Inactivated vaccine was more efficacious than live-attenuated vaccine in healthy adults.
Two types of seasonal influenza vaccines are currently licensed: an inactivated formulation, injected intramuscularly, and a live-attenuated formulation, administered by intranasal spray. Now, results from a double-blind trial conducted in Michigan during the 2007–2008 influenza season (and supported in part by the inactivated vaccine's maker) shed light on the comparative efficacy of these preparations in healthy adults. During that season, influenza A (H3N2) viruses predominated, with a slight antigenic drift from the vaccine strain.
A total of 1952 healthy adults aged 18 to 49 were randomly assigned to receive inactivated vaccine or matching intramuscular placebo, or live vaccine or matching nasal-spray placebo, in a 5:1 vaccine:placebo ratio. Throat-swab specimens were collected from individuals who developed influenza symptoms; culture, PCR, or both were used to confirm the diagnosis.
During the study period, 119 participants (6%) had laboratory-confirmed influenza; 91% of these cases involved influenza A. Efficacy was 68% for the inactivated vaccine and 36% for the live-attenuated preparation, compared with placebo. The inactivated vaccine was 50% more efficacious than the live-attenuated one.
Comment: Interestingly, these results in healthy adults are opposite those seen in young children. The authors speculate that the live-attenuated viruses may be unable to infect some adults because of these individuals' past exposure to similar strains. Of note, efficacy against influenza B could not be assessed because the circulating influenza B viruses were not included in the vaccine.
— Lynn L. Estes, PharmD
Published in Journal Watch Infectious Diseases September 23, 2009
Monto AS et al. Comparative efficacy of inactivated and live attenuated influenza vaccines. N Engl J Med 2009 Sep 24; 361:1260.