Influenza vaccine coverage and the attack rate of influenza-like illness among the elderly in Portugal: is there a correlation?

Abstract

Vaccination of the elderly (>=65 years of age) against influenza is recommended in all European countries

Excess pneumonia and influenza hospitalizations associated with influenza epidemics in Portugal from season 1998/1999 to 2014/2015

“…In 2016, a study that included 14 European Union countries, including Portugal, has analyzed the correlation between seasonal ILI incidence rates and vaccine coverage among all the population (except for individuals with 65 or more years of age) and showed no consistent results between the countries, identifying only negative correlation among the elderly population for England (−0.80), the Netherlands (−0.60), and Germany (−0.57). For Portugal, the study included seasons 1998‐1999 to 2013‐2014 showing correlation coefficients different from those obtained in our previous study that included seasons 1998‐1999 to 2006‐2007 . A possible explanation for the difference may be the data source for ILI rates, that is, the use of provisory ILI rates (in the European study) and definitive rates calculated in the end of the season (in the national study).…”

“…The influenza vaccine has proven to be moderately effective in reducing medically attended confirmed influenza . Using an ecologic approach, one would expect a reduction in excess hospitalizations/mortality with increasing vaccine coverage and this was already reported in previous influenza‐related outcomes studies …”

“…When considering all seasons, an increased dispersion was observed, reducing the strength correlation. Among the elderly (≥65 years), a negative correlation was also previously observed in another study that measured the correlation between vaccine coverage and seasonal ILI attack rates, and published a spearman rank coefficient of −0.359 and −0.899, respectively, for all seasons and only for the A(H3) predominant . In 2016, a study that included 14 European Union countries, including Portugal, has analyzed the correlation between seasonal ILI incidence rates and vaccine coverage among all the population (except for individuals with 65 or more years of age) and showed no consistent results between the countries, identifying only negative correlation among the elderly population for England (−0.80), the Netherlands (−0.60), and Germany (−0.57).…”

Long-Term Correlation between Influenza Vaccination Coverage and Incidence of Influenza-Like Illness in 14 European Countries

“…For example, a study in Portugal showed, in general, no significant correlations between vaccination coverage and ILI incidence trends. However, a significant negative linear correlation was found in analyses using only data from influenza A(H3)-dominant seasons [ 62 ].…”

The timing of influenza vaccination for older adults (65 years and older)

“…A search of the literature (including a 2006, Cochrane systematic review) found no clear correlation between influenza vaccination rates and influenza attack rates among the elderly. [13, 14] Therefore, without clear guidance on how to adjust the attack rate with earlier vaccination, we ran sensitivity analyses varying the effect that earlier vaccination would have on the influenza attack rate. In other words, this set of sensitivity analyses examined scenarios in which earlier vaccination would decrease the influenza attack rate by 1%, 5%, 10%, and 25%.…”