Estimating cumulative infection rate of COVID-19 after adjusting the dynamic zero-COVID policy in China

Background: At the end of 2022, China adjusted its coronavirus disease 2019 (COVID-19) prevention and control strategy. How this adjustment affected the cumulative infection rate is debated, and how second booster dose vaccination affected the pandemic remains unclear. Methods: We collected COVID-19 case data for China's mainland from December 7, 2022, to January 7, 2023, reported by the World Health Organization. We also collected cumulative infection rate data from five large-scale population-based surveys. Next, we developed a dynamic transmission compartment model to characterize the COVID-19 pandemic and to estimate the cumulative infection rate. In addition, we estimated the impact of second booster vaccination on the pandemic by examining nine scenarios with different vaccination coverages (0%, 20%, and 40%) and vaccine effectiveness (30%, 50%, and 70%). Results: By January 7, 2023, when COVID-19 was classified as a Class B infectious disease, the cumulative infection rate of the Omicron variant nationwide had reached 84.11% (95% confidence interval [CI]: 78.13%–90.08%). We estimated that the cumulative infection rates reached 50.50% (95% CI: 39.58%–61.43%), 56.15% (95% CI: 49.05%–67.22%), 73.82% (95% CI: 64.63%–83.02%), 75.76% (95% CI: 67.02%–84.50%), and 84.99% (95% CI: 79.45%–90.53%) on December 19, 20, 25, and 26, 2022, and on January 15, 2023, respectively. These results are similar to those of the population survey conducted on the corresponding dates, that is 46.93%, 61%, 63.52%, 74%, and 84.7%, respectively. In addition, we estimated that by January 7, 2023, the cumulative infection rate decreased to 29.55% (64.25%) if vaccination coverage and the effectiveness of second booster vaccination were 40% (20%) and 70% (30%), respectively. Conclusion: We estimate that, in late 2022, the cumulative infection rate was approximately 84% and that second booster vaccination before the policy adjustment was effective in reducing this rate.