Rapid risk assessment: circulation of drifted influenza A(H3N2) viruses in the EU/EEA, 22 December 2014

Risk assessment

European Centre for Disease Prevention and Control. Circulation of drifted influenza A(H3N2) viruses in the EU/EEA – 22 December 2014. Stockholm: ECDC; 2014.

Influenza surveillance data gathered since 1 October 2014 indicate that influenza viruses in EU/EEA countries have been predominantly A(H3N2) rather than A(H1N1)pdm09 and type B viruses. In previous seasons, influenza A(H3N2) viruses have been associated with more severe disease than A(H1N1) and type B viruses. 

Executive summary

Preliminary data suggest that more cases of severe disease could be expected in Europe during this influenza season​

Indications of drift for A(H3N2), good match for A(H1N1)pdm09

Furthermore, early season results from the US, where the season has progressed further than in Europe, indicate that circulating viruses will include strains that are antigenically distinct from the A(H3N2) virus in this season’s influenza vaccine. While very few influenza virus characterisations have been conducted so far in EU/EEA countries, the genetic information suggests the following: - Influenza A(H3N2) viruses circulating in EU/EEA countries this season will include strains that are antigenically distinct from the A(H3N2) vaccine virus.- Early indications are that circulating A(H1N1)pdm09 viruses are antigenically similar to the vaccine virus. - Too few type B viruses have been characterised to date to comment on their similarity to the type B vaccine component.  These observations indicate that the 2014-15 influenza season may be associated with a greater number of cases with more severe disease, given the higher proportion of A(H3N2) strains, but it must be noted that the pattern of strain distribution can change as the season progresses. In addition, the early evidence of drift indicates that vaccine effectiveness against disease due to A(H3N2) strains is likely to be reduced. 

Influenza vaccination protects the vulnerable

Influenza vaccination, which provides protection against at least three strains of influenza each season, remains the most effective measure to prevent illness and possibly fatal outcomes also this season, despite the drift in one of the influenza strains. In the EU, up to 40 000 people die prematurely during an average influenza season. Protecting those who are at risk of severe outcomes of influenza remains the priority of vaccination programmes, particularly the elderly and those vulnerable due to pre-existing illnesses or risk factors. 

Antivirals to be considered

The circulating viruses analysed so far show susceptibility to the antiviral drugs oseltamivir and zanamivir. As advised in previous seasons, physicians should therefore always consider treatment or post-exposure prophylaxis with antivirals when treating influenza-infected patients and exposed individuals in risk groups.  

Next steps by ECDC

ECDC will convene an expert panel in February 2015 to review the evidence currently available on the effectiveness of antivirals in public health use. ECDC will produce its seasonal influenza risk assessment once active circulation of influenza viruses starts in Europe.