Despite occasional misses, such as we saw over the winter of 2014-15 when a `drifted' H3N2 virus showed up in the early fall (see CDC HAN Advisory On `Drifted’ H3N2 Seasonal Flu Virus), the seasonal flu vaccine generally provides a moderate level of protection against currently circulating flu viruses.
The actual level of protection you get depends on many factors, including your own immune response to the vaccine, how close the vaccine strain matches the strain you've been exposed to, and the subtype of flu we are talking about.While flu vaccines have provided reasonably good protection against H1N1 and Influenza B viruses in recent years - often reaching VE (Vaccine Effectiveness) Ratings 50%-60% - the H3N2 vaccine component remains problematic.
As CIDRAP reported last April (see Review shows persistently low flu vaccine protection against H3N2):
A meta-analysis published yesterday of 56 studies showed flu vaccine effectiveness (VE) for the H3N2 strain over multiple seasons to be only 33%, compared with 54% for B strains and 61% for 2009 H1N1.Part of the reason is that influenza B viruses tend to evolve - via antigenic drift - more slowly than do influenza A viruses. And the H1N1 virus, which emerged as a pandemic in 2009, remained remarkably stable over its first 7 years in circulation.
H3N2, on the other hand, has shown remarkable diversity - something we've followed closely through the ECDC's ongoing Influenza Virus Characterization Reports. Since 2009 seven genetic groups based on the HA gene have been defined for H3N2. While all belong to clade 3C, they are divided into three subdivision; 3C.1 , 3C.2, and 3C.3. The most recent report stating:
In 2014 three new subclades emerged, one in subdivision 3C.2, 3C.2a, and two in 3C.3, 3C.3a and 3C.3b , with subclade 3C.2a viruses dominating in recent months.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.
Today Eurosurveillance published an early look of the flu vaccine's VE in Canada in this year's H3N2 dominated season. While clocking in with a non-spectacular 42%, this is still a major improvement over the near zero effectiveness during the 2014-15 H3N2 flu season, and slightly better than the VE derived in the meta-analysis mentioned above.
I've only included some small excerpts from a lengthy and detailed report. Click the link to read it in its entirety.
Eurosurveillance, Volume 22, Issue 6, 09 February 2017
Interim estimates of 2016/17 vaccine effectiveness against influenza A(H3N2), Canada, January 2017
DM Skowronski 1 2 , C Chambers 1 , S Sabaiduc 1 , JA Dickinson 3 , A Winter 4 , G De Serres 5 6 7 , SJ Drews 8 9 , A Jassem 1 2 , JB Gubbay 4 10 , H Charest 5 , R Balshaw 1 2 , N Bastien 11 , Y Li 11 , M Krajden 1 2
Citation style for this article: Skowronski DM, Chambers C, Sabaiduc S, Dickinson JA, Winter A, De Serres G, Drews SJ, Jassem A, Gubbay JB, Charest H, Balshaw R, Bastien N, Li Y, Krajden M. Interim estimates of 2016/17 vaccine effectiveness against influenza A(H3N2), Canada, January 2017. Euro Surveill. 2017;22(6):pii=30460. DOI: http://dx.doi.org/10.2807/1560-7917.ES.2017.22.6.30460
Received:02 February 2017; Accepted:09 February 2017
Using a test-negative design, the Canadian Sentinel Practitioner Surveillance Network (SPSN) assessed interim 2016/17 influenza vaccine effectiveness (VE) against dominant influenza A(H3N2) viruses considered antigenically matched to the clade 3C.2a vaccine strain. Sequence analysis revealed substantial heterogeneity in emerging 3C.2a1 variants by province and over time. Adjusted VE was 42% (95% confidence interval: 18–59%) overall, with variation by province. Interim virological and VE findings reported here warrant further investigation to inform potential vaccine reformulation.
We report interim VE of ca 40% for the 2016/17 influenza A(H3N2) epidemic in Canada, which is higher than in 2014/15 and consistent with expected but suboptimal VE estimates for influenza A(H3N2) more generally. Given that a substantial proportion of vaccinated people may remain unprotected against influenza A(H3N2) illness, other adjunct measures should be considered to minimise associated morbidity and mortality, particularly among high-risk individuals. Continued evolution in circulating 3C.2a variants and their derivatives, and the impact on vaccine protection, warrants ongoing monitoring to inform potential vaccine reformulation.
Even at a reduced vaccine effectiveness, some protection beats no protection at all. And there is some evidence that even when it doesn’t prevent infection, the flu vaccine may reduce the severity of illness (CDC: Flu Shots Reduce Hospitalizations In The Elderly).
As regular readers of this blog already know, I get one every year.
While I sometimes worry that the benefits (and effectiveness) of the flu vaccine are oversold, flu vaccines - along with practicing good flu hygiene (cover coughs, wash hands, stay home when sick, etc.) - remain our best protection against a virus that is estimated to kill a half million people around the globe each year.