Flu-like illness in US reaches 2009 pandemic levels

The latest FluView surveillance numbers from the Centers for Disease Control and Prevention (CDC) show that outpatient influenza-like illness (ILI) activity has now reached 7.7% of all clinic visits, which matches the highest level during the 2009-10 H1N1 pandemic.

"This year is not a pandemic, but the high ILI is a sign of the severity of this year's flu season," said Anne Schuchat, MD, the acting director of the Centers for Disease Control and Prevention (CDC).

In addition to the high ILI, Schuchat said the CDC received notification of 10 new pediatric deaths, raising the total this season to 63.

"We wish there could have been better news this week," said Schuchat. "But this season continues to be challenging, and we may be on track to break records."

Schuchat made the remarks during a media briefing this morning. She said that questions of how much longer the season will last are still unanswerable: The nation is in week 11 of heightened flu activity, but some seasons last 20 weeks. By every major indicator used by the CDC, flu activity is rising and not falling.

Rising influenza B cases

The only possible good news, Schuchat said, was that the nation is seeing rising cases of influenza B. While influenza A strains still accounted for 73.3% of all samples tested, influenza B accounted for 26.7%, up from 19.8% in last week's report.

Schuchat said she hoped more influenza B means better protection from the seasonal flu vaccine.

US researchers have not yet projected vaccine effectiveness for this season, but Canadian researchers estimate the vaccine is about 55% effective against influenza B, a vast improvement over the 17% effectiveness estimated for influenza A, a finding that wasn't even statistically significant.

Of the influenza A strains tested last week, 13.5% were subtyped as A(H1N1)pmd09, 78.3% were H3N2, and 8.2% were not subtyped. Flu seasons dominated by H3N2 tend to be more severe and result in more hospitalizations. The CDC said there is no evidence of antigenic drift in this year’s H3N2 strain, and the explanation for this year's particularly high case numbers is still unknown.

Hospitalizations for flu increased to an overall rate of 59.9 per 100,000 population, a jump from the previous week's rate of 51.4 per 100,000 population. Adults 65 and older had the highest hospitalization rate, which is typical.

The hospitalization rate for those 65 and older was 263.6 per 100,000 population; the previous week's rate was 226.8 per 100,000 population. Adults aged 50 to 64 were hospitalized at a rate of 63.1 per 100,000 population, and children aged 4 years and younger at 40.0 per 100,000 population.

Flu levels high in 43 states

As with the previous week, 48 states reported widespread ILI activity, with only Oregon and Hawaii reporting regional activity. The number of states reporting high flu activity rose by 1, from 42 to 43.

Schuchat said West Coast states are seeing more influenza B. "It's common for there to be dips and then more cases when we see a new subtype," she said.

In past severe years, Schuchat said roughly 34 million Americans contracted the flu. At this point, she said if you suspect flu, you're likely correct.

"Most don't need to be tested or see a doctor," Schuchat said. "But the elderly, pregnant women, and other high-risk groups should be seen." Schuchat said those groups needn't wait for laboratory confirmation before being treated with an antiviral drug.

"We have good evidence that if you start taking antivirals within 48 hours of symptoms, you can lessen the severity of your illness," she said. "Patients may have to call more than one pharmacy, but antivirals should be used."

See also:

Feb 9 CDC FluView

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