Influenza vaccine is generally available by the end of July, but some have wondered, when is it too early to administer a vaccination? In a new analysis assessing the association between influenza vaccine effectiveness (VE)—prevention of illness in vaccinated populations—and time since vaccination, researchers observed decreasing influenza vaccine protection with increasing time since vaccination across influenza types and subtypes.

CDC researchers examined the association between influenza VE and time since vaccination among enrollees in the U.S. Influenza Vaccine Effectiveness Network for the 2011–12 through 2014–15 influenza seasons. Patients aged 9 years and older who presented with an acute respiratory illness and received only one dose of the influenza vaccine per season were included in the analysis. Patients who tested positive for influenza were classified as cases, and patients who tested negative were the control group.

The researchers reported that the adjusted VE against influenza A(H3N2) virus infection decreased with increasing time since vaccination, with maximum VE of 35% at 14 days’ postvaccination, and VE reaching zero at 158 days’ postvaccination. This was equivalent to an approximate 7% decline in VE per 30-day period.

For influenza A(H1N1)pdm09 virus infection, similar findings were observed, with maximum VE of 80% at 14 days’ postvaccination and minimum VE of 37% at 128 days’ postvaccination. This was equivalent to an approximate 6%–11% decline in VE per 30-day period. Finally, adjusted VE against influenza B virus infection showed a maximum VE of 59% at 14 days’ postvaccination and minimum VE of 23% at 180 days’ postvaccination. This was also equivalent to an approximate 7% decline in VE per 30-day period.

The researchers discussed select limitations of their analysis and noted that bias or residual confounding could explain the results. They concluded, “While the possibility of waning vaccine effectiveness merits further investigation, the current uncertainty in its nature and magnitude makes drawing conclusions difficult and suggests that careful consideration of the risks and benefits of delaying vaccination is needed before contemplating changes to current vaccine recommendations.”

Other studies have reported similar findings, but an overall assessment of the literature shows that the duration of protection of influenza vaccination has been difficult to determine. In addition, delaying vaccination results in missed opportunities to vaccinate as well as difficulties in vaccinating a population within a more constrained time period.

Influenza viruses typically circulate widely in the United States annually, from late fall through early spring. CDC’s Advisory Committee on Immunization Practices (ACIP) currently recommends that influenza vaccine be given at the onset of influenza activity in the community and by October, if possible.

Visit www.pharmacytoday.org for the full article in the upcoming March 2017 issue of Pharmacy Today.