The Efficacy of the Flu Vaccine in Preventing Morbidity and Mortality Amongst the Elderly

Every year between 5 and 20 percent of the US population contracts influenza. In a typical year, complications from the flu lead to 200,000 hospitalizations and 30,000 deaths. The influenza vaccine has the potential to greatly reduce this public health burden. Randomized controlled trials demonstrate that vaccination substantially reduces the probability that a person develops influenza, but they lack sufficient power to estimate the vaccine’s efficacy in reducing rare events such as hospitalizations and deaths. Comparisons between the vaccinated and unvaccinated using observational data suggest that the vaccine reduces hospitalizations and deaths. However, these estimates suffer from selection bias.

We implement a quasi-experimental approach to estimating the influenza vaccine’s effect on flu-related doctors visits, hospitalizations, and deaths. This approach exploits age-based guidelines for influenza vaccination. In England prior to 1998, influenza vaccination was reserved for a small portion of the population, primarily those with chronic diseases or immunosuppression and those living in long-term care facilities. In the 1998-99 flu season, the policy was changed to introduce universal vaccination for those 75 years and older, and in the 2000-01 season the policy was extended to cover all individuals 65 and older. The guidelines generate a large and abrupt increase in vaccination rates when people reach the age threshold. This sharp increase allows us to implement a regression discontinuity design to estimate the effectiveness of the vaccine. The core insight in this approach is that, despite the increase in vaccination rates, individuals just under the age threshold are a plausible comparison group for those just over the age threshold.

Our preliminary results suggest that vaccination reduces the incidence of influenza-like illness among the elderly. However, there is no evidence of a reduction in hospitalizations or mortality rates. Guidelines that prioritize vaccination of the elderly thus may not achieve the intended goal of reducing severe events among this at-risk population.