What about kids between the ages of 7 and 18? It’s unclear which vaccine is best, because there’s been very little research on the nasal spray in this age group. A 2006 trial in asthmatic kids aged 6 to 17 found that the nasal vaccine had an edge over the shot—4.1 percent of kids given the nasal spray caught the flu, compared to 6.2 percent of kids who got the shot—but it’s impossible to say whether asthmatic kids’ responses are representative of the larger pediatric population. When Taiwanese researchers compared the effects of a live nasal vaccine versus an inactivated shot in a 1991 clinical trial, they found that the nasal spray worked best for kids under 9, but that the shot worked better in kids aged 10 through 18. But it’s also hard to extrapolate from these findings, as the nasal spray used in this study protected against half as many types of flu as the one approved in the U.S. today.
Indeed, the flu vaccines used this year are new and improved compared to the ones used (and tested) in years past. It used to be that U.S. flu shots and nasal vaccines protected against two types of influenza A viruses (the viruses that have wild birds as natural hosts) and one type of influenza B virus (the virus that primarily infects humans). But as scientists have known for decades, two distinct types of influenza B viruses circulate and sicken people, so last year, the Food and Drug Administration approved new versions of the shot and nasal spray containing two influenza B viruses and two influenza A viruses, which in theory should protect kids against more flu strains. Research suggests these “quadrivalent” vaccines are just as safe for kids as the former trivalent ones. (All the nasal sprays this year are quadrivalent; some, but not all, of the shots are.)
Speaking of safety, yes: Flu vaccines can pose risks. There are the mild side effects I already mentioned, and there can be rare complications, too: 1 or 2 in every million kids develops an immune condition called Guillain-Barré syndrome; another 1 in 1 million has a severe allergic reaction. But these reactions are far less common than the serious complications associated with the flu infection itself. Estimating that there are approximately 75 million kids living in the U.S. today, if they all got flu shots this year (last year only about half did), only about 225 children would suffer one of these major reactions. By contrast, about 20,000 children are hospitalized each year for flu complications such as pneumonia, meningitis, acute bronchitis, acute kidney failure, and sepsis. (As for the use of thimerosal, a form of mercury, as a preservative: Many, many studies have refuted any link between thimerosal and autism. Plus, five out of six flu vaccine manufacturers make at least one formulation that is thimerosal-free or contains only trace amounts.)
Now, what if you’re a parent to a child under the age of 2? The nasal spray is only approved for kids over 2, so you’re stuck with the shot. Yet only one controlled trial has tested the flu shot in infants, and scientists differ in their interpretations of what it found. Some researchers concluded from this trial that the flu shot is no more effective than a placebo in children under the age of 2. Yet the shot did work well at preventing flu in the first season assessed in the trial—its vaccine efficacy that year was 66 percent. Infants and young toddlers didn’t, however, derive much benefit from the vaccine the following year, when the flu happened to be much less common. When you combine the evidence from the two flu seasons, the overall benefits of the vaccine seem negligible, but the trial showed that the vaccine did provide protection in one out of the two years.
This raises another important point about the flu vaccine: Its efficacy varies from year to year, because it’s different from year to year. About nine months before flu season starts, scientists at the World Health Organization make an educated guess as to which viruses are likely to circulate the following year (influenza evolves rapidly), and they aren’t always right. When they’re wrong, the vaccine can still provide some cross-protection, but it often doesn’t work that well, and vaccinated people still get sick. Clinical trials on flu vaccines are, of course, affected by the luck of this draw, too—trials conducted during well-matched years conclude the vaccine is more effective than do trials conducted in mismatched years, which is why many trials try to test the vaccine over multiple flu seasons.
If all this information is starting to make you feel feverish, here’s some simple advice. Get your kid inoculated. If she’s over 2 but under 18, request the nasal spray, which seems to work much better than the shot and will certainly incite fewer screams—and you may want to do it soon, because supplies are limited: Less than a quarter of this year’s vaccine stock is the spray. (If you can’t get the spray, do the shot. Luckily, overall flu vaccine supply shouldn’t be affected by the government shutdown.) If you’re the mother of a baby or a 1-year-old—as I was last year—then yes, the science is less clear, but I think it’s still worth vaccinating. Absence of evidence doesn’t mean evidence of absence; if more trials were conducted on infants, scientists might find more evidence of benefit. Or they might not. But it’s a risk/benefit calculation, and the risks associated with inoculation seem quite low compared to the potential benefits of protection, especially since infants are at a high risk of suffering complications from the flu. Is it possible that the vaccine won’t work well this year or that this flu season will be extremely mild? Sure. But there’s no way to know. And I’d rather waste a morning at my pediatrician’s office—and yes, I’ve already made an appointment—than gamble with my family’s health.