PITTSBURGH (KDKA) — The Centers For Disease Control reports, overall, the flu vaccine is about 48 percent effective this year, where typically, the vaccine is 60 percent effective.

So why the difference? The recurrence of the H3 flu strain. Experts say that strain mutates a lot, and we don’t have as good immunity.

Dr. Richard Zimmerman, Director of the University of Pittsburgh flu and vaccine research team said the last time we saw the H3 strain in Pittsburgh was 2014-2015, the year of the vaccine mismatch.

“The vaccine worked if the strain hadn’t changed,” Dr. Zimmerman said.

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The good news is, in Allegheny County, we are seeing mostly the B strain of the virus, and against that strain, Dr. Zimmerman said the vaccine works great.

But we are seeing a second peak. After seeing a spike of flu cases in early January, we are starting to another increase approaching now.

UPMC had 81 confirmed flu cases in UPMC facilities as of Wednesday, compared to 67 the week before.

“They likely are issues dealing with humidity and weather. We don’t fully understand it. But the transmission can be different and obviously people crowd in when the weather is bad outside and so there can be both the crowding issues as well as humidity changes,” Dr. Zimmerman said.

There have been six flu deaths in Allegheny County this year.

“Flu attacks the pulmonary system, so people with underlying lung disease or people with underlying heart disease, obviously your heart needs the oxygen from your lungs, those two groups are specially at risk,”  Dr. Zimmerman said.

If you feel the flu coming on, Dr. Zimmerman said Tamiflu can shorten the duration of symptoms, but only when taken within 72 hours.