Flu vaccines don’t work as well as previously thought, although they’re still the best protection available against seasonal influenza, researchers report Tuesday.
Overall, flu shots in adults 18 to 65 are 59 percent effective, the scientists estimate. Evidence for protection in people 65 and older was lacking, they write in a paper published online by The Lancet.
They reached their conclusions by pooling the results—an approach called a meta-analysis--of 31 studies of flu vaccine. The effectiveness varied from year to year in the studies analyzed. In several flu seasons, there was no evidence of effectiveness.
The new finding contrasts with the conclusion of meta-analyses by the respected Cochrane Collaboration, which last year found that flu shots were 73 percent effective in adults.
“The studies we present are the very best studies that we have for the last 70 years,” says Michael Osterholm, lead author of the new report and director of the Center for Infectious Disease Research & Policy at the University of Minnesota.
A major difference between Osterholm’s meta-analysis and the Cochrane reviews is the type of studies they included. Osterholm’s used only studies that confirmed the presence of influenza virus in sick people with either sophisticated polymerase chain reaction technology, or PCR, or laboratory cultures. PCR can take a single piece of DNA and generate thousands to millions of copies.
The Cochrane reviews, on the other hand, included studies that determined whether sick people had the flu by checking to see whether they had elevated levels of flu antibodies in their blood, not whether influenza virus was present.
The problem, Osterholm says, is that the antibodies don't become noticeably elevated in three out of four people who contract influenza after getting a flu shot. So, he says, the studies in the Cochrane review probably missed diagnosing influenza in many people who became sick after getting flu shots.
While flu shots contain inactivated, or killed, influenza virus, the nasal-spray flu vaccine, sold as FluMist, contains live, albeit weakened, virus. Osterholm’s meta-analysis found that FluMist consistently was most effective in children 6 months to 7 years old. One study suggested it was also effective in people over 65, but FluMist is approved in the United States only for those age 2 through 49.
In related news, the Food and Drug Administration is cautioning against using jet injectors, such as PharmaJet, with flu vaccine. The only vaccine currently approved for use with a jet injector is the measles, mumps and rubella, or MMR, vaccine.
“Jet injectors represent a different method of delivery that has the potential to change the characteristics of an approved vaccine,” according to the FDA.
Kroger, Publix and Walgreens have administered flu vaccine with jet injectors, agency spokeswoman Shelly Burgess says. But, says Centers for Disease Control and Prevention spokesman Tom Skinner, neither the CDC nor the FDA think it’s necessary for people immunized with a jet injector to be re-vaccinated.
Earlier this week, news about a malaria vaccine that is 50 percent effective “drew cheers from the biomedical and policy communities,” notes Arthur Caplan, director of the Center for Bioethics at the University of Pennsylvania. It would be ironic if people decided not to get a flu shot because, though 59 percent effective, it’s not as effective as previously thought, Caplan says.
“We certainly need more effective vaccines, and governments should start putting serious resources into developing them,” Caplan says.
Osterholm agrees. He says vaccine manufacturers have told him they’re reluctant to invest in a new-and-improved flu vaccine because the one on the market now is cheap, safe and recommended for people of all ages. “We shouldn’t get rid of it,” Osterholm says, “but it’s not nearly the vaccine that we need for the future.”
Despite the lack of studies that included people over 65 in Osterholm’s meta-analysis, “the vaccine works reasonably well” in that age group, says Andrew Pavia, chair of the Infectious Disease Society of America Pandemic Influenza Task Force.
“The good news about this paper,” says Pavia, chief of the division of pediatric infectious diseases at the University of Utah, “is it hopefully will stimulate interest in new vaccines.”