摘要
The COVID-19 pandemic has impacted the world for 3 years.As of February 28,2023,more than 750 million confirmed cases and 6.8 million deaths have been reported globally by the World Health Organization.In China,approximately 9.9 million confirmed cases have been reported,including 120,000 deaths.Severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)continues to evolve,and the current circulating variant is Omicron.Infections with Omicron are usually mild with a low rate of severe disease,but it has high transmissibility,and the mean basic regeneration number(R0)of the variant is 9.5.During the Omicron era,more COVID-19 breakthrough infections have occurred than in previous variant eras.
