Influenza vaccination: how does timing influence effectiveness?

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.