What Makes This Flu Season So Bad?

Out of every 100,000 hospitalizations in the U.S., 22.7 were for the flu in the first week in January. According to the CDC, the number had doubled from the week before. During the severe flu season that ended in 2015, rates of hospitalizations reached 29.9 for every 100,000.

The winter season that began in 2017 and will end in 2018 “unquestionably falls into a bad year,” says Anthony Fauci, director of the National Institutes of Health National Institute of Allergy and Infectious Disease. First is the nature of the most dominant flu strain infecting people – H3N2. Flu viruses mutate every year, and it’s common to see several strains of flu during the same season. A similar strain called H1N1 was responsible for the 1918 flu pandemic, and the 2009 “swine flu” outbreak. “H3N2 is historically the bad actor among influenzas,” he says. “It’s also associated with complications.”

The second reason H3N2 has been so pervasive, says Fauci, is people have less exposure to it. When the same flu strain strikes repeatedly, people and thus regions tend to build up immunity.

The third reason this year’s flu has been so bad is complications with the vaccine. Most influenza vaccines are grown in chicken eggs, and when this year’s vaccine was being incubated, the virus mutated while it was growing and became less effective. Scientists think it may only be about 30% effective against H3N2. In Australia, the vaccine was only 10% effective.

At best, says Fauci, flu vaccines are only ever about 60% effective. While that makes it a least 60% more effective than not getting a vaccine at all, it still means the rapidly mutating virus has a fighting chance. Every year, the World Health Organization (WHO) meets to determine what specific flu strains should receive vaccines in the northern hemisphere. H3N2 was one of several identified as a threat.

There are four different types of influenza virus, three of which infect people. Of those three, influenza A and B are the most common and each of those subsets develops different strains. H1N1 and H3N2, for example, are strains of influenza A, and they adapt by constantly changing their surface proteins.