Prior Flu Immunization Does Not Reduce Vaccine Effectiveness

Prior-season influenza vaccination history does not reduce the effectiveness of the vaccine, and could even improve its effectiveness, according to the results of a recent study.

Following expanded recommendations for annual vaccination,the effects of prior influenza vaccination on vaccine effectiveness (VE) has been called into question. Some previous studies have reported negative effects on VE with prior vaccination, but overall, this relationship is not well understood.

To explore this association further, the researchers conducted a multiseason, test-negative case-control study in outpatient clinics at 4 US sites involving children aged 2 to 17 years. The children had confirmed influenza by reverse-transcription polymerase chain reaction, while controls were those with negative test results for influenza.

Using medical records, the researchers determined the participants’ vaccination history, including vaccine type during the enrollment season and the season before enrollment. They measured the effectiveness of the live attenuated influenza vaccine (LAIV) and the inactive influenza vaccine (IIV) using a logistic regression model estimated as 100 × (1 − odds ratio).

Of the 3369 participants, 772 had a positive test result for influenza and 1674 were vaccinated during the enrollment season. Among those participants given LAIV, VE against influenza A (H3N2) was higher in those vaccinated both during the year of enrollment and the prior year than those only vaccinated during the enrollment year (50.3% vs −82.4%).

However, the effectiveness of LAIV against influenza A(H1N1)pdm09 was not associated with prior season vaccination (47.5% vs 7.8%). Prior season vaccination was also not associated with the effectiveness of IIV against influenza A(H3N2) or ([H1N1)pdm09.

“Influenza VE varied by influenza type and subtype and vaccine type, but prior-season vaccination was not associated with reduced VE. These findings support current recommendations for annual influenza vaccination of children.”

—Michael Potts

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