The impact of targeting all elderly persons in England and Wales for yearly influenza vaccination: excess mortality due to pneumonia or influenza and time trend study

Abstract

Objective: To investigate the impact on mortality due to pneumonia or influenza of the change from risk-based to age group-based targeting of the elderly for yearly influenza vaccination in England and Wales.

Design: Excess mortality estimated using time series of deaths registered to pneumonia or influenza, accounting for seasonality, trend and artefacts. Non-excess mortality plotted as proxy for long-term trend in mortality.

Setting: England and Wales.

Participants: Persons aged 65-74 and 75+ years whose deaths were registered to underlying pneumonia or influenza between 1975/1976 and 2004/2005.

Outcome measures: Multiplicative effect on average excess pneumonia and influenza deaths each winter in the 4-6 winters since age group-based targeting of vaccination was introduced (in persons aged 75+ years from 1998/1999; in persons aged 65+ years from 2000/2001), estimated using multivariable regression adjusted for temperature, antigenic drift and vaccine mismatch, and stratified by dominant circulating influenza subtype. Trend in baseline weekly pneumonia and influenza death rates.

Results: There is a suggestion of lower average excess mortality in the six winters after age group-based targeting began compared to before, but the CI for the 65-74 years age group includes no difference. Trend in baseline pneumonia and influenza mortality shows an apparent downward turning point around 2000 for the 65-74 years age group and from the mid-1990s in the 75+ years age group.

Conclusions: There is weakly supportive evidence that the marked increases in vaccine coverage accompanying the switch from risk-based to age group-based targeting of the elderly for yearly influenza vaccination in England and Wales were associated with lower levels of pneumonia and influenza mortality in older people in the first 6 years after age group-based targeting began. The possible impact of these policy changes is observed as weak evidence for lower average excess mortality as well as a turning point in baseline mortality coincident with the changes.

Keywords: Aged; Influenza; Mass Vaccination; Mortality; Trends.

Figure 1

Excess mortality in the age groups (A) 65–74 and (B) 75+ years in each influenza season in England and Wales, stratified by the dominant influenza virus in circulation (black ‘x’=A/H3N2 viruses were dominant or codominant; grey ‘+’=A/H1N1 or B viruses were dominant). Vaccine coverage in the respective age group from published data is also shown (dots and asterisks, right axis). Asterisks indicate that vaccine coverage in these years was inferred from observed vaccine coverage in the 65+ years age group and the average ratio of coverage in the age group 65+ to 65–74 years (or 75+ years). P&I, pneumonia or influenza.

Figure 2

Summary of estimated vaccine impact from log(excess +1) regression models of the binary effect of the switch to age group-based targeting of yearly influenza vaccination on all those aged 65 years and older (from 2000/2001 onwards) compared to before 2000/2001. Coefficients are shown on the original scale and are therefore multiplicative. Squares, circles and triangles indicate the age groups 65–74, 75+ and 45–64 years, respectively. Filled symbols represent seasons dominated by influenza A/H3N2 viruses and open symbols indicate seasons dominated by influenza A/H1N1 or B viruses.

Figure 3

Model fit: the observed time series of weekly pneumonia or influenza (P&I) deaths in the age groups (A) 65–74 years and (B) 75+ years in England and Wales between 1975 and 2005 (light dotted line on each plot). The fitted curve from the log-linear Simonsen-like model fitted excluding December to April and accounting for seasonality, long-term trend and artefacts is overlaid (dark line). Vaccine coverage (in the age groups 65–74 and 75+ years) adapted from published data is shown on the right axis of each plot (dots and asterisks). Asterisks indicate that the vaccine coverage in these years was inferred from the observed vaccine coverage in the 65+ years age group and the average ratio of coverage in the 65–74 (or 75+) to 65+ years age group. The fitted curve can be deconstructed into its constituent parts. Thus, (C) and (D) showing the long-term trend (ie, cubic spline) component of the fitted curve (dark lines), with its 95% CI (light lines), for the 65–74 and 75+ years age groups, respectively. This allows a better visualisation of the shape of the long-term trend. P&I, pneumonia or influenza.