It's looking to be a bad flu year, it there anything to be done?


2015 is looking like it is going to be a bad influenza season with a new H3 strain wandering the US. I figure when school starts up in a week influenza will really take off:

The closure of schools has been postulated as a means of reducing transmission between children and hence reducing the number of cases at the peak of an epidemic; this is supported by the marked reduction in cases during school holidays observed across the world during the 2009 pandemic.

and exacerbated by people going back to work since

commuting volume is highly correlated with the spread of ILI.

So while influenza is bad in some parts of the US now, it is likely to get a lot worse in January.

Do you need to get the influenza vaccine aka the flu shot this year?  The CDC says the vaccine will not work this year because there is no match  right?  Why bother? It is, as my kids would, the meme.

Short answer, there is still benefit from getting the flu vaccine despite the mismatch between the vaccine and circulating strains.  For the long answer, keep reading.

Let’s talk influenza immunity.  Like most of medicine, it is complicated and nuanced.  

Every year the circulating strains of influenza change, either by a drift (small changes in the virus) or by shift (the virus gets a new and unique chunk of DNA).  Both drift and shift lead to new hemagglutinin and neuraminidase proteins on the virus, the ‘H’ and ’N’ that identify the virus. 

The vaccine contains hemagglutinin and neuraminidase viral proteins and since the vaccine has to be produced months before we know the circulating influenza strains, they have to make an educated guess as to which strain to put in the vaccine.

The year the vaccine contains

- Influenza A/California/7/2009 (H1N1) virus
- Influenza  A/Texas/50/2012 (H3N2) virus
- Influenza B/Massachusetts/2/2012 virus.
- Some of the 2014-2015 flu vaccine also protects against an additional Influenza B virus (B/Brisbane/60/2008 virus).

What is circulating?  

About 80% of the current influenza isolates are H3N2 of which 43% is a “drifted” version of H3N2 (A/Texas/50/2012) that poorly matches the vaccination strain (these percentages will likely change as the season progresses). 

So of the H3N2 strains circulating, about 43% are a poor match for the vaccine.  Or put another way, the vaccine covers still about 57% of the circulating H3N2 strains. 

So the vaccine is worthless, right?


The vaccine still covers 2.5 or 3.5 of the circulating influenza strains, depending on if you get the trivalent or quadravalent vaccine.  And even with the drift,  the vaccine is  37 percent to 42 percent effective against H3N2 viruses, about half what the would be expected from a well matched vaccine. So the vaccine covers more like 2.75 or 3.75 of the circulating strains.

And just because there is not an exact match does not mean that the vaccine does not have effect.  Flu vaccination is far more complicated than a one viral protein leading to one antibody.  You make multiple antibodies directed against multiple sections of viral proteins, so while you may not make an antibody to the new, drifted, part of the H3N2, you may make antibody to non-drifted parts of the protein.  Plus, due to a phenomenon known as back boosting you can get increase in antibodies against prior unrelated strains of influenza to which you have been exposed over your lifetime from vaccination or disease that may cross react with the current circulating strains.  

As the recent Science (Science 2014;346:919–920) review noted, the interaction of influenza and the immune system is 

a multidimensional immunological landscape that is created by potentially complex interactions between the immune system and influenza antigen over a lifetime of vaccination and infection.

This is why even when there is a mismatch between the vaccine and circulating strain, you may still get benefit from the vaccine:

In not well-matched years, the overall risk of getting the flu drops to 2 people out of 100. The flu vaccine cuts that number in half, for a 50% risk reduction. If you do the math, one person out of 100 benefits from the flu shot.

And that is just for 43% of the H3H2 strains. You get full vaccine benefit for the other strains in the vaccine and for 57% of the H3N2 strains.  And that is  for just getting the flu, not taking into account the downstream beneficial effects of flu vaccination:

  • you do not pass on disease
  • you do not get complications of disease. There is  less flu, fewer flu deaths, fewer flu related complications,  fewer flu related hospitalization, fewer ICU admissions, fewer stillbirths, larger babies, less influenza in newborns, smarter, richer children (really), and fewer vascular events. Influenza vaccination could potentially reduce influenza related occurrence of sudden death, AMI, and stroke by 50%; and it is the H3 influenzas, the current circulating strains, that may have the largest risk for a precipitating a vascular event.

I like the car metaphor.  I would rather be in a car accident with seat beats and an airbag than without them.  I would rather have a vaccine  during flu season than not.  This year, we don’t have the metaphorical side airbag.  That doesn’t mean I want to give up my seatbelt and front airbag as well.

Points of Interest 12/31/2014
Points of Interest 12/30/2014

Related Posts