Effectiveness of influenza vaccine among the population in Chongqing, China, 2018–2022: A test negative design-based evaluation

Influenza vaccination is the most cost-effective strategy for influenza prevention. Influenza vaccines have been found to be effective against symptomatic and medically attended outpatient influenza illnesses. However, there is currently a lack of data regarding the effectiveness of inactivated influenza vaccines in Chongqing, China. We conducted a prospective observational test-negative design study. Outpatient and emergency cases presenting with influenza-like illnesses (ILI) and available influenza reverse transcription polymerase chain reaction (RT-PCR) were selected and classified as cases (positive influenza RT-PCR) or controls (negative influenza RT-PCR). A total of 7,307 cases of influenza and 7,905 control subjects were included in this study. The overall adjusted influenza vaccine effectiveness (IVE) was 44.4% (95% confidence interval (CI): 32.5–54.2%). In the age groups of less than 6 years old, 6–18 years old, and 19–59 years old, the adjusted IVE were 32.2% (95% CI: 10.0–48.9%), 48.2% (95% CI: 30.6–61.4%), and 72.0% (95% CI: 43.6–86.1%). The adjusted IVE for H1N1, H3N2 and B (Victoria) were 71.1% (95% CI: 55.4–81.3%), 36.1% (95% CI: 14.6–52.2%) and 33.7% (95% CI: 14.6–48.5%). Influenza vaccination was effective in Chongqing from 2018 to 2022. Evaluating IVE in this area is feasible and should be conducted annually in the future.