Effectiveness of influenza vaccine in the community-dwelling elderly. Nichol K, Nordin JD, Nelson DB, et al. NEJM 2007; 357: 1373-81.
Explanatory note: Influenza is a major cause of illness, hospitalizations and death in the elderly. In the US, about 40,000 excess deaths/year are attributed to influenza—largely among the elderly. Many studies have demonstrated that the inactivated vaccine protects against laboratory-confirmed influenza. However, because of the variability of annual outbreaks of influenza it has been difficult to make reliable estimates of the benefits of immunization in this group.
How effective is inactivated influenza vaccine at protecting seniors from hospitalizations for pneumonia or influenza-like illness and from death from all causes?
The investigators combined the data from multiple HMOs over a period of ten years, looking at 713,872 person-seasons for members of the plans who were older than 64 years and who were not institutionalized. They determined vaccinated and unvaccinated members and any pre-existing medical conditions. Outcomes of the study were hospitalization for pneumonia or influenza and death from any cause during the influenza seasons. Influenza seasons were defined as the CDC-defined regional periods (plus two weeks) of influenza activity. They looked at the results among many subgroups of individuals.
The influenza vaccinated persons were older and had more medical conditions than unvaccinated persons. Influenza vaccination was associated with statistically significant reductions in hospitalizations for pneumonia or influenza as well as fewer deaths from any cause during influenza seasons. However, there was no difference in hospitalizations between the vaccinated and unvaccinated groups during the summer (non influenza) seasons.
There was variability in influenza activity season to season. The impact of vaccination was less in seasons when the influenza strains in the vaccine did not match well with the types of influenza circulating in the community. All sub-groupings that they examined demonstrated reduced morbidity and mortality among the vaccinated seniors, even if they assumed a large unrecognized confounding factor.
Influenza related hospitalizations and deaths among the elderly are reduced during influenza seasons by immunization with inactivated influenza vaccine.
Inactivated influenza vaccine reduces illness and deaths among the most vulnerable population, the elderly. Although confounding factors could have had an impact on the findings, they are unlikely to have influenced the results appreciably.