As the 2016-2017 flu season winds down in the Northern Hemisphere, let’s take a look at which virus strains had been predicted to be predominant this season and were included in the 2016-2017 influenza vaccine. Which strains were actually seen and how well did the vaccines do?
By late February, nearly 146 million doses of flu vaccine had been distributed in the U.S. – about average for the past few years -- and the Centers for Disease Control (CDC) reported that the overall risk of getting the flu had been reduced by about half in those that received the vaccine.
Flu viruses are constantly mutating, a phenomenon known as drift, or a gradual change, or shift, which is a less common but sudden and substantial change. The composition of flu vaccines is updated every year to help match as precisely as possible those viruses that could be circulating over the next season. Researchers base their predictions on strains that had dominated the previous flu season in Asia, particularly in the Southern Hemisphere.
Predicting which viruses will dominate in any given season is challenging if not impossible – there is no absolute certainty -- and researchers must make their best guess months in advance so that the vaccine can be produced and distributed before the season begins.
This season we saw that 70% of circulating strains were type A flu, of which 93% where H3N2. Of the 30% of B strains, 79% were of the Yamagata lineage. Among these strains 98% and 100% of the H3N2 and H1N1 were the same strain as found in the 2016-17 Northern Hemisphere vaccine. Of the B strains 91% and 100% of the Victoria lineage and Yamataga lineage were the same strain as found in the 2016-17 Northern Hemisphere vaccine. Overall, the World Health Organization (WHO) and CDC made nearly perfect picks this year, which is not always the case.
For this Northern Hemisphere winter, trivalent vaccines were designed to protect against Type A strains H1N1 and H3N2 plus one influenza B strain, while quadrivalent vaccines contain the two A strains as well as two influenza B viruses, all those anticipated to be in circulation.
An additional challenge is that flu viruses can change not only from one season to the next but also right before a season begins or even once it is underway. This means there is always the possibility of a less than optimal match between circulating viruses and those in the vaccine. CDC studies influenza virus samples throughout the season to help determine how close a match was achieved.
The U.S. Flu Vaccine Effectiveness (VE) Network estimated this year’s flu vaccine would be 48% effective in preventing medically-attended influenza A and B illness infection – that meant 43% effectiveness for the predominant H3N2 and 73% for the influenza B viruses. VE can also vary among different age and risk groups and even by vaccine type. When vaccines are less than an optimal match, VE is reduced, of course, but, still, antibodies made in response to vaccination with one flu virus can help provide limited protection against different but related viruses.
On another note, CDC recommended only injectable flu shots for this season -- the injectable flu shot (inactivated influenza vaccine or IIV) and the recombinant influenza vaccine (RIV). Not recommended was the nasal spray flu vaccine (live attenuated influenza vaccine or LAIV) because of ongoing concerns about effectiveness.