Thursday, January 04, 2018

Eurosurveillance: Changes In Timing Of Influenza Epidemics - WHO European Region 1996-2016


The annual two-week drought in new scientific papers published during the second half of December has ended, and we are beginning to see some new papers emerge.  Today, from Eurosurveillance, we have an analysis of the changes in the peak of influenza season across Europe over the past 20 years.
Curiously, while Western European countries have seen a discernible shift towards later flu season peaks, the opposite is true in Eastern Europe and Russia. 
As a result the overall duration of influenza seasons across the entire WHO European region has shortened over the past 20 years.  While the reasons behind these changes aren't immediately apparent, their impact on influenza prevention and control efforts is something that needs to be examined going forward.

Some excerpts from a much longer research paper follow.  Click the link to read the paper in its entirety.

Important changes in the timing of influenza epidemics in the WHO European Region over the past 20 years: virological surveillance 1996 to 2016 
Saverio Caini1, François Schellevis1,2, Clotilde El-Guerche1,2,3, Séblain3, John Paget1 

The global epidemiology of many infectious diseases has changed in recent years; a number of concomitant and mostly anthropogenic factors play a role in this process, including climate change, increased urbanisation, population mobility, deforestation, agricultural intensification and forced displacement of human populations [1]. Most research has focused on malaria and other vector-borne infections [2-4] or on and food- and waterborne diseases [5,6]. The question of whether the timing of seasonal influenza epidemics has changed in recent years has received comparatively little attention. However, influenza seasonality is known to be linked to many of the above factors [7-12], and the temporal characteristics of influenza epidemics may evolve over time as a result of changes in these factors [13,14].

Seasonal influenza epidemics in the northern hemisphere are typically characterised by a short epidemic period of 8 to 12 weeks that varies in intensity during the winter months (November to March) [15] and are associated with substantial morbidity and mortality. Annual vaccination is the most effective measure to reduce the burden of influenza and is most effective when vaccination campaigns coincide optimally with seasonal epidemics. Considering that 2 to 4 weeks may be required to develop an immune response to the vaccine [16] and protection may wane within 6 months [17], the timing of the epidemic peak is an important element that should be considered to optimise the effectiveness of influenza vaccination campaigns.

To assess whether the epidemiology of influenza has changed in the World Health Organization (WHO) European Region (900 million inhabitants), we examined for each country and the Region as a whole whether the timing of epidemic peaks has changed (i.e. occurs earlier or later) between 1996 and 2016.

Timing of the influenza epidemic peak
The peak of the influenza epidemics occurred progressively later during the study period in 25 countries (Figure 1 and Table 2). The trend was statistically significant in Belgium, the Czech Republic, Portugal, Spain and Switzerland. In 13 other European countries, the influenza epidemic occurred progressively earlier, with a statistically significant trend in the Russian Federation and Ukraine.

This study showed that a key characteristic of seasonal influenza epidemics – the timing of the peak – has changed in the WHO European Region between 1996 and 2016. Unexpectedly, however, the timing of the peak did not change uniformly across the Region, but instead according to a longitudinal gradient, with influenza epidemics tending to peak progressively later in Western European countries and progressively earlier in Eastern European countries. These results were confirmed in several sensitivity analyses.
Our findings have implications for influenza control and prevention in countries across the WHO European Region. The implementation of vaccination campaigns should be synchronised with the timing of influenza epidemics, considering that the optimal immune response to vaccination may take 2 to 4 weeks to develop [16] and may decline substantially within 6 months [17]. Accordingly, failure to consider a systematic shift in the timing of epidemics (i.e. the timing of their onset, peak and end) may gradually reduce the effectiveness of influenza vaccination programmes. Influenza vaccination campaigns may need to be planned later in Western Europe and earlier in Eastern Europe, especially if the observed trends persist in the coming years. Importantly, we observed exceptions to these general patterns, and each country needs to carefully assess their situation at a national level, i.e. verify that the current timing of vaccination campaigns is still optimal. For example, the peak in Denmark, which is considered to be a country in Western Europe, has remained fairly stable over the years, and this would suggest that the timing of the vaccination campaign may not need to be modified.
One important consequence of these changes is that the overall duration of influenza activity in the WHO European Region (as a whole) has shortened over the past twenty years, with the average interval between peak influenza activity in western and eastern countries declining from nearly 2 months in 2004/05 (with influenza activity first peaking in the west and then in the east) to less than 3 weeks in 2015/16 (with peaks typically occurring in February and March in most countries [25]). Our findings are particularly relevant for influenza vaccination programmes, as they suggest that the timing of epidemics and, therefore, the optimal time of vaccination has become better aligned across the whole WHO European Region, which in turn allows more coordinated and efficient management of surveillance and prevention efforts.

We found that the timing of the peak of influenza epidemics has changed in countries of the WHO European Region between 1996 and 2016. The main drivers behind this phenomenon remain to be clarified, but how the changes might affect influenza prevention and control efforts in Europe demands further attention.
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