Comparison of the disease severity with infection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and Omicron variants: A meta-analysis

The increased transmissibility of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and Omicron variants due to their complex mutations has been confirmed by many studies, but the disease severity caused by these two variants and vaccine efficacy have not been clearly concluded. This meta-analysis evaluated and compared disease severity and vaccine efficacy for infection with these two variants. Databases were searched on July 29, 2022, for original studies. Data extracted included participants, hospitalization rate, intensive care unit (ICU) admission rate, mortality rate, and vaccination status. Total 15 studies (570,713 subjects) were included. The risk of hospitalization, ICU admission and death in patients with Delta were increased compared with patients with Omicron, especially hospitalization (Log risk ratio [RR]: 1.09, 95% confidence interval [CI]: 0.65–1.53). The Delta was associated with a higher risk of death in the unvaccinated population (Log RR: 0.48, 95% CI: 0.04–0.93). No significant differences in the risk of severe illness between BA.1 and BA.2 were observed. All three risks were reduced in fully vaccinated populations infected with Delta and Omicron. The existing evidence shows that the inherent virulence and disease severity of Omicron are reduced compared with Delta, and fully vaccination is effective in preventing severe infection of both variants.