether its generation time is shorter than that of previous variants.Method: We used a Bayesian approach to analyze 23,122 SARS-CoV-2 infected individuals clustered in 8,903 households as determined from contact tracing operations in Reggio Emilia, Italy, throughout January 2022. We estimated the distribution of the intrinsic generation time (the time between the infection dates of an infector and its secondary cases in a fully susceptible population), realized household generation time, realized serial interval, and contribution of pre-symptomatic transmission.Results: We estimated a mean intrinsic generation time of 6.84 days (95% CrI 5.72-8.60), and a mean realized household generation time of 3.59 days (95%CrI: 3.55-3.60). The household serial interval was 2.38 days (95%CrI 2.30-2.47) with about 51% (95%CrI 45-56%) of infections caused by symptomatic individuals being generated before symptom onset.Interpretation: These results indicate limited differences between the intrinsic generation time of the SARS-CoV-2 Omicron variant as compared to previous estimates on ancestral lineages, Alpha and Delta. Like for previous lineages, pre-symptomatic transmission appears to play a key role for Omicron transmission.Funding Information: MM, AZ, FR, PPe, GG, and SM acknowledge funding from EU grant 874850 MOOD (catalogued as MOOD 000). Declaration of Interests: MA has received research funding from Seqirus. The funding is not related to COVID-19. All other authors declare no competing interest.Ethics Approval Statement: Covid-19 surveillance data collection, including specific studies on contact tracing, was authorized by the Italian Presidency of the Council of Ministers on 27 February 2020 (ordinance No 640, article 1 [15]).