“So far this season, influenza A -- H3N2 viruses -- have been detected most frequently, and in almost all states,” said CDC Director Thomas Frieden, MD, MPH, in a telebriefing with reporters.
“Unfortunately about half of the viruses in cases that we’ve identified this season are different than the ones that are included in this year’s vaccine. The vaccine won’t protect against these viruses.”
Each year, several months before flu season starts, scientists and vaccine manufacturers from around the globe make an educated guess about which strains of flu will be most common in the coming year, giving them time to make the vaccines before the season begins.
But in some cases, these guesses can be wrong, or viruses can mutate while the vaccine is being made. Even in a “well-matched” flu vaccine year, the vaccine is only 60%-90% effective.
“The flu virus can be unpredictable, and what we’ve seen so far this year is concerning,” Frieden said.
Because vaccines take at least 4 months to make, there is little that can be done to adjust this year’s vaccine.
William Schaffner, MD, of the Vanderbilt University School of Medicine, says every flu season brings more than one strain of the virus.
“Although we have a slight mismatch with one strain, which I have to admit is so far the dominant strain, there’s a good match with the others,” Schaffner says.
Flu Hot Spots Cropping Up
There have been over 1,200 confirmed flu cases and 5 deaths, with the most cases being in Alaska, the Pacific Northwest, and the southern central parts of the United States.
Flu seasons where H3 viruses, including this year’s H3N2, are the predominant flu strain tend to be worse, with longer-lasting and more severe illnesses, leading to more hospitalizations and more deaths, Frieden said.
And, he said that of the deaths in children typically seen from the flu, about 90% of those children did not get the vaccine.
Despite a poorly matched vaccine, Frieden says the CDC is still recommending that people get this year’s flu shot.
“In situations such as this, we continue to recommend the vaccine, because although it’s far from perfect, it still offers us the best chance at prevention,” he said.
“We always knew the influenza vaccine is an imperfect vaccine, but there is no doubt that people who have not been vaccinated should get vaccinated,” Schaffner says.
“There are other strains around,” Schaffner says. “It is really quite possible that even though the match is imperfect, people will get partial protection to make a more serious disease less serious.”
In addition to the vaccine, Frieden emphasized general protective measures: washing your hands, covering your coughs, and staying home when you're sick. He also highlighted the importance of antiviral drugs such as oseltamivir (Tamiflu) and zanamivir (Relenza) for treating the disease.
“This year, treatment with antivirals is especially important, in particular for people at high risk of serious complications,” Frieden said. “They work best when you get them within 2 days of starting symptoms, so it’s important to get antivirals quickly.”
He added that doctors should not wait until they get flu test results before prescribing the drugs to their patients. “We need to get the message out that treating early with these drugs can make the difference between having a mild illness and having a very severe illness,” he said. “It is not a miracle drug, but we believe it is an effective drug.”
While some studies have questioned the effectiveness of antiviral medications, Frieden said CDC scientists believe that antiviral drugs can cut 1 day off the length of illness for many patients, and can reduce flu hospitalizations and deaths.
Flu season typically runs December through February, according to the CDC, but can start as early as October and end as late as May.