Influenza vaccination: how does timing influence effectiveness?

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Takeaway

  • Waning protection with time since influenza vaccination most pronounced among patients with prior season vaccination.

Why this matters

  • Careful consideration of risks and benefits of delayed vaccination needed before changes to current vaccine recommendations.

Study design

  • US Influenza Vaccine Effectiveness Network.
  • Pooled data from 2011-12 through 2014-15 seasons.
  • Patients ≥9 y old with laboratory-confirmed influenza.
  • Considered vaccinated if dosed at least 14 d before illness onset.
  • Funding: US Centers for Disease Control and Prevention.

Key results

  • Vaccine effectiveness (VE) against influenza A(H3N2) (n=11,200),  influenza A(H1N1)pdm09 (n=4100), and influenza B (n=5525).
  • Adjusted VE decreased with increasing time since vaccination for influenza A(H3N2) (P=.004), influenza A(H1N1)pdm09 (P=.01), and influenza B viruses (P=.04).
  • Maximum VE observed shortly after vaccination, declining about 7% per month for influenza A(H3N2) and influenza B and 6%-11% per month for influenza A(H1N1)pdm09.
  • Decline in VE was more pronounced among patients with prior season influenza vaccination.

Limitations

  • Possibility of a drifted variant less well-matched to the vaccine strain; confounding factors associated with vaccine timing and influenza risk; heterogeneity of the at-risk population and the leaky vaccine effect.