PHILADELPHIA (WPVI) -- There's new disappointment on this season's flu vaccine.
Federal health officials say it is only 23% effective in protecting people from the H3N2 strain - the dominant strain in the current outbreak.
And it performs even worse in some age groups.
Age 6 mo to 17 years: 26%
Age 18 to 49: 12%
Age 50+: 14%
The Centers for Disease Control and Prevention says the H3N2 strain "drifted," or changed slightly from what it was when the vaccine was formulated in March.
So it is not a good match for the vaccine, leaving more people vulnerable to the flu.
Infectious disease experts do say it can, however, lessen the severity of those who do catch this strain, and that will prevent hospitalizations and even deaths.
And it can protect in the event there's a repeat of the 2013-2014 flu season, and a second strain - usually a B strain - circulates in late winter or early spring.