Impact of influenza vaccination on seasonal mortality in the elderly population of the United States

Impact of influenza vaccination on seasonal mortality in the elderly population of the United States
  • Lone Simonsen, Thomas A. Reichert, Cecile Viboud, William C. Blackwelder, Robert J. Taylor, Mark A. Miller
  • DOI: 10.1001 / archinte.165.3.265
  • February 2005

Abstract

Context:  Observational studies report that influenza vaccination reduces the risk of winter mortality from any cause among the elderly by 50%. Influenza vaccination coverage among the elderly (≥65 years old) in the United States increased from 15% to 20% before 1980 to 65% in 2001. Unexpectedly, estimates of influenza mortality in this age group also increased in this period. We tried to reconcile these conflicting results by adjusting the excess mortality estimates for aging and the increased circulation of influenza A (H3N2) viruses.

Methods: We used a cyclical regression model to generate seasonal estimates of national influenza-related mortality (excess mortality) among the elderly for both pneumonia and influenza and all causes of death for the 33 seasons from 1968 to 2001. We stratified the data by age groups of 5 years and separated the seasons dominated by viruses A (H3N2) from the other seasons.

Risultati: For people aged 65 to 74, excess mortality rates in A-dominated seasons (H3N2) declined between 1968 and the early 80s, but remained roughly constant thereafter. For people aged 85 or older, the death rate remained flat throughout the period. The excess mortality in seasons A (H1N1) and B did not change. For people aged 65 and over, excess mortality never exceeded 10% of all winter deaths.

conclusions: We attribute the decline in influenza-related mortality among people aged 65-74 in the decade following the 1968 pandemic to the acquisition of immunity to emerging virus A (H3N2). We could not correlate the increase in vaccination coverage after 1980 with the decline in mortality in any age group. Since less than 10% of all winter deaths were attributable to influenza in any season, we conclude that observational studies substantially overestimate the benefit of vaccination.

Accurate assessment of the seasonal impact of influenza on mortality is a difficult task. Diagnosis of influenza virus infection is rarely confirmed in the laboratory, and influenza infection is often cleared before the onset of the secondary complications that actually cause the patient's death.1,2 Consequently, influenza-related mortality must be determined indirectly, using statistical models that estimate winter seasonal excess of pneumonia and influenza (P&I) or total mortality above a predicted mortality baseline (Figure 1). 3-8 The study of trends in influenza-related mortality over time is further complicated by both the substantial season-to-season variation in the number of deaths from 0 to 70.000 since 19687,8 - and the fact that mortality is much higher in seasons dominated by influenza A (H3N2) viruses than in seasons dominated by influenza B and A (H1N1) viruses .5

Influenza vaccination in the United States has long been recommended for all people aged 65 and over.9 Vaccination coverage for this age group increased from 15% to 20% before 1980 to 65% in 2001.10 However, the 3-year moving averages of uncorrected excess P&I mortality rates among people aged 65 and over compiled for the Healthy People 200011 initiative12 to track the effect of vaccination on influenza-related mortality in the United States - rose substantially during this period.13 This was surprising because influenza vaccination is believed to be highly effective in reducing influenza-related mortality.19-XNUMX

We therefore decided to analyze the data on influenza-related mortality over the past 3 decades and to adjust the data to 2 important factors that could influence the observed trends. First, although the risk of influenza mortality increases rapidly with age6, the moving averages have not been adequate for the substantial increase in the average age of the U.S. elderly population over the past 3 decades.20 Second, the averages have not adapted to the increased frequency with which A (H3N2) viruses dominated flu seasons in the 90s. We then adjusted the estimates of influenza-related mortality by age and analyzed mortality trends over time for seasons dominated by influenza A (H3N2) viruses separately from those dominated by A (H1N1) or B viruses. influence.