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CDC Warns of Severe Flu Season

Kate Traynor

The federal government is stepping up efforts to promote influenza vaccination amid indications that influenza is hitting early and hard this season.

"This is probably the earliest season that we've had on record" since the mid-1970s, said Keiji Fukuda, chief of the influenza branch at the Centers for Disease Control and Prevention (CDC) Epidemiology and Surveillance Section, during a conference call for clinicians yesterday.

Texas and Colorado are current hotbeds of influenza activity in the United States, according to CDC. Agency Director Julie Gerberding said Monday that "a high proportion of flu this year" in the United States is caused by influenza type A H3N2 viruses, which are associated with a greater burden of illness and death than other influenza strains.

In Australia, where influenza activity is winding down, the flu season was "moderately severe to severe" and was characterized by the presence of H3N2 viruses, Fukuda said. This observation, along with data on current influenza activity in this country, forebodes that "we may see a more severe season on the United States than we have seen in the last few years," he said.

Fujian drift strain. Of special concern is the emergence of a "Fujian-like" influenza virus, a so-called drift strain that is similar to but genetically distinct from the H3N2 Panama strain included in this season's vaccine.

"A major characteristic of influenza viruses is that they tend to drift, or they tend to change and produce new strain variants," Fukuda said. "Right now, we are in the midst of a change from the older H3 viruses to the newer H3 viruses, which can be termed Fujian-like viruses. And typically what we might expect to see is that this group of viruses will become the dominant viruses both in the United States and worldwide for a couple of years, and then they'll be replaced by another virus."

The Fujian-like viruses emerged at the end of the 2002–03 influenza season. "They really appeared too late to be included in this year's vaccine, so that's why we have a mismatch this year," Fukuda said.

During the vaccine strain-selection process at the beginning of the year, an advisory committee for the Food and Drug Administration debated including a Fujian-like virus in the 2003–04 vaccine. The committee reluctantly concluded that, because no acceptable seed stocks of the variant were available for vaccine production, this season's vaccine should instead include the H3N2 Panama strain contained in the previous season's vaccine.

Fukuda said that, in past years when a mismatch has occurred between circulating and vaccine strains, the vaccine strain was "somewhere between 30 and 80 percent" effective at preventing influenza infection by the circulating virus.

"Antibodies produced to the Panama viruses—the virus which is contained in this year's vaccine—should provide some coverage level against the Fujian viruses," Fukuda predicted.

The push to vaccinate. With a potentially severe influenza season underway, Fukuda said that "it's absolutely critical for health care providers to really be pushing on their vaccination activities now and to continue them throughout the season or until the vaccine supplies run out."

Although much of the concern about influenza-related illness centers on the circulating H3N2 strain, Fukuda said that, as the season unfolds, the H3N2 strain could be overtaken by the other influenza type A virus or the type B virus that are contained in the current vaccine.

This year, CDC is emphasizing the need for health care workers to get vaccinated against influenza. Walter Orenstein, director of CDC's National Immunization Program, said during the conference call that influenza vaccine coverage for health care workers "has been frankly, abysmal—at about 36 2001."

Orenstein emphasized that health care workers can contract the flu from patients and pass the virus on to vulnerable persons—with potentially deadly results. About 36,000 people, on average, die from influenza-related complications each year in the United States, according to CDC data.

CDC recommends that health care workers receive the traditional inactivated influenza vaccine instead of FluMist, MedImmune Inc.'s live attenuated intranasal vaccine, because that product poses theoretical exposure risks for immunocompromised patients.

Vulnerable patients. As was the case in the southern hemisphere, children in the United States seem to be unusually at risk for complications of influenza. Fukuda said there have been "some reports of deaths and severe illnesses in children," particularly in Texas, but these reports are "somewhat anecdotal." Nonetheless, CDC is "keeping a watch" on the situation, he said.

"Kids are the first group of people who get affected when influenza viruses sweep into a community, and that seems to be what we're seeing now," Fukuda said. He added that he expects to see more reports of severe influenza and related complications among elderly people as the season progresses.

Fukuda urged clinicians to be alert for influenza-related neurological complications, such as encephalitis, or sudden death, and to report these instances to CDC.