CDC: Dominance of H3N2 viruses may portend more deaths this flu season

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The director of the Centers for Disease Control and Prevention has a warning for Americans: Get ready for a potentially nasty flu season.

So far this flu season, the influenza strains most active in the U.S. are versions of H3N2 viruses, Dr. Tom Frieden said Thursday.

“We know that in seasons when H3 viruses predominant, we tend to have seasons that are the worst flu years, with more hospitalizations from flu and more deaths from the flu,” Frieden said.


A total of 103 flu samples have been analyzed in laboratories since Oct. 1, and 82% of them were of the H3N2 variety, according to data from the CDC’s influenza surveillance report for the week ending Nov. 22, the most recent report available.

Making matters worse, this year’s flu vaccines are a good match for only 48% of these these H3N2 strains. That means the vaccine – though still the best defense against the flu – is likely to offer less protection than in years past, Frieden said.

The flu vaccines in the U.S. are designed to prime the body’s immune system to fight three or four strains of influenza. This year’s vaccines protect against two types of influenza A viruses – one H1N1 and one H3N2 – as well as one or two influenza B viruses.

The CDC’s FluView report says that only one type of H1N1 virus has been tested since Oct. 1. It was a good match with the A/California/7/2009 strain in this year’s flu.

An additional 17 influenza B viruses have been analyzed, and they are good matches as well. Ten of them were of the Yamagata/16/88 lineage that’s in all North American vaccines, and the seven others were of the B/Victoria/02/87 lineage that’s in the quadrivalent versions of the vaccines.

But most of the flu strains subjected to laboratory testing have been of the H3N2 variety, according to the CDC. And slightly fewer than half matched the A/Texas/50/2012 strain that’s in the vaccines given in the U.S.


Most of the rest of the H3N2 samples were similar to a strain called A/Switzerland/9715293, which was picked for the flu vaccine for the Southern Hemisphere, according to the FluView report.

Flu watchers first detected these strains in the U.S. in March. By then, “it was already too late to include them in this season’s vaccine,” Frieden said.

It’s not clear that vaccine makers would have made the switch at that point, even if it were possible. Back then, the A/Texas strains were “still by far the most common of the H3N2 viruses,” he noted. The A/Switzerland strains appeared in “significant numbers” only in September, he said.

The dominance of H3N2 viruses and the mismatch with the vaccine mean that doctors and patients should be vigilant this flu season, Frieden said. That’s especially true for people who are more vulnerable to influenza because they have lung diseases, asthma, heart disease or diabetes or are pregnant. Five children have died of the flu this year, he said.

Antiviral medications like Tamiflu and Relenza can make the flu “milder and shorter,” and “reduce the likelihood that you’ll end up in a hospital or in intensive care,” Frieden said. “We believe treatment with antiviral drugs can reduce the risk of dying from influenza.”

Yet only 1 in 6 patients who really need the drugs gets them, Frieden said.

“We strongly recommend that if doctors suspect the flu in someone who may be severely ill from the flu, they don’t wait for the results of a flu test before starting antivirals,” Frieden said.


But these drugs are only the second line of defense against influenza. The first, Frieden said, is the flu vaccine.

“Though far from perfect, it still offers us the best chance for prevention,” he said.

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