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Rise of H3N2 Could Worsen This Year’s U.S. Flu Season

This winter’s flu season has come in like a lion, and is already neutralizing the effectiveness of some flu shots and could lead to a large increase in deaths and hospitalizations.

The Centers for Disease Control and Prevention reported in early December that an aggressive type of influenza is circulation and some strains aren’t covered by this year’s flu vaccine. The CDC report states flu viruses in the H3N2 family are proven to be the most prevalent type of viruses this season.

Recent history has proven that a rise of H3N2 strains leads to rise of influenza deaths. In the three deadliest flu seasons of the last decade, H3N2 strains were the most common forms of influenza.

“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” CDC director Dr. Tom Frieden said at a December press briefing. “We can save lives with a three-pronged effort to fight the flu: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home when you’re sick.”

The scariest news about the rise of H3N2 viruses this season is the limited effectiveness of vaccines. About 50 percent of the H3N2 viruses the CDC has studied are “drift” variants – strains different than those targeted by this year’s vaccine.

The problem: The World Health Organization’s list of targeted flu viruses this year went out in mid-February. But flu viruses continued to mutate, and the H3N2 drift variants didn’t appear until late March.

The United States saw a spike in H3N2 cases during the harsh 2007-08 flu season. That year, the CDC reported, the vaccine guarded 40 percent of those who got sick from H3N2.

Dr. Trevor Bedford of Fred Hutchinson Cancer Research Center called H3N2’s emergence just after the approval of this year vaccine a case of “bad luck and bad timing.” Bedford said H3N2 could make this year’s flu season especially deadly.

“This year may be more deadly for two reasons: more infections, and a more virulent strain,” Bedford said.

The trouble new strain of H3N2 is an influenza A strain, which generally produces more severe symptoms than an H1N1 strain – which was the cause of most flu cases during the 2013-14 flu season. A more transmissible virus and a less consistent vaccine could lead to more infections and possibly more deaths this season.

According to CDC data, an estimated 32,000 people died of the flu annually from 1997-2007.

“We know that in seasons when H3 viruses predominate, we tend to have seasons that are the worst flu years, with more hospitalizations from flu and more deaths from the flu,” Frieden said. “The rate of hospitalization and death can be twice as high or more in a flu season when H3 doesn’t predominate.”

The good news is flu activity (as of mid-December 2014) is currently low in the U.S. The World Health Organization reported on November 27 that global influenza activity remains low, with the exception of some Pacific Islands. Annually, the flu strikes about 15 percent of the population of the Northern Hemisphere and strikes greatest during autumn and winter.

But the CDC is still recommending that Americans, especially high risk patients including young children and people with chronic health conditions, get the flu vaccine this season.  And experts like Bedford say the fly vaccine – even this year’s weakened version – as a strong option to reduce your chances of acquiring the flu.

“The vaccine will almost certainly provide some protection [against the new H3N2 variants], which will help,” Bedford said. “Plus, you’ll get good protection against the other types of influenza. It just won’t be quite as effective as in other years.”

photo credit: lorenkerns via photopin cc

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