Influenza vaccine, while not 100% effective, may reduce the severity of flu symptoms

Embargoed Until: August 26, 2015, 12:30 pm EDT

Annual vaccination remains the best strategy for the prevention of influenza and its complications. It is recognized that influenza vaccines are moderately (~60%) effective against disease, meaning that receipt of influenza vaccine reduces by approximately 60% a person’s risk of developing an influenza-associated illness that would require a visit to the doctor. Individuals who have been recently vaccinated may still get infected with influenza (“vaccine failure”). Among these individuals, it is not known whether recent vaccination can attenuate the course of illness, that is, lessen the severity and/or duration of symptoms associated with influenza.

A study led by the Infectious Disease Clinical Research Program (IDCRP), Uniformed Services University of the Health Sciences in Bethesda, Maryland, examined the association between recent receipt of influenza vaccine and symptom severity among so-called vaccine failures.

Of the 155 participants, 72% had recently received influenza vaccine. When all cases of influenza were analyzed, there were no differences between vaccinated and unvaccinated participants with respect to symptom severity at the baseline visit. However, among patients whose illness was caused by a specific influenza subtype (i.e. A/H3N2), the severity of upper and lower respiratory symptoms, and systemic symptoms (e.g. chills, muscle aches, etc.), was significantly lower among those who had recently received influenza vaccine as compared to those who had not. The overall symptom score was also lower among vaccinated individuals. Moreover, the differences in symptom severity persisted for the 7-day observation period. No such differences were observed for participants with influenza A/H1N1 or B infection.

The findings of this study suggest that annual receipt of influenza vaccine, while not 100% effective in preventing disease, may be of some benefit by lessening the severity of symptoms among those who do get influenza. This study was conducted among young, otherwise healthy active duty military personnel and beneficiaries, and it is not known whether these findings would be consistent among individuals who are known to be at high-risk for complications due to influenza infection (e.g. pregnant women, elderly persons, and persons with underlying medical conditions).

These results were presented at the International Conference on Emerging Infectious Diseases, held in Atlanta, GA, from August 24-26, 2015.

Corresponding Author: Eugene Millar