As vaccines go, the flu shot is no exemplar. A comprehensive new review of the research, published online by The Lancet Infectious Diseases, finds that the vaccine prevented illness in 59% of adults aged 18 to 64, and in eight of 12 flu seasons studied.
The authors said that evidence of effectiveness was particularly lacking in children aged 2 to 17 and in certain populations that are especially vulnerable to flu like elderly people over 65.
Still, said lead author Michael Osterholm, director of the Center for Infectious Disease Research & Policy at the University of Minnesota, 59% is better than zero, so people should still be getting their annual flu shots.
In their analysis, Osterholm and his team included 31 previously published studies (out of 5,707 they considered) — 17 randomized controlled trials and 14 observational studies — chosen based on strict study design criteria. Pooling data from various studies, the researchers found that the H1N1 vaccine was somewhat more effective than the seasonal flu shot, preventing infection in 69% of adults under 65. Nasal spray vaccines, which contain weakened live flu viruses, were more effective still, protecting 83% of children under age 7.
Because of the stringent criteria Osterholm and his team used to exclude trials from their analysis, their findings veer from those of the most recent review by the Cochrane group, which estimated that the flu vaccine was 73% effective when circulating flu strains matched those in the vaccine, and 44% when they didn’t.
The findings may come as a surprise to the average consumer, but public health officials and vaccine makers are familiar with the flu shot’s imperfect performance. Based on its own recent, unpublished data on the 2010-11 flu season, the Centers for Disease Control and Prevention (CDC) estimates that the vaccine was 60% effective in all age groups combined, cautioning that protection may vary widely:
At least two factors play an important role in determining the likelihood that influenza vaccine will protect a person from influenza illness: 1) characteristics of the person being vaccinated (such as their age and health), and 2) the similarity or “match” between the influenza viruses in the vaccine and those spreading in the community. During years when the viruses in the vaccine and circulating viruses are not well matched, it’s possible that no benefit from vaccination may be observed. During years when the viruses in the vaccine and circulating viruses are very well matched, it’s possible to measure substantial benefits from vaccination in terms of preventing influenza illness. However, even during years when the vaccine match is very good, the benefits of vaccination will vary across the population, depending on characteristics of the person being vaccinated and even, potentially, which vaccine was used.
Indeed, a study published on Monday found that the vaccine may be even less effective than previously thought in overweight and obese people. About 11 months after it was administered, the vaccine had worn off by a factor of four in twice as many heavy people (50%) as in those of healthy weight (25%), according to the study by researchers from the University of North Carolina at Chapel Hill. What’s more, when blood samples from the 461 participants were exposed to the flu virus, 75% of samples from normal-weight people showed the appropriate immune response, compared with only 25% of obese people’s blood samples.
“It’s clear that what we really need is to develop new and better vaccines,” Osterholm told NPR’s Shots blog. But because the current vaccine, while not ideal, is safe and moderately effective, and because it would cost so much to introduce a new vaccine that works significantly better, it’s held up development.
Scientists are now working on a universal flu vaccine that would offer protection against all flu strains for years. The current flu shot must be reformulated and manufactured anew each year, with the three strains that health officials predict will be the most commonly circulating.
There’s another way to boost the potency of flu shots: by adding adjuvants. In a recent study published in the New England Journal of Medicine, researchers found that seasonal flu shots with adjuvants were 86% effective in children aged 6 or younger, compared with a 43% protection rate for conventional shots. No flu shots with adjuvants are currently approved for use in the U.S.
On balance, though, getting the regular old flu shot is better than not getting one. The CDC recommends the vaccine for everyone over 6 months old. It may not eradicate all influenza infections, but it can help prevent illness, hospitalizations and deaths.