Effectiveness of inactivated COVID-19 vaccines against severe illness in B.1.617.2 (Delta) variant–infected patients in Jiangsu, China

•The largest study of inactive COVID-19 vaccine in preventing severe illness in China.•Those who received 2 doses of inactivated vaccine had an 88% reduced risk in severe illness.•The full immunization offered 100% protection from severe illness among women.•The effect of the vaccine was potentially affected by underlying medical conditions. The SARS-CoV-2 B.1.617.2 (Delta) variant has caused a new surge in the number of COVID-19 cases. The effectiveness of inactivated vaccines against this variant is not fully understood. Using data from a recent large-scale outbreak of B.1.617.2 SARS-CoV-2 infection in Jiangsu, China, we conducted a real-world study to explore the effect of inactivated vaccine immunization on the course of disease in patients infected with the Delta variant. Of 476 patients with B.1.617.2 infection, 184 were unvaccinated, 105 were partially vaccinated, and 187 were fully vaccinated. A total of 42 (8.8%) patients developed severe illness, of whom, 27 (14.7%), 13 (12.4%), and 2 (1.1%) were unvaccinated, partially vaccinated, and fully vaccinated, respectively (P