This season’s flu vaccine offers limited protection against the viruses sweeping the country, with its overall effectiveness of 36 percent falling to 25 percent against the most virulent and predominant strain, according to a government report released Thursday.
The report from the Centers for Disease Control and Prevention on the vaccine’s midseason effectiveness confirms what federal health officials and infectious-disease experts have suspected for some time. In unrelentingly bad flu seasons such as the current one, which is dominated by the most dreaded flu strain, vaccines are less effective. That’s because that strain, the influenza A virus known as H3N2, can change more rapidly than any other flu viruses transmitted among humans, allowing it to evade the body’s immune system.
Still, experts said the current vaccine offers some protection against H3N2. Its 25 percent effectiveness rate means that one in four people who get the shot reduce their risk of becoming sick enough to need to see a doctor. In Australia, interim estimates showed the vaccine to be only 10 percent effective against H3N2, and researchers recently reported that early data show that the vaccine is about 17 percent effective in Canada against that strain. The vaccine’s effectiveness may have changed as it is measured at the end of the season.
Seasons when this flu bug is dominant are associated with more severe complications, especially for people older than 65, young children and others with certain chronic conditions. The number of people going to doctors’ offices and emergency rooms has surged to levels not reported since the peak of the 2009 swine flu pandemic. Hospitalizations are also at record levels.
Each year, health officials must choose the three or four influenza strains that vaccine makers should target for an upcoming season. The decision is made months in advance, when it is hard to know what strains will be circulating.