This study compared the efficacy of the injectable inactivated (TIV) and the intranasal attenuated (LAIV) influenza vaccines in healthy adults.
Prevention of Antigenically Drifted Influenza by Inactivated and Live Attenuated Vaccines. Ohmit SE, Victor JC, Rotthoff JR et al. New England Journal of Medicine, 2006;355:2513-22.
How efficacious are the injectable trivalent inactivated (TIV) and the intranasal attenuated (LAIV) influenza vaccines in healthy adults?
This study included 1,247 healthy adults 18 to 46 years of age divided into three groups. Prior to the 2004-2005 winter season, the first group received TIV, the second LAIV, and the third placebo. The participants were then followed for any signs of influenza-like illness.
Viral cultures, molecular sampling (PCR), and serology were compared as measures of determining influenza infection.
Most of the strains of influenza A (H3N2) and B viruses circulating during the winter of 2004-2005 were not the same as the strains in that season’s vaccines.
Using culture and/or PCR to define infection, the study found that the vaccine efficacy for both vaccines for influenza A was about 74%. Vaccine efficacy for type B influenza was 80% for TIV and 40% for LAIV
The results also suggested that the efficacy of TIV would have been overestimated and the efficacy for LAIV would have been underestimated had the researchers used serum antibody titers alone to detect influenza infections.
Both TIV and LAIV influenza vaccines were efficacious at preventing laboratory-confirmed influenza in healthy adults, although LAIV was less so.
It is reassuring, but also surprising, that these influenza vaccines were as efficacious as was observed given the differences between the strains causing illness and those in the vaccines. Until other influenza seasons are similarly studied, however, it will not be possible to predict that these interesting observations will extend to the experience of future years. These vaccines would not be expected to protect against very different influenza strains, such as might be seen during a pandemic.