Effectiveness of Inactivated COVID-19 Vaccines Against COVID-19 Pneumonia and Severe Illness Caused by the B.1.617.2 (Delta) Variant: Evidence from an Outbreak in Guangdong, China

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Background: Real-world evidence (RWE) of a vaccine supplements clinical trial data by providing information in populations differing from clinical trial populations, under different epidemiological situations, on alternative outcomes, or against different pathogen lineages. To date, RWE on inactivated COVID-19 vaccines against the highly transmissible SARS-CoV-2 B.1.617.2 (Delta) variant is limited, leaving an important gap in the evidence base of inactivated COVID-19 vaccines for use by immunization programs. Methods: Between May and June 2021, an outbreak of the B.1.617.2 variant was discovered and traced in Guangdong, China. Before this outbreak, Guangdong province had started mass vaccination using inactivated vaccines approved by China’s regulator for use in adults. Using surveillance and vaccination data from the outbreak, we assessed the real-world effectiveness of inactivated vaccines against pneumonia and severe illness caused by the B.1.617.2 variant. We enrolled 10813 subjects who were close contacts of laboratory-confirmed cases, categorizing them as an unvaccinated group, a partially vaccinated (1-dose) group, and a fully vaccinated (2-dose) group. We estimated relative risk (RR) and vaccine effectiveness (VE) of the vaccinated groups in relation to the unvaccinated group. Findings: Unadjusted and adjusted VE of full vaccination against pneumonia were 77·7% (95% CI 45·1–90·9) and 69·5% (95% CI 42·8–96·3), respectively. Full vaccination was 100% effective against severe illness. Unadjusted and adjusted VE of partial vaccination against pneumonia were 1·4% (95% CI -79·7–45·9) and 8·4% (95% CI -47·6–64·4). Interpretation Full vaccination with inactivated vaccines is effective against pneumonia, severe, and critical illness caused by the B.1.617.2 variant. Effort should be placed to ensure full vaccination of target populations. Funding: National Natural Science Foundation of China and Key-Area Research and Development Program of Guangdong.Declaration of Interest: None to declare. Ethical Approval: This study was approved by the institutional ethics committee of the Guangdong Provincial Center for Disease Control and Prevention (GDCDC). Data in the study were collected per administrative requirements of disease control and surveillance, and were anonymized for analysis. Participants were informed about the requirements of disease surveillance and provided oral consents.