Important changes in the timing of influenza epidemics in Western and Eastern Europe

A study carried out by Nivel and published in Eurosurveillance has found that the timing of influenza epidemics has changed in Europe over the last 20 years. These findings have important implications for influenza vaccination programmes in the region. Climate change and/or changes in population movement may be underlying factors that explain these developments.

Main findings
The Nivel study found that the timing of seasonal influenza epidemic peaks has changed in Europe according to a longitudinal gradient, occurring progressively later in Western Europe (e.g. by 2.8 days/season in Spain) and progressively earlier in Eastern Europe (e.g. by 3.5 days/season in the Russian Federation). This means, for example, that over a ten year period seasonal influenza epidemics have got, on average, 28 days (nearly a month) later in Spain.

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 to less than 3 weeks in 2015/16.

John Paget, Senior Researcher at Nivel, noted that the findings were ‘very surprising and that whilst the implications have not been discussed at a European level they have been acted upon at a local level. For example, Portugal has already moved their influenza vaccination programme to a later date - from September to October - so that it is better aligned with influenza epidemics in this country.’ He added that ‘it is now important to assess what factors explain these changes and Nivel is currently carrying out a research project to assess the impact of climate change on the epidemiology of influenza activity in the Netherlands’.

About the study
The Nivel study investigated whether the timing of the peak of seasonal influenza epidemics has changed over twenty years (July 1996 to June 2016) in countries of the World Health Organization (WHO) European Region, a region of 900 million inhabitants stretching from Portugal in the west to Russia in the east. Data for 38 countries and more than 600,000 influenza cases were included in the study. The study was performed as part of the GIBS project and used surveillance data obtained from the WHO FluNet database.

GIBS is funded by an unrestricted research grant from Sanofi Pasteur.