This year's flu vaccine is not very effective at preventing the flu, particularly among adults, according to new estimates from the Centers for Disease Control and Prevention.
In a study of more than 2,000 people in the United States, including both children and adults, researchers found that those who got this year's flu shot were just 23 percent less likely to go to the doctor for flu symptoms than people who didn't get a shot. This level of protection is quite a bit lower than the level of protection seen in some previous seasons — for example, during the 2012 to 2013 flu season, getting a flu shot reduced people's risk of needing a doctor's visit for flu by 56 percent.
This year's vaccine was most effective for children. But among people ages 18 and older, those who got a flu vaccine were just as likely to go to the doctor for flu as those who were not vaccinated, according to the new study.
The findings confirm what health officials have suspected for weeks — that this year's flu shot offers limited protection against the disease — and underscores the need to give patients who may have the flu early treatment with antiviral drugs, if they are at high risk for complications from the disease, the CDC said. High-risk patients include young children, pregnant women, older adults and those with chronic diseases such as asthma, diabetes or heart disease. [ 6 Flu Vaccine Myths ]
Most of the flu viruses circulating this year belong to a strain known as H3N2. But about 70 percent of the H3N2 viruses in circulation are different from the H3N2 strain included in this year's flu shot. These differences can happen because the viruses' genetic material changes slightly, or "drifts" over time.
The poor match between the vaccine and the viruses in circulation likely explain the low effectiveness of the vaccine, the CDC said.
However, the CDC still recommends that people get a flu shot this year if they haven't already. That's because the shot may still prevent some people from getting sick (particularly if another strain of flu starts to predominate later in the season). And people who get vaccinated and then do get sick with the flu may have less severe symptoms — and lower likelihood of hospitalization — than those who skip the flu shot, the CDC said.
For example, during the 2012 to 2013 flu season, the flu vaccine was not very effective at preventing illness in older adults, but older adults who were vaccinated were 77 percent less likely to be hospitalized as a result of the flu, according to a study published in the journal Clinical Infectious Diseases.
The CDC researchers noted that, in the new study, the estimates of the current flu vaccine's effectiveness are preliminary, and larger studies may be needed to detect significant protection among adults.