Mid-season risk assessment: Seasonal influenza 2015–2016 in the EU/EEA countries

A(H1N1)pdm09 dominant influenza strain in Europe

This year’s seasonal influenza risk assessment identifies type A viruses, in particular A(H1N1)pdm09, as dominant thus far in EU/EEA countries. There are strong indications from some EU/EEA countries that the A(H1N1)pdm09 virus is responsible for the hospitalisation of a large number of severe cases. This includes hospitalisations for severe outcomes for both risk groups and otherwise healthy young adults. A similar pattern of severity is likely to be observed in other countries as the season progresses.

The season started in EU/EEA countries in week 52/2015, with the Netherlands reporting regional spread, while Sweden reported widespread activity. The A(H1N1)pdm09 virus is the most prevalent so far this season overall but B viruses predominated in four countries, and three countries had an even distribution of both A and B viruses. B viruses could emerge later and become dominant by the end of the season. In previous seasons, B viruses have tended to be more prevalent in the second half of the season.

The A(H1N1)pdm09 virus is responsible for the vast majority of patients in intensive care units due to influenza; 61% of those were in the 15–64 years old age group. This contrasts with the 2014–15 season where the predominant A(H3N2) virus affected the elderly more.

Seasonal influenza vaccine effectiveness

The composition of influenza vaccines in the southern hemisphere in 2015 and in the northern hemisphere in 2015–16 were identical and thus provide an indication of how effective vaccination could be in Europe. Estimates of vaccine effectiveness in New Zealand are encouraging, with an overall effectiveness against hospitalisations of 50%.

For Europe, the vaccine effectiveness is expected to be lower than in the 2015 season in New Zealand. Europe is seeing a higher prevalence of B/Victoria virus circulating, which is not included in the widely used trivalent vaccine, and it is unclear if the emergence of a new genetic subgroup of A(H1N1) virus might compromise vaccine effectiveness.

Susceptibility to antiviral drugs

Almost all viruses tested for neuraminidase inhibitor (antiviral) susceptibility, showed no reduction in effectiveness.

ECDC advises:

• Simple measures such as self-isolation, good hand hygiene and cough etiquette can reduce transmission and protect others.
• Early treatment and post-exposure prophylaxis with neuraminidase inhibitors (antivirals) can assist in protecting the elderly and people in risk groups against serious influenza illness.
• EU Member States are encouraged to report ICU-admitted, laboratory-confirmed influenza cases to the European Surveillance System (TESSy) in a timely fashion in order to facilitate the assessment of the severity of the season.