Flu season gets a late start

February was the first time more than 10% of respiratory specimens tested positive for influenza, indicating the beginning of flu season, the CDC says.

By — Posted Feb. 27, 2012

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Although the influenza season is off to the slowest start in nearly three decades, physicians should continue administering flu vaccine to unimmunized patients to protect them against a possible uptick in illness in the next few weeks, infectious diseases experts say.

Flu intensity for the 2011-12 season is low, "but the risk for infection is still there. The viruses are spreading," said W. Paul Glezen, MD, professor of molecular virology and microbiology at Baylor College of Medicine in Houston. "The good news is there's still time to get vaccinated."

Surveillance data from Jan. 29 through Feb. 4 show that, for the first time this season, 10.5% of respiratory specimens tested positive for influenza, according to the Centers for Disease Control and Prevention. The figure was 7.6% the previous week.

Surpassing 10% generally indicates that influenza season is beginning, the CDC said. In the past 29 years, there was only one period -- 1987-88 -- when flu season started this late, the agency said.

The season can begin as early as October and last as late as May, but cases of the illness typically peak in February, according to the CDC. The CDC recommends that all people 6 months and older be immunized against influenza.

Contributing to the late start in 2011-12: Circulating virus strains are very similar to those present during 2010-11, Dr. Glezen said. As a result, a good proportion of the population has some level of immunity to the viruses, which slows their spread, he said.

Most of the season's infections have involved influenza A (H3N2), the CDC said. Researchers also have identified illness due to 2009 H1N1 and influenza B viruses.

Fortunately, the seasonal flu vaccine appears to be a good match to a majority of the circulating strains, said Lynnette Brammer, MPH, an epidemiologist with the CDC's influenza division.

The immunization contains the pandemic 2009 influenza A(H1N1)-like virus, an A/Perth/16/2009 (H3N2)-like virus and a B/Brisbane/60/2008-like virus. These are the same viruses that were included in the 2010-11 vaccine.

Also likely stalling the spread of influenza across the country is the record number of flu vaccine doses available this season, Dr. Glezen said. About 132 million doses were distributed as of Feb. 3, according to the CDC's most recent data.

Possible increases of the flu

Despite the late start to the season, cases of the illness have begun to increase. The season's first report of widespread geographic influenza activity came from California during the week that ended Feb. 4, the CDC said. The agency projects there will be further increases in flu activity in the coming weeks.

But experts hesitate to make any specific predictions about how the remainder of the season will progress because of the unpredictable nature of influenza.

"A hallmark of flu virus is that the virus is constantly changing. ... That's why we do surveillance," Brammer said. "We don't know what it's going to do, so we watch it."

The CDC is keeping an eye on the swine-origin influenza A(H3N2)v virus that infected 12 people, most of whom were children, between Aug. 17 and Dec. 23, 2011. The cases were reported in Indiana, Iowa, Maine, Pennsylvania and West Virginia. Everyone who was infected has recovered, the CDC said.

Epidemiologists also are watching Mexico, where they say the influenza A(H1N1) virus has been active this season. However, they said they are not overly concerned.

"That virus is what caused the pandemic in 2009," Brammer said. "But since then, that virus has basically become one of our regular seasonal influenza viruses."

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External links

Situation Update: Summary of Weekly FluView, Centers for Disease Control and Prevention, Feb. 17 (link)

FluView, Centers for Disease Control and Prevention, Feb. 17 (link)

"Update: Influenza A (H3N2) v Transmission and Guidelines -- Five States, 2011," Morbidity and Mortality Weekly Report, Jan. 6 (link)

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