CDC: Flu Hitting Harder, Too Early to Tell If It's Peaking

January 11, 2013

Influenza is hitting the nation harder than it did at this point last year, but it could be peaking in some parts of the country, the Centers for Disease Control and Prevention (CDC) announced today.

CDC Director Thomas Frieden, MD, MPH, said his agency will not be able to pin down the trend for sure until it gathers a few more weeks of data. Meanwhile, Americans who have not yet gotten vaccinated against seasonal influenza should do so.

"Better late than never," said Dr. Frieden at a press conference today.

And the unvaccinated might want to get their dose sooner than later: The CDC stated in the latest issue of the Morbidity and Mortality Weekly Report that influenza vaccine makers were expected to supply the US market with some 135 million doses. As of January 4, roughly 128 million doses had been distributed. Given reports of spot shortages, vaccine seekers may have to call more than a single healthcare provider to receive their dose.

That same report puts the effectiveness of the current seasonal influenza vaccine at 62%, which means that 38% of Americans who get vaccinated could still contract influenza. Dr. Frieden said those are still good odds, even though he wants to see influenza vaccine effectiveness someday match that of childhood vaccines, which top 90%.

"The flu vaccine is far from perfect, but it is the best tool by far for preventing the flu," he said. "It's well worth the effort."

Joseph Bresee, MD, who heads the epidemiology and prevention branch of the CDC's influenza division, noted during the press conference that the vaccine effectiveness rate of 62% resembles those for previous years. Rates of 50% to 70% represent a "reasonable range." The seasonal influenza vaccine for 2011-2012 was less than 60% effective, Dr. Bresee said.

Fear of "Bad Year" Becoming a Reality

In early December 2012, Dr. Frieden said that given the early onset of the 2012-2013 influenza season and the particular mix of viruses in circulation, "this could be a bad year." The ensuing weeks have proved him right. Patients presenting with influenza-like symptoms have flooded hospital emergency departments in the Northeast, for example — so much so that the mayor of Boston, Massachusetts, declared a public health emergency on January 9.

The CDC's latest weekly surveillance report of influenza activity, covering the first week of 2013 and ending January 5, continues to support the "bad year" outlook. According to the report, called FluView , 47 states reported widespread geographic influenza activity, which is 6 more than the week before. In comparison, no state reported widespread geographic influenza activity for the first week of 2012.

In addition, the percentage of deaths attributed to influenza and pneumonia in 122 benchmark cities rose to 7.3% as of January 5, 2013, crossing the epidemic threshold of 7.2%. That grim statistic encompasses the deaths of 20 children so far in this influenza season.

Another key indicator, the proportion of outpatient visits for influenza-like illness (ILI), told a different story. The ILI rate at the end of the first week of 2013 stood at 4.3%, which is above the national baseline of 2.4% and well above the 1.4% for the same week in 2012. However, the latest ILI rate is down from the 5.6% posted for the week before, which ended December 29, 2012. The number of states reporting high levels of ILI decreased from 29 to 24.

Dr. Frieden said the ILI trend suggests that influenza is peaking or waning in some states. However, the findings for the last week of 2012 preclude certainty because they can represent an artificial holiday spike. Americans make fewer trips to physician offices in December for routine matters such as check-ups, so the patients who do end up in the waiting room "have a different pattern of illness." As a result, the ILI rate can swell up.

It will take a few more weeks of influenza surveillance and more data points to confirm the trend, Dr. Frieden said.

Predominant A(H3N2) Virus Associated With More Severe Flu Seasons

Of the influenza viruses in circulation as of January 5, the most common is A(H3N2), which accounts for 42.2% of all influenza-positive respiratory specimens. Influenza seasons tend to be more severe when this virus predominates, according to the CDC. Another 36.7% of specimens test out as influenza A of unknown subtype, unknown because subtyping was not performed. The next most prevalent culprit is some subtype of the influenza B virus at 20.2%.

The once dreaded pandemic influenza A(H1N1) virus from 2009, now classified as part of the seasonal virus mix, has shown up in less than 1% of specimens.

"We're not seeing much of the (pandemic) virus yet, though Europe and other places in the world are," said Dr. Bresee at today's briefing. "We know that influenza viruses in a given country, city, or region will vary from year to year in unpredictable ways. It doesn't mean we'll see less of it all during the season, and it certainly doesn't mean it vanished."

Arnold Monte, MD, an epidemiologist at the University of Michigan in Ann Arbor, said the pandemic influenza A(H1N1) virus may not be a factor this year because much of the population probably has gained an extensive immunity against it. The trivalent seasonal flu vaccine has included the pandemic virus since the 2010-2011 season. The current vaccine also guards against influenza A(H3N2) and influenza B.

The United States has not experienced a major A(H3N2) outbreak since the 2007-2008 season, Dr. Monto told Medscape Medical News. "This may explain why this year's spread is so extensive, without a big change in the circulating virus."