Background: Live, attenuated influenza vaccine (LAIV) has been reported to be more effective in children than the trivalent inactivated influenza vaccine (TIV); however, comparative effectiveness in adults is unknown. Monto and colleagues previously studied the protective effect of the two vaccine types for the 2004–2005 and 2005–2006 influenza seasons. Although the data suggested that TIV was more effective in adults, a conclusive answer was hampered by relatively low influenza rates in these years. In contrast, influenza-related morbidity was higher in the 2007–2008 influenza season, which allowed for additional analysis by the authors in this study.
The Study: The authors conducted a randomized, double-blind trial of healthy adults 18 to 49 years of age. Patients were excluded for having health conditions for which TIV was specifically recommended and for whom either vaccine was contraindicated. Participants were randomized to receive one of four interventions: an intramuscular injection of TIV (Fluzone, Sanofi Pasteur) or placebo, or an intranasal spray of LAIV (Flumist, Medimmune) or placebo. The primary end point was symptomatic illness that was confirmed as influenza A or B by a cell culture or a real-time polymerase chain reaction assay.
Results: The study followed 1,952 participants, in whom no serious vaccine-related sequelae were reported. Overall, 119 participants (6.1 percent) had laboratory-confirmed symptomatic influenza, with 108 (90.8 percent) having influenza A and 11 (9.2 percent) having influenza B. Absolute effectiveness in preventing influenza A was 72 percent for TIV, compared with 29 percent for LAIV; TIV was 60 percent more effective. Too few cases were identified to compare vaccine effectiveness in preventing influenza B.
Conclusion: TIV and LAIV protect healthy adults against laboratory-confirmed symptomatic influenza A, although the inactivated vaccine is markedly more effective in this population. The authors mention that prior evidence has suggested that LAIV is more effective in children, particularly in those younger than six years, and that we may be entering a new era of influenza control in which different types of vaccines may be appropriate for different age groups.