February 16, 2008

CDC Reports Flu Vaccine Less Effective This Season

The U.S. Centers for Disease Control and Prevention (CDC) reported Friday that this year's flu vaccine is only effective for about 40 percent of flu viruses, and flu season is rapidly worsening this year as a result.  

Typically, in a good year, the flu vaccine is effective against 70 to 90 percent of the flu viruses. 

Dr. Joe Bresee, Branch Chief of Epidemiology of the CDC's Influenza Division said that flu infections from an unexpected strain have been thriving this year, and now account for most lab-confirmed flu cases.

"Every area of the country is experiencing lots of flu right now," Bresee said.  "This week 44 states are reporting widespread influenza activity.  This is up from 31 states reporting widespread activity last week.  Five states are reporting regional activity, making 49 states overall in one of our top two categories of activity," he explained in a conference call Friday.

Those numbers aren't considered alarming, and Bresee said it is too soon to know whether this year will prove to be a particularly bad flu season overall.

Early February is the time of year when flu cases tend to peak. Ten pediatric deaths have occurred, but are about the same number as was reported at this time in the last two flu seasons, Bresee said.

He said the biggest surprise was how poorly the vaccine has performed.

Each winter, influenza experts try to predict which strains of flu will circulate so they can develop an effective vaccine for the following season. They choose three strains- two from the Type A family of influenza, and one from Type B.
Usually this approach produces good results, and indeed the vaccines have been a match in 16 of the last 19 flu seasons, Bresee explained. 

However, this year the vaccine's Type B component turned out not to be a good match for the B virus that has been most common this season.   And one of the Type A components turned out to be ill suited for the Type A H3N2/Brisbane-like strain that now accounts for the largest portion of lab-confirmed cases.

Over the years, the H3N2 flu has tended to cause more deaths, Bresee said.

H3N2 strains are treatable by Tamiflu and other antiviral drugs, but the other H1N1 Type A strains are more resistant.   In fact, 4.6 percent have been resistant to antiviral medications, a substantial increase from last year's rate of less than 1 percent.

"This represents a real increase in resistance," Bresee said.

The World Health Organization (WHO) recommended this week that next year's flu vaccine contain a completely different makeup, a decision the U.S. Food & Drug Administration will make next week.

For now, Bresee advises people to take steps in addition to vaccination that will reduce the likelihood of catching the flu. 

"We'd like to remind folks that other methods are available to prevent or treat the influenza, including protective "“ everyday protective actions like staying home from work when you're sick and use of "“ appropriate use of antiviral medications," said Bresee.

Even with the reduced effectiveness of this year's vaccine, CDC officials said they expect some effectiveness. For example, with partial protection, the vaccine may be protect against hospitalizations, or against death, so it's still important to get vaccinated even in years where there is a less than ideal match against the circulating strains, particularly in patients who are more likely to get severely ill with flu.

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On the Net:

The CDC's flu season update

The World Health Organization (WHO)

U.S. Food & Drug Administration