Overshadowed by Europe's unprecedented HPAI H5 epizootic - and China's record-setting 5th wave of H7N9 - the EU has nonetheless just finished a prolonged, and severe winter flu season.
One which began unusually early, and which was dominated by a recently emerged subclade (3C.2a1) of H3N2 (see Eurosurveillance: Emergence Of A Novel Subclade Of Seasonal A/H3N2 - London).
H3N2 has shown remarkable diversity - something we've followed closely through the ECDC's ongoing Influenza Virus Characterization Reports. Since 2009 at least seven genetic groups based on the HA gene have been defined for H3N2. While all belong to clade 3C, they are divided into three subdivisions; 3C.1 , 3C.2, and 3C.3.
This diversity, and the rapid shifting of dominance between one subclade and another, makes it difficult for any single vaccine strain to protect against a multi-faceted and continually evolving H3N2 threat.As we've seen in other, recent H3N2 dominated flu seasons, the H3N2 vaccine component struggled to provide even moderate protection against this continually evolving strain.
Follow the link to read the full ECDC report:.
Review of influenza season 2016–2017 in the EU/EEA: severe season with A(H3N2) dominance
12 May 2017
The influenza season 2016–2017 has now come to an end in Europe. In week 17/2017, all EU/EEA countries reported low influenza activity and the positivity rate (proportion of influenza virus positives among all tested specimens) falling below the 10% benchmark to 9.9%. For the previous six weeks, almost all EU/EEA countries had reported low influenza activity.
This season’s dynamics
The season started in EU/EEA countries in week 46/2016, the earliest start to the season in the past five years. As in the previous season, it was the Netherlands who first reported regional spread and medium intensity. The season lasted for 27 weeks, which is longer than recent past seasons (20–22 weeks).
The peak of the season, when the positivity rate exceeded 50% in the EU, occurred between weeks 52/2016 and 5/2017. For recent past seasons, the positivity rate exceeded 50% around 3–7 weeks after the start of the season (defined as positivity rate higher than 10%) and the >50% peak lasted 1–11 weeks.
Influenza A(H3N2) virus dominated, accounting for 76% of all sentinel specimens during the season. ECDC’s monitoring picked up signals that A(H3N2) was dominant as early as week 43. For intensive care unit admissions, A(H3N2) was the most commonly identified virus subtype and, as expected, given that A(H3N2) is known to affect older people more severely, about two-thirds of intensive care unit patients were over 65 years of age. This contrasts with the 2015–16 season where the predominant virus was A(H1N1)pdm09 and around 60% of influenza-related ICU admissions were 15–64 years.
Two-thirds of the A(H3N2) viruses genetically characterised belong to subclade 3C.2a1, although they remained antigenically similar to the clade 3C.2a vaccine virus. Preliminary estimates of influenza vaccine effectiveness for the 2016–2017 season show moderate effectiveness (38%) against A(H3N2) for all age groups, but suboptimal effectiveness (23.4%) for those over 65 years, although this is similar to other A(H3N2)-dominated seasons.