BACKGROUND: The objective of this study was to estimate influenza vaccine effectiveness (VE) for the 2016/17 epidemic of co-circulating influenza A(H1N1)pdm09 and A(H3N2) viruses in Beijing, the capital of China.

METHODS: The surveillance-based study included all swabbed patients through influenza virological surveillance, between November 2016 and April 2017. A test-negative case-control design was used to estimate influenza VE against medically-attended laboratory-confirmed influenza in outpatient settings. Cases were influenza-like illness (ILI) patients who tested positive for influenza, and controls were influenza negative patients.

RESULTS: A total of 10,496 ILI patients were enrolled and swabbed. Among them, 735 tested positive for influenza A(H1N1)pdm09, 1851 for A(H3N2), and 40 for type B. Of the 45 randomly selected specimens out of 1851 influenza A(H3N2) viruses, 2(4.4%) belonged to the H3N2 3C.2a1 clade, and 43(95.6%) belonged to A/Hong Kong/4801/2014-like 3C.2a clade. Among the 43 viruses of the 3C.2a clade, 32 viruses clustered in one subgroup carrying T131K, R142K and R261Q substitutions. The adjusted VE against all influenza was low at 25% (95% confidence interval (CI): 0-43%), with 54% (95%CI: 22-73%) for influenza A(H1N1)pdm09, and 2% (95%CI: -35% to 29%) for influenza A(H3N2).

CONCLUSIONS: Our study suggested a moderate VE against influenza A(H1N1)pdm09, but low VE against influenza A(H3N2) in Beijing, 2016/17 season. Amino acid substitutions in the hemagglutinin may contribute to the low VE against influenza A(H3N2) for this season.