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

BACKGROUND: 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. METHODS: 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. RESULTS: 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 <0.001). All 15 (3.2%) patients who required mechanical ventilation were unvaccinated. After adjusting for age, sex, and comorbidities, fully vaccinated patients had an 88% reduced risk of progressing to severe illness (OR(adjusted): 0.12, 95% CI: 0.02-0.45). However, this protective effect was not observed in partially vaccinated patients (OR(adjusted): 1.11, 95% CI: 0.51-2.36). Full immunization offered 100% protection from severe illness among women. The effect of the vaccine was potentially affected by underlying medical conditions (OR(adjusted): 0.26, 95% CI: 0.03-1.23). CONCLUSION: Full vaccination with inactivated vaccines is highly effective in preventing severe illness in Delta variant–infected patients. However, partial vaccination does not offer clinically meaningful protection against severe disease.