Transmission risk of asymptomatic SARS-CoV-2 infection: a systematic review and meta-analysis

BACKGROUND: Global evidence on the transmission of asymptomatic SARS-CoV-2 infection needs to be synthesized. METHODS: A search of 4 electronic databases (PubMed, EMBASE, Cochrane Library, and Web of Science databases) as of January 24, 2021 was performed. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. Studies which reported the transmission rate among close contacts with asymptomatic SARS-CoV-2 cases were included, and transmission activities occurred were considered. The transmission rates were pooled by zero-inflated beta distribution. The risk ratios (RRs) were calculated using random-effects models. RESULTS: Of 4923 records retrieved and reviewed, 15 studies including 3917 close contacts with asymptomatic indexes were eligible. The pooled transmission rates were 1.79 per 100 person-days (or 1.79%, 95% confidence interval [CI] 0.41%–3.16%) by asymptomatic index, which is significantly lower than by presymptomatic (5.02%, 95% CI 2.37%–7.66%; p<0.001), and by symptomatic (5.27%, 95% CI 2.40%–8.15%; p<0.001). Subgroup analyses showed that the household transmission rate of asymptomatic index was (4.22%, 95% CI 0.91%–7.52%), four times significantly higher than non-household transmission (1.03%, 95% CI 0.73%–1.33%; p=0.03), and the asymptomatic transmission rate in China (1.82%, 95% CI 0.11%–3.53%) was lower than in other countries (2.22%, 95% CI 0.67%–3.77%; p=0.01). CONCLUSIONS: People with asymptomatic SARS-CoV-2 infection are at risk of transmitting the virus to their close contacts, particularly in household settings. The transmission potential of asymptomatic infection is lower than symptomatic and presymptomatic infections. This meta-analysis provides evidence for predicting the epidemic trend and promulgating vaccination and other control measures. Registered with PROSPERO International Prospective Register of Systematic Reviews, CRD42021269446; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=269446.