This post has been updated.
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.
