By Aaron E. Carroll
Image Credit: Dominic Kesterton
Jan. 11, 2018
Even a “less effective” vaccine packs a payoff in averting illness and death.
This year’s flu season is shaping up to be a bad one. Much of the country endured a bitterly cold stretch, causing more people to be crowded together inside. The strain that has been most pervasive, H3N2, is nastier than most. And, we’re being told, the vaccine this year is particularly ineffective.
That last fact has had many people wondering if they should still get a flu shot. If you read no further in this column, know this: The answer is yes, you should still get a flu shot. The flu season typically peaks December through February but can last until May, and it usually takes about two weeks for the shot’s immunity to kick in.
It’s worth exploring what we mean by effectiveness when we’re discussing the flu vaccine.
The flu virus is unstable, and it changes a lot each season. This means that the immunity you gained from a shot a year ago doesn’t work so well this time around. Each summer, scientists gather and try to make a best guess on which variants are going to be more common in the coming year. They look at data from countries like Australia (whose flu season comes before ours), then they make the shot to match.
The scientists’ guess wasn’t bad, as it included H3N2, the strain making most of the news right now. Vaccines don’t work as well against it in general because it tends to mutate more than other strains. It’s also harder to produce a targeted vaccine for H3N2 than for other variants, because of the way we produce the vaccine using eggs. That, along with other factors, makes for more infections and more severe illnesses.
In any year, even when you’re vaccinated, you can get the flu. The shot is about reducing your risk, not eliminating it. Still, even when the flu vaccine is “less effective,” it’s a good bet.
One reason is that “less” and “more” effective are relative terms. We need to be careful about these words and focus more on absolute risks.
In 2010, researchers published a meta-analysis of all available flu shot studies. They showed that when a vaccine is considered effective, 1.2 percent of vaccinated people had the flu, while 3.9 percent of unvaccinated people had the flu. That’s an absolute risk reduction of 2.7 percentage points. This means that the number of people who needed to be treated for one person to see the benefit — a concept known as N.N.T. — was 37. Given the millions who are vulnerable to flu and the thousands of deaths each year, this is a big payoff in public health.
In studies in which the flu shot was considered ineffective, 1.1 percent of vaccinated people had the flu compared with 2.4 percent of unvaccinated people. The absolute risk reduction was 1.3 percentage points, and the N.N.T. was 77.
Let’s say that this year’s flu vaccine is even worse than we think. Maybe the absolute risk reduction will be as low as 1 percentage point, making the N.N.T. 100. That’s still not that bad. Even at an N.N.T. of 100, for every 100 people who get a flu shot, one fewer will get the flu. That’s a pretty low N.N.T. compared with many other treatments that health experts recommend every day.
Further, a Cochrane review published in 2016 showed that the N.N.T. in children 6 years old or younger to prevent one flu case was just six — an astonishing payoff in medical terms. It has even been shown to be effective in preventing death in children from flu-related causes.
The negatives of a flu shot are almost nonexistent, and significant side effects are very rare. Even in an ineffective year, the benefits greatly outweigh the harms. The Centers for Disease Control and Prevention estimates that 9 million to 36 million people become ill with the flu each year in the United States. Somewhere between 140,000 and 710,000 of them require hospitalization, and 12,000 to 56,000 die each year.
No vaccine is perfectly protective in any year, so it’s important to practice good infection control. That includes regular hand washing — especially before eating — and limiting contact with others when you or they are sick. But to minimize your chances of illness, yes, you should still get the flu shot this year, and any year.
Correction: January 11, 2018
An earlier version of this article described incorrectly how the effectiveness of flu vaccine is gauged. When a vaccine was considered effective, 3.9 percent of unvaccinated people contracted the flu. It is not the case that when a vaccine was considered effective, 3.9 percent of people did not contract the flu.
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