Credit NIAID
# 9474
While we often talk about seasonal strains (H1N1, H3N2, Influenza B) as if they were individual entities – in truth within each subtype there are many clades and variants - and they are all evolving over time. Geographically, these viruses can vary widely, and so the dominate strains in Europe may differ from the dominant strains in North America or Asia.
Over time, new, more biologically `fit’ viruses replace older strains as community immunity drives them closer to obsolesce.
All which makes the flu world dynamic and ever-changing, and presents a genuine challenge for vaccine manufacturers to stay ahead of. NIAID has a terrific 3-minute video that shows how influenza viruses drift over time, and why the flu shot must be frequently updated, which you can view at this link.
Over the summer it was becoming apparent that a new, `drifted’ H3N2 virus was making inroads in Europe and around the globe (see ECDC: Influenza Characterization – Sept 2014) – one that differed antigenically from this year’s H3N2 vaccine strain.
In September the WHO announced a strain change for next year’s Southern Hemisphere vaccine to meet this viral challenge, but this virus emerged far too late in the year to allow changes to this fall’s Northern Hemisphere vaccine.
In early November, in A `Drift’ In A Sea Of Influenza Viruses, I wrote about early concerns over this year’s vaccine, and in the first week of December the CDC issued a HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus along with a warning that Early Data Suggests Potentially Severe Flu Season.
Today the ECDC has issued their own Risk Assessment on this `drifted’ flu strain.
22 Dec 2014
Available as PDF in the following languages
This document is free of charge.
Abstract
Surveillance data gathered since 1 October 2014 indicate that in the first ten weeks of the 2014–15 influenza season, 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 were associated with more severe disease than A(H1N1) and type B viruses; they were also associated with several outbreaks in long-term care facilities.
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 among isolates typed to date and the early evidence of drift that is likely to be associated with reduced vaccine effectiveness.
Influenza vaccine coverage among the elderly and the risk groups in most parts of Europe is low. However, the benefits of vaccination are considerable in protecting these population groups, even if vaccine effectiveness against one of the circulating viruses may turn out to be low.
I’ve excerpted the following from the full report:
Main conclusions and recommendations
Surveillance data gathered since 1 October 2014 indicate that in the first ten weeks of the 2014–15 influenza
season, 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 were associated with more severe disease than
A(H1N1) and type B viruses; they were also associated with several outbreaks in long-term care facilities.
The recently published US CDC health alert network notification on antigenically drifted influenza A(H3N2) viruses is the first signal from a northern hemisphere country that circulating viruses will include strains that are antigenically distinct from the A(H3N2) vaccine virus, A/Texas/50/2012, which was recommended by WHO for the northern hemisphere 2014–15 season at the February 2014 strain selection meeting.
Very few influenza virus characterisations have been conducted to date in EU/EEA countries, and the majority of them have been genetic rather than antigenic. The genetic information reported so far suggests the following:
- Influenza A(H3N2) viruses circulating in EU/EEA countries this season will be antigenically distinct from the northern hemisphere 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 the likely effectiveness of the B/Massachusetts/2/2012 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 among isolates typed to date and the early evidence of drift that is likely to be associated with reduced vaccine effectiveness.
Despite the expected low vaccine effectiveness (VE) of the A(H3N2) vaccine virus component in the vaccines administered for protection in the 2014–15 influenza season, the current tri- and quadrivalent vaccines are likely to provide protection against infection by other currently circulating influenza viruses. Even with low VE of the A(H3N2) vaccine virus components, the vaccine may ameliorate or shorten the duration of influenza disease in infected individuals and is likely to reduce the number of severe outcomes and mortality. Influenza vaccination remains the most effective measure to prevent illness and possibly fatal outcomes.
The circulating viruses are susceptible to the antiviral drugs oseltamivir and zanamivir. Physicians should therefore always consider treatment or post-exposure prophylaxis with antivirals when treating influenza infected patients and exposed individuals in risk groups.
Influenza vaccine coverage among the elderly and the risk groups in most parts of Europe is low.However, the benefits of vaccination are considerable in protecting these population groups, even if vaccine effectiveness against one of the circulating viruses may turn out to be low.
