New estimates from the Centers for Disease Control and Prevention reveal that this year’s flu vaccine is only 23 percent effective, meaning that it only cuts the chances of getting sick by about a quarter. Still, health experts say the vaccine can still protect against strains that may become dominant later in the season, and may reduce the severity of flu if you do get sick.
The CDC had predicted this year’s vaccine wouldn’t work very well because the dominant strain of the flu virus that’s circulating this year, known as an H3N2 virus, mutated slightly after the vaccine was created. That enables the virus to evade the immune system response created by getting vaccinated.
The effectiveness of the flu vaccine varies dramatically from year to year, but can be as high as 50 percent to 60 percent, the CDC says. So if the early estimate of this year’s vaccine’s effectiveness, reported in this week’s Morbidity and Mortality Weekly Report, holds up, it would put the protection at the low end.
Because flu vaccine takes months to make, experts select the strains for the following winter’s shot in February each year. If a virus mutates — or, drifts — after production starts, sub-optimal protection can result.
The CDC report said that about two-thirds of the H3N2 viruses seen this year in the U.S. were not well matched to the vaccine virus.
Nevertheless, the CDC and others are still urging people to get their flu shots because the vaccine does appear to work well against other, less common strains of the virus that are circulating, and they could become more common as the flu season progresses. We’re only about halfway through and we’ve already reached epidemic levels, with widespread flu activity reported across most of the country.
“Even when vaccine effectiveness is reduced, vaccination still prevents some illness and serious influenza-related complications, including thousands of hospitalizations and deaths,” the CDC report says. “Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated, including persons who might already have been ill with influenza this season.”
The CDC has been predicting that this year’s flu season could be more severe than others because of the expectation that the vaccine wouldn’t work very well and because H3N2 viruses tend to be more aggressive. So the CDC has been urging doctors to be more aggressive about using antiviral drugs like Tamiflu and Relenza when their patients get the flu, especially those at high risk for complications.
“Physicians should be aware that all hospitalized patients and all outpatients at high risk for serious complications should be treated as soon as possible with one of three available influenza antiviral medications if influenza is suspected, regardless of a patient’s vaccination status and without waiting for confirmatory testing,” the CDC’s Joe Bresee said in a statement.