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This Year’s Super Bad Flu Season In 20 Maps

Influenza isn’t just widespread — the strains in circulation are also severe. As the following chart illustrates, the share of doctor visits for flu and flu-like illnesses has not been this high since the 2009-10 season, when the flu hit early and hard but then quickly declined. (The flu season typically begins around October, peaks somewhere between December and February and peters out by the end of May.)

Still, there’s some good news out this week. Data released Friday shows that, after a steep and steady rise over the past weeks, doctor visits for flu and flu-like illnesses are finally dropping.

The CDC tracks “flu-like illnesses” because viruses other than influenza, such as respiratory syncytial virus, can provoke flu-like symptoms too. Making a definitive diagnosis requires lab testing that takes time and isn’t done in all cases. But what makes influenza on its own noteworthy is that it can become severe enough to kill you — and there’s a vaccine against it.

Prior to each flu season, researchers try to predict which strains will circulate in the coming year so that they can include these in that season’s vaccine. But some years their predictions are better than others. On Feb. 16, the CDC released its latest data on the effectiveness of this year’s vaccine. The numbers showed that the U.S. flu vaccine’s overall effectiveness was 36 percent, which means that a vaccinated person reduced the risk of getting sick enough with the flu to seek a doctor visit by about one-third.

But the vaccine was only about 25 percent effective against a strain called H3N2, a particularly nasty subtype associated with higher rates of hospitalizations and deaths than other strains. And that’s a problem, because the H3N2 strain has been the most predominant this year. “We see lower protection against the H3N2 strain than we see against others, and that’s a consistent finding from year to year,” said Edward Belongia, an epidemiologist at the Marshfield Clinic Research Institute in Wisconsin who has tracked the vaccine’s effectiveness for the CDC over the past 14 flu seasons. The reasons for this aren’t entirely clear, but there are a few preliminary and puzzling clues:

  1. There’s some evidence to suggest that the first flu virus you’re exposed to as a child may orient your immune system to respond best to that kind of flu virus. This suggests that people who were exposed to H3N2 as children may mount a better immune response to it, Belongia said. If that’s true, people older than 50 may be especially vulnerable: “Before 1968, no one was being exposed to H3N2,” he said.
  2. Numerous studies going back to the 1970s suggest that people who are vaccinated every year may not get as much protection as those who get the vaccine one year but not the previous one or two years before then. This so-called repeated vaccination effect is not always present, and “we don’t know what’s driving it,” Belongia said. “It’s very complicated and may be different in children than in older adults.”
  3. The H3N2 viruses can mutate when grown in eggs for producing the vaccines, and that can lead to meaningful changes in how well the vaccine works, even if it was well-matched to the strains in circulation.

What does this all mean? The flu vaccine is helpful, but it’s not enough to get everyone through the winter without coming down with the crud. Even at the peak of the season, Belongia said, only something like 40 percent to 50 percent of the people who seek care for flu-like symptoms actually have influenza. And until scientists develop a universal flu vaccine that’s effective against all strains, the vaccines we have will only reduce the severity of flu season, not eliminate it.

If we want to get serious about preventing flu deaths — particularly if there’s a pandemic or a dangerous new strain — we should also give serious thought to quarantine strategies. The word quarantine carries a lot of political and ethical baggage, but it doesn’t have to mean restricting travel. It can also include promoting policies that enable people to stay home when they are sick.

People who have the flu are very infectious, and if they can stay home from work or school (and the grocery store and post office and everywhere else), they can reduce the spread of the disease. But our societal and workplace norms can make it hard to stay home when you’re sick. People who have a respiratory illness should avoid passing things like money or food back and forth, and yet workers in jobs that require tasks like these may find it especially hard to get (or afford) time off.

“We have many workplaces where people don’t have an opportunity to take any paid sick leave, so you have a strong incentive to come into work no matter how sick you are,” Belongia said. “I don’t know what the solution is, but this needs to be looked at in terms of sick leave policies.”

As this brutal flu season barrels on, taking precautions to stop its spread remains crucial. In many cases, that can mean avoiding public places when you’re ill. Yes, you can spread the flu in the early stages, before you’re flat-out sick, but you’re particularly contagious when you’re in the thick of the symptoms. And in that case, unless you require urgent medical care, the best thing you can do is stay the hell home.

Christie Aschwanden was a lead science writer for FiveThirtyEight. Her book “Good to Go: What the Athlete in All of Us Can Learn from the Strange Science of Recovery” is available here.