Type B viruses rise as overall flu activity declines

Mar 23, 2009 (CIDRAP News) – The second week of March brought a slight decline in influenza activity across the country, with 30 states still reporting widespread cases, down from 35 states a week earlier, according to the Centers for Disease Control and Prevention (CDC).

As overall activity has decreased, however, the season has seen an increase in the proportion of influenza B viruses, the one type of flu that is not very well-matched by this year's vaccine, the CDC reported. The spread of B viruses also poses a challenge for decisions about antiviral treatment, the agency said.

Eighteen states reported regional flu activity, and two—Utah and West Virginia—reported only local activity. Seven percent of all deaths reported through the CDC's mortality reporting system were attributed to pneumonia and influenza, which is below the epidemic threshold of 8%.

However, the proportion of medical visits attributed to flu-like illness in the CDC's flu surveillance network was 2.9%, still above the national baseline of 2.4%.

Six more flu-related deaths in children were reported, bringing the total for the season to 32, the CDC report said. That compares with 88 fatal cases in children in 2007-08 and 78 in 2006-07. The six deaths occurred between Feb 15 and Mar 7.

Influenza A viruses have predominated through the season overall, making up 70.8% of those that have been typed, the CDC reported. But type B viruses have increased lately: of 1,102 lab-confirmed flu cases for the week, 580 (52.6%) were type B.

Of 210 type B viruses that have been antigenically tested by the CDC this season, only 44 belonged to the Yamagata lineage targeted by the B strain in this year's vaccine. The other 166 isolates belonged to the Victoria lineage and are not related to the vaccine, the agency said. However, all the A/H1N1 and A/H3N2 isolates tested so far have been related to the corresponding strains in the vaccine.

Among type A viruses, H1N1 has been predominant this season. That pattern continued in the latest report, with 107 of 120 type A isolates that were subtyped identified as H1N1. Nearly all H1N1 isolates (98.9%) tested for drug resistance this season have been resistant to the antiviral oseltamivir (Tamiflu) but sensitive to zanamivir and to the adamantine drugs.

In contrast, all A/H3N2 viruses have been susceptible to oseltamivir and zanamivir but resistant to the adamantanes. Type B viruses are also susceptible to oseltamivir and zanamivir, but the adamantanes are not effective against them.

Given the differences in drug sensitivity among the three types of flu, the growing proportion of B viruses "presents challenges for the selection of antiviral medications for the treatment and prophylaxis of influenza," the CDC report said. "Health care providers should be aware of the possibility of increased influenza B circulation in their area, and continue [to] test patients for influenza and consult local surveillance data when evaluating patients with acute respiratory infections during the influenza season."

The CDC revised its recommendations on antiviral treatment for flu last December, after discovering the high rate of oseltamivir resistance in H1N1 viruses.

In other developments, the Rapid City Journal in South Dakota today reported a flood of patients with flu-like symptoms in local clinics and said many of them had received flu shots early in the season.

A physician's assistant at an urgent care center said about half of her patients who tested positive for flu had been vaccinated, but most of the shots were early in the season, the report said. A physician quoted in the story suggested that the immunity conferred by vaccination starts to wane after about 3 months, though it can still lessen the severity of flu after that.

However, online CDC information says the protection provided by a flu vaccine lasts the whole season. Researchers have found no benefit from getting a second shot later in the season, it says.

William Schaffner, MD, a flu immunization expert and chair of the Department of Preventive Medicine at Vanderbilt University School of Medicine in Nashville, agreed.

"Immunization even in September ought to have you protected throughout the flu season," he told CIDRAP News. "What we're seeing is a very late influenza season. We know that the vaccine is a pretty good vaccine but not a perfect one. So some people will acquire a milder case despite vaccination. Because it's occurring so late in the season, the question arises about the duration of protection."

If flu activity had peaked in December or January, there would have been cases in people who had been vaccinated and were only partially protected, he said. But in that case the question of duration wouldn't have come up, simply because of the date.

Schaffner also allowed that the recent increase in type B viruses, many of which are not well-matched by the vaccine, may be playing a role in cases in vaccinated people. "A late season flu tends to be Bs, and indeed we only have one [B] strain in the vaccine. So the vaccine once again may provide only partial protection," he said.

He noted that there has been considerable discussion of putting both type B lineages in the flu vaccine, making a quadrivalent (four-strain) vaccine. "I'm in favor of that," he said.

See also:

CDC flu update

Dec 19, 2008, CIDRAP News story "With H1N1 resistance, CDC changes advice on flu drugs"

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