Moderate effectiveness of influenza vaccine in outpatient settings: A test-negative study in Beijing, China, 2023/24 season.

Introduction: The objective of our study was to estimate the influenza vaccine effectiveness for 2023/24 epidemic of co-circulating influenza A(H3N2) and B(Victoria) viruses in Beijing, China.

Methods: The surveillance-based study included all swabbed patients through influenza virological surveillance in Beijing, between October 2023 and March 2024. A Test-Negative Design(TND) was used to estimate influenza vaccine effectiveness(VE) against medically- attended laboratory-confirmed influenza in outpatient settings, also calculated the influenza vaccination rate(IVR). Cases were influenza-like illness (ILI) patients who tested positive for influenza, and controls were ILI who influenza negative patients.

Results: A total of 18,665 ILI patients were enrolled and swabbed. Among them, 6362(34.1 %) tested positive for influenza, major epidemic strain was A(H3N2) and B(Victoria). The overall IVR was 8.7 %, and the differences of IVR by gender, age, region, chronic conditions and month of onset were statistically significant(P<0.05). The adjusted VE against all influenza was moderate at 44.8 %, with the highest for B (Victoria) at 52.2 %, the highest for 19-59 age at 72.4 %, and the highest when vaccinated only in current season at 48.3 %.

Conclusion: Our study suggested the influenza vaccine has moderate effectiveness, with the best VE against B(Victoria), followed by A(H3N2) and A(H1N1)pdm09 in Beijing, 2023/24 season. Meanwhile, the influenza VE was relatively high in school-age children and the elderly. Consistent long-term studies are required in the future to evaluate the protect effect of influenza vaccine.