Influenza Vaccine Effectiveness among Primary and Secondary School Students in Shenzhen during the 2023/24 Influenza Season

Evidence on influenza vaccine effectiveness (VE) among children in southern mainland China remains sparse. To evaluate influenza VE among students aged 6-18 years in Shenzhen during the 2023/24 influenza season, a test-negative case-control study was conducted. The study involved students with influenza-like illness (ILI) symptoms (acute onset with fever ≥38°C, sore throat or cough) who visited two designated hospitals during the winter influenza peaks. Data were collected through questionnaires and oral-pharyngeal swabs, with rt-PCR testing performed in Shenzhen Center for Disease Control and Prevention. Multiple logistic regressions were used to adjust for covariates including sex, age, geographic location, underlying diseases, and vaccination history from the previous season. Additionally, we conducted an exploratory evaluation of influenza VE for the spring-summer peak using the same method. During the winter influenza peaks, a total of 2689 ILI patients were collected from the two hospitals, with 58.61% of the participants vaccinated. Of the ILI patients, 761 (28.3%) were laboratory-confirmed influenza infection, with 76.7% being A(H3N2) and 22.8% B/Victoria. After adjusting for covariates, the adjusted VE (aVE) was 57.06% (95% CI, 48.59 - 64.13%). In the subtype/lineage and age subgroup analysis, influenza VE against B/Victoria was higher than A(H3N2), and decreased with age. More, influenza VE (aVE = 19.63% [95% CI, -29.13 to 49.98%]) against all subtypes/lineages were not statistically significant for the spring-summer peak. Overall, influenza vaccines provided significant protection against influenza-associated outpatient visits among students in Shenzhen during 2023/24 winter influenza season. The findings underscore the importance of maintaining high vaccination coverage.