Epidemiological characteristics of SARS-CoV-2 infection outbreak in Shanghai in the Spring of 2022

Objective To investigate the epidemiological characteristics and analyze the incidence trend of SARS-CoV-2 infection in Shanghai， China， and compare with the characteristics of the infection in Jilin Province of China during the same period in 2022 and Wuhan at the beginning of 2020.

Methods Information of new locally-transmitted confirmed SARS-CoV-2 cases， imported confirmed COVID-19 cases， local asymptomatic SARS-CoV-2 carriers and imported asymptomatic SARS-CoV-2 carriers in Shanghai from March 1 to April 18， 2022 was collected for descriptive analysis.

Results From March 1 to April 18， 2022， a total of 397 933 locally-transmitted SARS-CoV-2 cases were reported in Shanghai. Of those， 27 613 were clinically confirmed cases and 21 were severe cases. Ten deaths were related to COVID-19. The pathogen is Omicron variant BA.2 of SARS-CoV-2. The number of the infected subjects increased rapidly after March 24 and lead to a disease outbreak. Severe and deceased cases had severe comorbidity and were mostly unvaccinated with SARS-CoV-2 vaccines. Asymptomatic SARS-CoV-2 carriers accounted for 93.06%， which is significantly higher than that in Jilin Province during the same period （48.07%， P<0.001）. Daily increase in the number of clinically confirmed COVID-19 cases in Shanghai in 2022 was much lower than that in Wuhan， Hubei Province， in 2020. Number of daily newly imported confirmed COVID-19 cases and imported asymptomatic SARS-CoV-2 carriers declined during this period.

Conclusion The Omicron variant in Shanghai 2022 is highly infectious and less pathogenic. Omicron variant BA.2 replicates rapidly in asymptomatic carriers， which makes the carriers the major source of infection. Full-term vaccination of inactivated SARS-CoV-2 vaccine might decrease the pathogenicity and fatality of SARS-CoV-2 variants. SARS-CoV-2 of the Omicron BA2 strain is likely transmitted through aerosols and droplets， which poses a great challenge to the control of the COVID-19 pandemic in large cities with high population density and sophisticated public transportation.