Producing flu shots is a long, complex process. The U.S. Food and Drug Administration meets every February to choose what to put into the year’s flu shot based on recommendations from the World Health Organization. They include three or four strains, since multiple viral strains can circulate throughout the season.
The FDA may pick a good match, but the influenza virus often mutates, sometimes so much that the vaccine elicits immune responses that do not recognize it and therefore struggle to fight it.
How much protection the influenza vaccine provides overall varies every year. Over the past 10 years, at best it was 60 percent effective and at worst was about 20 percent effective, according to CDC data.
Still, the CDC recommends getting vaccinated, even at this point in the year, because some protection is better than none. The agency expects the flu season to last for several more weeks.