As described in a recent editorial, influenza virus is elusive — for three reasons (Paules CI, Sullivan SG, Subbarao J, and Fauci AS. Chasing Seasonal Influenza — The Need for a Universal Influenza Vaccine. N Engl J Med. 2018 Jan 4;378(1):7-9.).
First: Every year the influenza virus hemagglutinin (the cell-attachment protein that sits on the surface of the virus) mutates so much so that immunization or natural infection the previous year doesn’t protect against disease the following year. For this reason, the Centers for Disease Control and Prevention (CDC) recommends that anyone more than 6 months of age receive a yearly influenza vaccine. The requirement of a yearly vaccine is difficult for patients and burdensome to the health system.
Second: Every year the Food and Drug Administration (FDA) must determine which strains of influenza to include in the vaccine. They do this by evaluating which strains are circulating in South America — generally a reliable predictor of the strains that will later enter the United States. But sometimes influenza viruses in South America mutate before entering the United States. This is what happened during the 2014-2015 season, when the influenza A (H3N2) strain in circulation didn’t match the strain in the vaccine. As a consequence, protection afforded by the vaccine was only 13 percent. This mismatch contributed to the severity of the infection as well as the high morbidity and mortality among people over 65 years of age.
Third: Influenza vaccine viruses can mutate while they are being propagated in eggs. This phenomenon isn’t observed when vaccine viruses are grown in mammalian cells or when they are made using recombinant DNA technology (i.e., FluBlok®). Some researchers believe that the capacity of influenza viruses to mutate (or drift) during manufacture accounted for the 43 percent efficacy rate of the vaccine during the 2016-2017 season.
The solution to these problems would be a universal flu vaccine — one that uses a conserved region of the hemagglutinin (or other influenza virus proteins) that is not subject to constant mutation. Researchers have been working on this idea for more than 40 years, so far without success.