Each year millions of people are infected with the influenza (flu) virus, which causes many to become ill and can pose serious harm to those who are young and elderly. Even though each year a new flu vaccine is created, targeting the three or four most likely strains of the virus to be circulating in the particular year, many think that because the vaccine is not foolproof and may not protect against a prominent strain, it's not worth it to get the immunization. But new research, out of St. Michael's Hospital and published in the journal BMC Medicine, took a long-term look at the effectiveness of the vaccine and found that it was protective even if the strains didn't match.

Each year, a panel of members from the Food and Drug Administration (FDA), World Health Organization (WHO), U.S. Centers for Disease Control and Prevention (CDC), and other institutions gets together to choose which flu strains are most likely to become prominent in the upcoming flu season. As it requires months of growing the virus under special conditions in chicken eggs, the decision has to be made almost a year before the actual flu strains in circulation are known. Although some researchers are looking into a pan-flu vaccine, which would protect an individual against all flu strains for their entire life, the current system has been working well for decades. But how well?

"It's quite common for people to say they are not going to get the flu shot this year because they've heard it does not match the strain of flu going around," said Dr. Andrea Tricco, the lead author of the paper and a scientist at the Li Ka Shing Knowledge Institute of St. Michael's Hospital, in a press statement. "However, we've found that individuals will be protected regardless of whether the flu strain is a match or not."

The research team headed by Tricco reviewed scientific studies spanning more than 40 years from 1971 to 2011, 47 flu seasons, and 95,000 healthy people. The team had a particular interest in seasons where the experts got it wrong and the flu vaccine did not match the circulating virus strains. They wanted to see if people would be protected from infection despite the vaccine and flu not matching.

The study found that in years when the vaccine was matched well to the flu strains in the wild, the effectiveness was between 65 and 83 percent. And, when there was not a good match, flu protection was still relatively high, standing at 52 to 54 percent, far higher than no vaccine at all.

"Looking at matches and mismatches can be a difficult process because it's not a yes or no variable," Tricco said. "Often we're looking at the degree of match between a flu strain and what's included in a vaccine because strains drift from year to year."

This evidence gives more credence to the recent change in guidelines, saying that not only the young and elderly should be getting their yearly flu shot, but everyone should. Every year, the flu causes more than a combined 111 million workdays lost due to sick leave and more than $7 billion in lost economic output in the U.S. alone.

Source: Tricco A, Chit A, Soobiah C, et al. Comparing influenza vaccine efficacy against mismatched and matched strains: a systematic review and meta-analysis. BMC Medicine. 2013.