In late 2022, China transitioned from a strict ‘zero-COVID’ policy to rapidly abandoning nearly all interventions and data reporting. This raised great concern about the presumably-rapid but unreported spread of the SARS-CoV-2 Omicron variant in a very large population of very low pre-existing immunity. By modeling a combination of case count and survey data, we show that Omicron spread extremely rapidly, at a rate of 0.42/day (95% credibility interval: [0.35, 0.51]/day), translating to an epidemic doubling time of 1.6 days ([1.6, 2.0] days) after the full exit from zero-COVID on Dec. 7, 2022. Consequently, we estimate that the vast majority of the population (97% [95%, 99%], sensitivity analysis lower limit of 90%) was infected during December, with the nation-wide epidemic peaking on Dec. 23. Overall, our results highlight the extremely high transmissibility of the variant and the importance of proper design of intervention exit strategies to avoid large infection waves.
From the beginning of the SARS-CoV-2 pandemic until fall 2022, China maintained a strict ‘zero-COVID’ set of policies that implemented strong non-pharmaceutical interventions such as city-wide lock-downs, travel and movement restrictions, contact tracing of direct and secondary contacts, and compulsory quarantine of infected individuals and their contacts in centralized facilities. In addition, to track the spread of the virus, the population was tested on a regular bases (every 2–4 days)