Most Influenza Strains Do Not Match Current Vaccine

from WebMD — a health information Web site for patients

Daniel J. DeNoon

February 11, 2008

February 11, 2008 — As the U.S. flu season soars, the CDC says this year's flu vaccine doesn't match two of the three main types of flu bugs now in circulation.

That doesn't mean the vaccine won't work. Vaccinated people who catch the flu get a much milder disease. This can make a life-or-death difference to people who are at high risk of flu complications, such as the elderly.

Flu season is officially under way. No U.S. state is flu free, says Joe Bresee, MD, the CDC's chief flu tracker. As of Feb. 2, flu was widespread in 31 states, regional in 17 states, and local in the two remaining states and in the District of Columbia.

"I don't think we are peaking yet," Bresee tells WebMD. "If you look at hospitalizations or outpatient illnesses or mortality, everything still has an upward slope. I wouldn't be surprised if we continue to trend upward for the next few weeks."

Will this be an especially bad flu season? It's too soon to tell. So far, the surge in flu cases is typical for this time of year.

But there's an ominous finding coming from flu surveillance labs. The predominant type A flu virus this year is the H3N2 strain; 87% are the "Brisbane" strain. And 93% of this year's type B flu bugs are from the "Yamagata" lineage.

The current flu vaccine's H3N2 component is the "Wisconsin" strain; the type B component is from the "Victoria" lineage. That means reduced protection against the Brisbane and Yamagata bugs. Last year, when the Brisbane strain was predominant in Europe, the "Wisconsin" vaccine was only 52% protective against infection.

"Protection may not be optimal, but flu vaccination can still protect enough to make illness mild and prevent complications," Bresee says. "While vaccination is the best way to prevent flu, good hand hygiene and flu etiquette are also effective in preventing flu transmission."

Since the 2003-2004 flu season, when news reports called attention to child flu deaths, the CDC has tracked the number of children under age 18 who die from flu complications. Since then, the annual number of confirmed child flu deaths has ranged from 44 to 153.

So far, there has been one confirmed child flu death this year, but Bresee says the CDC received unconfirmed reports of five more child flu deaths in the week ending Feb. 2.

Last year, staph infections -- especially the scary, drug-resistant MRSA bug -- complicated many fatal child flu infections. Bresee says it's too soon to know what role MRSA will play in this year's flu season.

Also alarming is that a small but significant percentage of this year's type A flu bugs have become resistant to one of the two flu drugs. About 4.5% of circulating flu bugs -- all of them the type A H1N1 strain against which the flu vaccine remains fully effective -- have developed Tamiflu resistance.

There's been no resistance to Relenza, the other major flu drug. Both Relenza and Tamiflu can shorten flu duration if taken soon after a person develops flu symptoms.

SOURCES: Joe Bresee, MD, chief, epidemiology and prevention branch, CDC Influenza Division, Atlanta. CDC news conference. Feb. 8, 2008. CDC web site.

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