The effectiveness of the flu vaccine can be directly linked to the number of hospitalizations and sometimes the number of deaths during the flu season.
The Associated Press reported that this season’s flu vaccine has proven particularly ineffective against a certain strain.
The vaccine is only 25 percent effective against the H3N2 strain, which is causing most of the flu cases across the country and the state. The vaccine also carries other strains, and the AP reported that the trivalent vaccine is 67 percent effective against H1N1 (swine flu) and 42 percent effective against Type B (Victoria lineage).
The quadrivalent vaccine included those three strains and a Type B (Yamagata lineage) strain, about which the AP did not have statistics.
With the lowered protection against one of the strains, the total effectiveness of the vaccine is at 36 percent.
That number is lower than in previous years when vaccines tend to be around 40 to 60 percent effective.
The vaccine was 40 percent effective in 2016-17, 48 percent effective in 2015-16, 53 percent effective in 2013-14 and 49 percent effective in 2012-13.
However, in 2014-15, the flu vaccine was only 19 percent effective, which caused a spike in hospitalizations and mortality, especially among those 65 and older. In that year, according to the CDC, H3N2 was also the reason the vaccine was ineffective, with the virus having drifted differently than what was grown for the vaccine.
In 2014-15, flu-associated deaths made up 10.8 percent of the deaths in Pennsylvania when it peaked in the first week of January, compared to the peak rate of 8 to 9 percent.
Across the country, flu-associated deaths made up 12.2 percent of all deaths in the first week of January 2015 and stayed above 9 percent until mid-March among those 65 and older, according to the CDC. Most seasons peak at 9 or 10 percent mortality rate for seniors.