This season's flu shot may offer limited protection since two of the four strains included in the vaccine do not match the most dominant strains seen in the Southern Hemisphere's flu season, which ended in September, according to STAT.
Flu experts analyze global data twice a year to predict which flu strains should be included in next season's vaccines for the Northern and Southern Hemispheres. Officials met in late September and determined that the H3N2 and B/Victoria components of the vaccine should be changed for next year to better protect against the dominant strains seen in the Southern Hemisphere's winter. The Northern Hemisphere's current vaccine does not contain the dominant strains experienced in the southern winter, suggesting that the current vaccine may not provide optimal flu protection this season.
Health experts delayed issuing a recommendation for what virus strains to include in this season's vaccine to conduct more research on H3N2 strains. The H3N2 strain they selected differs from the one that was most dominant the Southern Hemisphere this season. Therefore, the strain is more likely to cause the flu this winter than it was last season, Scott Hensley, PhD, associate professor of microbiology at the University of Pennsylvania in Philadelphia, told STAT.
While the best flu vaccine composition is difficult to predict, Mr. Hensley said that flu vaccines "often protect against severe disease even when … mismatched."