Public-health promotions pushing seasonal influenza vaccination usually begin popping up in September. A new study, however, questions whether that timing is appropriate. The article in Clinical Infectious Diseases notes that in the United States, healthcare providers are usually advised to begin offering influenza vaccination by October, if possible.

In light of that, a study team from Kaiser Permanente raised a key issue: If the vaccine’s effectiveness begins to wane quickly, that might be too early for vaccination.

Accordingly, the researchers sought to determine whether the effectiveness of influenza vaccine wanes during the influenza season with increasing time since vaccination, and, if so, by how much.

To do that, they identified Americans who were vaccinated with inactivated influenza vaccine from September 1, 2010, to March 31, 2017, and who were subsequently tested for influenza and respiratory syncytial virus (RSV) by a polymerase chain reaction test. Defined as the primary outcome was test-confirmed influenza, whereas days-since-vaccination was the predictor of interest in conditional logistic regression. RSV was used as a negative control outcome. The study also took into account age, calendar day, and geographic area.

Compared with people vaccinated 14 to 41 days prior to being tested, those vaccinated 42 to 69 days prior to being tested had 1.32 (95% confidence interval [CI]: 1.11-1.55) times the odds of testing positive for any influenza.

In addition, researchers point out that the odds ratio (OR) increased in linear fashion by approximately 16% for each additional 28 days since vaccination. In fact, the OR jumped to 2.06 (95% CI: 1.69-2.51) for persons vaccinated 154 or more days prior to being tested.

No evidence of waning was found for RSV, however.

“Our results suggest that effectiveness of inactivated influenza vaccine wanes during the course of a single season,” the study authors conclude. “These results may lead to reconsideration of the optimal timing of seasonal influenza vaccination.”

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