What Offers Better Immunity: The Flu Vaccine or the Flu Itself?

This year’s flu season has been fairly mild so far, as predicted. But also as predicted, flu cases have been picking up, in line with the expected February peak this year. The most recent report from the Centers for Disease Control and Prevention shows an increase in influenza-like illnesses and notes that it’s never too late to get vaccinated.

But the flu vaccines varies widely in its effectiveness from year to year. So, how worthwhile is it, really? You could instead take your chances with the flu, and if you get it, you’ll have even better immunity to the flu in the future anyway, right?

Unfortunately, it’s not that simple.

There are two things to unpack here: how effective the flu vaccine is and whether getting the flu might protect someone more than the vaccine from future illness. Both of these start with understanding the virus itself.

“What makes the flu virus so confusing and tricky is it mutates,” explained Dr. Kawsar R. Talaat with the Center For Immunization Research at Johns Hopkins Bloomberg School of Public Health. “When it makes copies of itself, it does so sloppily, and there are mistakes made,” Talaat said. “Some of those mistakes don’t work — the virus doesn’t leave the cell or dies — but occasionally in the copying process, it allows the virus to be a little bit better at infecting the next person.”

The influenza virus also mutates very rapidly, explained Litjen Tan, Ph.D., chief strategy officer for the Immunization Action Coalition.

“With flu, we’re dealing with a very cool virus,” Tan said. “It’s got these great ways of switching around its genes so it can look different, and it wants to look different so it can better evade the immune system.”

These rapid mutations account for the different strains among the four basic types of flu viruses that infect humans. That variety of strains partly explains the variation in the flu vaccine’s effectiveness. To make an effective vaccine, public health officials must predict which strains will be circulating almost a year ahead of time, all while the virus is a moving target.

The vaccine’s effectiveness falls short for other reasons too, Talaat said.

A person's "immune response to flu vaccine is not perfect, so effectiveness will never be 100 percent even if the strains perfectly match,” Talaat said.

Further, even if strains or antibodies match well, each new vaccine’s ability to induce an immune response varies.

Finally, individuals respond differently to the vaccine. Older adults do not mount as strong a defense because their immune systems are older. Infants don’t mount a strong response because their immune systems aren’t fully developed yet. And if you're in the middle -- being sick, tired, sleep-deprived or otherwise run down in some way may affect how well your own body responds.

Then there’s the evidence suggesting annual flu shots are less effective when you’ve had one the previous year. One possible explanation for this is that the body’s response to a previous year’s vaccine blunts the response the following year, Tan said, but scientists are still trying to understand it. A blunted immune response from the vaccine, however, still offers more protection than not getting the vaccine at all.

“Until we get a true handle on what the mechanism could be, the current recommendation is the best one we have,” Tan said.

At this point you might be thinking -- maybe I'll just take the immunity I would get from contracting the flu itself.

That might make some sense — except it requires actually getting sick, and the flu is not a mild illness.

“The first problem with this idea is that you can get really, really sick and die from the flu,” Talaat said. “If you get the flu vaccine, even if it doesn’t protect you from getting sick, hopefully it protects you from severe disease and from death. It tends to mitigate the illness somewhat.”

It is true that a flu infection will stimulate the immune system more strongly than the vaccine will, said Tan, but that comes with a price.

“As a country, we don’t think flu is a big deal, but it is deceivingly serious,” Tan said. Deaths from flu each year vary widely, but they’re always in the thousands, ranging from 3,000 to 49,000 people a year in the U.S. “It makes absolutely no sense when we have proven safe and effective vaccine against flu -- and all these other diseases -- that we don’t use them,” Tan said. “It’s like throwing the dice with your child’s, your own or your grandparent’s life.”

Even aside from that gamble, however, the immunity gained from a bout of the flu only protects against that particular subtype and won’t last long because of those rapid mutations described above.

“Getting the flu and surviving is the best immunity against that particular strain of flu, but that’s just one of the four lineages, and it drifts. That one may be circulating for only three or four years,” Talaat said.

“Are you willing to get the flu every three or four years to maintain immunity to it? That doesn’t make a lot of sense.”