Although influenza has been late in arriving this year in North America (and Western Europe) for the past couple of weeks we've been watching reports of unusually severe influenza in Russia and Eastern Europe, with Russian epidemiological reports indicating a new subgroup of A(H1N1) has recently emerged.
Genetic characterization. 55 investigated influenza A(H1N1)pdm09 virus strains were A/South Africa/3626/2013-like. All viruses bear clade 6B specific mutations in HA (S84N, S162+N and I216T) and formed new genetic group according to phylogenetic analysis. Two A(H1N1)pdm09 sequences obtained directly from autopsy sample showed the presence of additional mutation D222G in HA1.
We've discussed the significance of the D222G mutation a number of times (see here & here), but essentially it promotes lower lung infections, and is linked to increased virulence. The Russian epi report also mentions reduced titers against the current vaccine strain for some viruses sampled.
Despite the lack of North American flu cases so far, last Monday the CDC issued a HAN Advisory: Severe Influenza Illness Reported.
Today, the ECDC cites - 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 - and published the following mid-season risk assessment.
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.
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.
Almost all viruses tested for neuraminidase inhibitor (antiviral) susceptibility, showed no reduction in effectiveness.
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.
Although influenza season normally peaks by February, every flu season is different, and we may still be on pace to see a late onset flu season. It isn't too late to get a flu shot, and as always, it is important to practice good flu hygiene.
As for the ultimate impact and significance of the changes being reported in the H1N1 virus, we'll just have to wait and see.
Last week we looked at similar changes reported in India in 2015, in Eurosurveillance: Emergence of A(H1N1)pdm09 Genogroup 6B In India, 2015. There you will also find further discussion of the H275Y mutation which confers resistance to Oseltamivir.