SARS‐CoV‐2 Omicron variant seemed to cause milder disease compared to previous predominated variants. We aimed to conduct a meta‐analysis to assess the pooled proportion of nonsevere disease and asymptomatic infection among COVID‐19 patients infected with Omicron and Delta. We searched PubMed, Embase, Web of Science, and China National Knowledge Infrastructure (CNKI) databases. We included studies of SARS‐CoV‐2 Omicron infection from November 1, 2021, to April 18, 2022, and studies of Delta infection from October 1, 2020, to June 30, 2022. Studies without corresponding data, with less than 50 patients, or obviously biased concerning main outcome were excluded. Meta‐analysis was performed in R 4.2.0 with the “meta” package. Subgroup analyses were conducted by study group and vaccination status. The pooled proportion of asymptomatic infection and nonsevere disease with Omicron were 25.5% (95% confidence interval [CI] 17.0%–38.2%) and 97.9% (95% CI 97.1%–98.7%), significantly higher than those of Delta with 8.4% (95% CI 4.4%–16.2%) and 91.4% (95% CI 87.0%–96.0%). During Omicron wave, children and adolescents had higher proportion of asymptomatic infection, SOTR and the elderly had lower proportion of nonsevere disease, vaccination of a booster dose contributed to higher proportion of both asymptomatic infection and nonsevere disease. This study estimates the pooled proportion of asymptomatic infection and nonsevere disease caused by SARS‐CoV‐2 Omicron compared to other predominant variants. The result has important implications for future policy making.

During March 2022 to January 2023, two Omicron waves hit Shanghai and caused a massive number of reinfections. To better understand the incidence and clinical characteristics of SARS‐CoV‐2 reinfection in Shanghai, China, we conducted a multicenter cohort study. COVID‐19 patients first infected with BA.2 (March 1, 2022–May 23, 2022) who were quarantined in Huashan Hospital, Renji Hospital, and Shanghai Jing'an Central Hospital were followed up for reinfection from June 1, 2022 to January 31, 2023. Of 897 primary infections, 148 (16.5%) experienced reinfection. Incidence rate of reinfection was 0.66 cases per 1000 person‐days. Female gender (adjusted odds ratio [aOR]= 2.19, 95% confidence interval [CI]: 1.29–3.83) was a risk factor for reinfection. The four most common symptoms of reinfections during the circulation of BA.5 sublineages were cough (62.59%), sore throat (54.42%), fatigue (48.98%), and fever (42.57%). Having received a booster vaccination was not associated with reduced severity of reinfection in comparison with not having received booster vaccination. After matched 1:1 by age and sex, we found that reinfections with BA.5 sublineages had significantly lower occurrence and severity of fever, fatigue, sore throat, and cough, as compared to primary infections with BA.5 sublineages. SARS‐CoV‐2 Omicron reinfections were less severe than Omicron primary infections during the circulation of the same subvariant. Protection offered by both vaccination and previous infection was poor against SARS‐CoV‐2 reinfection.