It turns out the rumors were true: This year’s flu shot is indeed less effective than usual.
An unusually resilient strain of influenza called H3N2 has been the predominant assailant this season, and the vaccine rolled out last year was ill-suited to protect against it. While previous analyses from Canada and Australia on its H3N2 effectiveness lent some non-significant support to suspicions about the shot, a new report from the Centers for Disease Control and Prevention laid all doubt to rest: This season’s vaccine reduced illness caused by H3N2 by only 25 percent.
Still, it’s better to get a shot because it offers some protection, especially against other strains that are just starting to emerge. The vaccine was particularly effective for children ages six months to eight years, reducing their risk of contracting H3N2 by more than half. Overall, however, this season’s vaccine is more than 40 percentage points less effective against H3N2 than it is for H1N1, another flu strain that is also spreading.
Weaker than usual vaccine protection is among the reasons this season features record levels of flu. The most recent data show the amount of influenza-like illness currently being reported matches the peak of the 2009 swine flu epidemic.
There were 40,414 deaths in the U.S. during the third week of 2018, the most recent data available, and 4,064 were from pneumonia or influenza, according to the CDC data. The number for that week is expected to rise as more reports are sent to the agency. The death toll in future weeks is expected to grow even higher because flu activity is still rising-and the number of deaths follow the flu activity. Hospitalization rates are already approaching total numbers usually seen at the end of the flu season-which may not be for months.
Deaths caused by the flu and pneumonia were responsible for 1 out of every 10 deaths at the beginning of February. The number of deaths from this year’s outbreak will likely far outpace those from the 2009 season.
There is a reason why it is more difficult to inoculate against H3N2. Flu shots usually include inactivated flu viruses grown in chicken eggs. The H3N2 virus has circulated since 1968 and is well-adapted to human beings. So when it’s grown inside a chicken egg, it mutates in a way better suited to that environment, but is less likely to create the same reaction in people.
Last year’s flu vaccine had about 30 percent effectiveness against H3N2 in the U.S.
While the vaccine’s ineffectiveness has filled emergency rooms and burdened the health-care system, it hasn’t damaged profits across the pharmaceutical industry. Vaccine makers, hospitals and drug distributors are among those that have seen the epidemic boost revenues.