The flu vaccine may have a worrisome problem that US scientists can't fix
The US government is unwavering in its support of the annual flu vaccine.
"The single best way to protect against the flu is to get vaccinated each year," the US Centers for Disease Control and Prevention, or CDC, says in bold letters on its website.
But researchers are concerned about what appears to be a troubling trend. Repeated vaccinations against the flu might make the newest shot less effective than the last, Helen Branswell reports at Stat.
When researchers followed 328 households during the 2010-11 flu season, they found — much to their surprise — that the only people who seemed to benefit from immunization were the ones who hadn't gotten a flu shot the year before. These "unexpected findings ... require further study," the researchers wrote in 2013.
A larger and more robust study, published last year in Clinical Infectious Diseases, added more evidence that the 2013 study was onto something important. Researchers followed more than 7,000 people for eight yearly flu seasons, and they learned that people got the strongest protection against the flu only when they were vaccinated for the current season — and at no other time during the previous five years.
Considering that some vaccines require multiple doses to be effective, as Branswell notes, "the fact that repeated vaccination against flu might diminish rather than enhance the vaccine's protection is perplexing." But it's possible that "antibodies produced in year one may neutralize some of the vaccine in year two's shot before it can trigger a full immune response," she writes.
Unfortunately, US scientists interested in getting to the bottom of this are in a tough spot. They can't buck the CDC's recommendations and ask groups of people to forgo the flu vaccine in certain years just so they can compare them to people who get it every year. It would be considered unethical, as Branswell points out.
Fortunately, researchers in other countries without such ironclad recommendations are working on studying this problem in controlled environments.
Figuring out what's going on "is a critically important component of the effort to improve our control of influenza," Drs. John Treanor and Peter Szilagyi of the University of Rochester Medical Center wrote in 2013.
Until those much-needed, carefully controlled studies happen, questions about the effect of repeated vaccinations on vaccine effectiveness are just that: questions. As the researchers behind the Clinical Infectious Diseases study noted: "Additional studies are needed."
For now, it's too soon to change recommendations, and the weight of evidence is still greatly in favor of getting your flu shot.
"In every scenario, it is better for people to be vaccinated than not vaccinated," Dr. Edward Belongia, one of the researchers behind the study of 7,000 people, told Branswell. "It would not be, I think, accurate or helpful for people to take away from this, 'Oh, well, I shouldn't get vaccinated because I got vaccinated in the past and that's a bad thing.'"