Rogue flu strain threatens high risk groups with severe infections

March 18th, 2015

An H3N2 A influenza strain not covered in the current vaccine is circulating in the U.S. and threatens high risk groups with severe infections, the Centers for Disease Control and Prevention reports.

The vaccine includes an H3N2 strain, but the stain of concern has antigenically “drifted,” meaning it has mutated enough to render the vaccine ineffective. That said, the CDC still recommends vaccination to cover for other circulating strains and to possibly minimize the severity of infections with the H3N2 strain. In addition to the mutated H3N2 strain there is another H3N2 A flu virus circulating that is well matched with the vaccine. These two strains, split in roughly equal proportions, comprise about 90% of the currently circulating flu virus in the U.S.

“We know that in seasons when H3 viruses predominate, we tend to have the worst flu years, with more hospitalizations and deaths,” CDC Director Tom Frieden, MD, said at a Dec. 4 press conference. “Unfortunately, about half of the H3N2 viruses that we've analyzed this season are different from the H3N2 virus that's included in this year's flu vaccine.”

Influenza antiviral medications may take on a more important role this year, and those at risk of severe complications of flu infection (e.g., those with asthma, diabetes, heart disease, lung disease, pregnancy) should be prepared to get them from their doctor if they acquire influenza. There are two FDA approved drugs recommended for use in the U.S. during this season, Oseltamivir and Zanamivir, Frieden said, noting that antiviral drugs work best when they are begun within 48 hours of illness onset.

“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.

Of course, flu is a particular threat to the elderly and frail and there have already been five pediatric deaths this flu season.

“We need to get the message out that treating early with the [antiviral] drugs makes the difference between a milder illness or a very severe illness,” he said. “Treatments with antiviral drugs for influenza can make your illness milder and shorter. It can reduce the likelihood you'll end up in a hospital or in intensive care, and we believe treatment with antiviral drugs can reduce the risk of dying from influenza.”

Antiviral drugs, however, are greatly under prescribed, particularly for people who are at very high risk of flu infection complications.

“Probably fewer than one in six people who are severely ill with the flu get antiviral drugs,” he said.