Ten Percent Is Nothing To Sneeze At

As you might have read recently, review of the influenza season in the Southern Hemisphere demonstrated that this year’s flu vaccine was only 10% effective there. It is too early in the Northern Hemisphere to predict whether the vaccine that we have been giving will be any more effective. We do know that the vaccine composition is similar, and presumably, the flu viruses that are circulating will be similar as well. In a recent article in the NEJM, the authors question whether we should be looking at different approaches to producing flu vaccines so that there isn’t as great a chance of missing the mark.

Our current vaccines cover influenza A and B varieties. The two influenza A virus subtypes in general circulation in humans are H1N1 and H3N2. The H and N designations are two proteins on the surface of the virus – hemagglutinin and neuraminidase. In Australia, there have been record numbers of hospitalizations and deaths this year – four times as many cases as in the 2009 H1N1 flu pandemic. This year’s viruses are predominantly H3N2, which is not as well covered by the vaccine.

I was surprised to learn that in the U.S. alone, there are 140,000 to 710,000 flu-related hospitalizations and 12,000 to 56,000 deaths each year, with those most affected being the very young, very old and people with co-existing medical conditions. Here, decisions about what strains to cover are made in February for the vaccines to be produced for the fall/winter season. In a year when the predominant circulating virus and vaccine are poorly matched, the effectiveness may be around 10%, in a good year, maybe 40-60%.  Even the latter is lower than the rate for most vaccines against other illnesses. Effectiveness in any one particular person is affected by many factors, including the host’s past vaccine and infection experience, age, and co-existing conditions.

Most available flu vaccines can not be given to people who are allergic to eggs. That is because almost all flu vaccines make use of viruses that are propagated in eggs, although some are produced in cell culture. Other vaccines use proteins that have been produced using recombinant DNA technologies, without having to grow the whole virus.

During the egg-based production process, the vaccine virus acquires certain protein changes that allow easier replication in eggs – the changes are often in hemagglutinin (HA) protein. Although these changes are good for growing the virus, it changes how useful the proteins are for creating the antibody response we need to be ready to attack the flu virus when we encounter it. It would be better for us if the vaccine presented a true version of the HA.  So the egg-based vaccine ends up being less effective. Using egg-based production, then, might be part of the problem with our vaccines. This has been hypothesized, but not yet proven.  Another issue is having to predict in February what the predominant flu strain will be in the following winter.

The article concludes, “However imperfect, though, current influenza vaccines remain a valuable public health tool, and it is always better to get vaccinated than not to get vaccinated.” The CDC estimates that influenza vaccines avoided 40,000 deaths between the 2006 winter season and 2014. The authors encouraged changes in manufacturing strategies to achieve a more universal vaccine that will not require us to chase antigenic drift from year to year and perhaps not require viruses to be propagated in eggs.  

We expect that Tamiflu will still be effective in this year’s influenza season.  If you have the flu or are fairly certain that you have been exposed to the flu, and especially if you are in one of the high risk categories (extremes of age, immune deficiencies, co-existing illness…), be sure to contact your physician sooner rather than later to see if Tamiflu might be beneficial.  There are regimens available for treatment as well as prophylaxis following an exposure. 

REFERENCES: 

Paules, C., et al; NEJM November 29, 2017. DOI: 10.1056/NEJMp1714916

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