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Make A Flu Shot Your Halloween Tradition

Last year’s super bad flu season is behind us, and the numbers are in — 80,000 people died from flu or its complications, and 900,000 people ended up in the hospital. Much of last year’s misery came from H3N2, a particularly bad strain of influenza that has caused more severe symptoms than other strains and was poorly matched to last season’s vaccine. Add to that a growing population of people age 65 and older, who are most vulnerable to developing complications from the flu, and it made for a record year.

So what should we expect this year? Experts are asking themselves the same question. “The flu is predictably unpredictable,” said Alicia Budd, an epidemiologist in the Centers for Disease Control and Prevention’s influenza division. “We really have no way to predict exactly what we’re going to see.” And that makes it tricky to make the vaccine, which is produced months in advance. Flu viruses mutate and evolve very rapidly, so it’s hard to predict what will happen.

Still, there’s reason for cautious hope. The Southern Hemisphere had a “stunningly mild” flu season this year, said William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. The predominant virus in the Southern Hemisphere was H1N1, and that strain is what has turned up so far this flu season among the first influenza viruses in the U.S. “Put those two pieces together, and we can hope for a mild season, but I’m always nervous about saying that,” Schaffner said. “The flu is fickle.” We won’t know how this year’s season will turn out until it’s over.

Also fickle: the flu shot. But it’s still worth it. A flu shot can cut your risk of influenza infection and reduce the chance of spreading the virus. While it’s true that the flu vaccine isn’t nearly as effective as childhood immunizations like the measles, mumps and rubella vaccine, that’s not a reason to skip it, said Michael Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota. Last year’s flu vaccine was about 40 percent effective (which means that a vaccinated person had about 40 percent less of a risk of getting sick enough with the flu to see a doctor). And that’s about average over the past 14 flu seasons. That may sound unimpressive, but “it’s a lot better than zero,” Osterholm said. And studies suggest that even if it doesn’t prevent the flu in all cases, it may still reduce the severity of symptoms, he said.

How well the vaccine prevents the flu depends in part on how well matched it is to the circulating strains. Every year, experts from around the world get together to decide what will go into the season’s vaccine. “Decisions about the vaccines going into people’s arms now were made in February of this year,” Budd said. That’s because it takes time to produce and distribute the vaccine in time for flu season. This year’s vaccine was adjusted in part to address a manufacturing wrinkle that seemed to have lessened the effectiveness of last year’s vaccine.

Most flu vaccines are made by growing flu viruses in eggs, and this process can end up causing slight mutations in the viruses. “What you started with is not what you end up with,” and this may reduce a vaccine’s effectiveness, said Edward Belongia, an epidemiologist at the Marshfield Clinic Research Institute in Wisconsin. The problem seems most important for H3N2 viruses, which tend to evolve more rapidly than other strains. This year’s vaccine contains four strains, and its H3N2 strain appears to be less prone to these “egg-adapted” changes than last year’s, so the hope is that it will offer more protection against these types.

Another important but imperfectly understood problem is that influenza vaccines seem to lose effectiveness relatively fast. A study published last month in the journal Clinical Infectious Diseases examined data from seven flu seasons and found that the effectiveness of the flu vaccine waned by 16 percent every four weeks after the vaccine was delivered. So if you get the shot in October, you won’t be as strongly protected in December as you were in November. The study is just one of several concluding that the more time has elapsed since a flu vaccination, the less protection it offers, said Marc Lipsitch, a professor of epidemiology at the Harvard T.H. Chan School of Public Health who wrote an editorial that accompanied the paper. But these studies all share methodological flaws, he said: Calculating a flu vaccine’s effectiveness is a difficult challenge. “As a scientist, I would say we don’t really know yet.”

When I spoke with him Wednesday, Lipsitch said he happened to have a doctor’s appointment that day, but he wasn’t planning to get a flu shot just yet. “I’ll probably wait a month or two,” he said. “I consider the evidence, flawed though it is, convincing enough that I think there’s almost certainly no harm in waiting.” That is, unless putting it off means that you never get around to the vaccine, he said. In other words, if you’re a procrastinator like me, it’s probably better to get it when it’s in front of you than to assume you’ll get around to it later.

But the places that offer vaccines could help by adjusting their timing. (My local pharmacy started offering it in August.) To that end, the CDC’s Advisory Committee on Immunization Practices recently changed its recommendations on timing to say not to get it as soon as it’s available, but to get it by the end of October, Osterholm said. It might seem easy to just wait until the season has started to get the vaccine, but it takes about two weeks to build immunity, so waiting too long could easily backfire. Getting the timing right is “kind of like surfing the big waves of Maui,” Osterholm said. “Too far forward, you’re dead. Too far back and you’re dead.” Osterholm said the past 10 flu seasons — except for the 2009 pandemic — all started in mid- to late December, which makes October a pretty safe bet.

Ultimately, though, what we really need is a better vaccine. The most recent iterations are “not the vaccine we need for the future,” Osterholm said. In 2012, Osterholm was part of a group that put out a report that called for prioritizing the search for a universal vaccine, one that could be given less often and remain effective across a wide range of flu strains. It’s a major challenge, but Osterholm said we’re inching closer. “When we put out that report, it was like shouting into the Grand Canyon. Now we’re seeing substantial investments in both money and intellect.”

Earlier this year, the National Institute of Allergy and Infectious Diseases released a strategic plan for developing a universal flu vaccine that would be at least 75 percent effective, protect against two of the major groups of influenza viruses, provide protection that lasts at least a year and be suitable for people of all ages. Achieving this goal will take years, and the clock is ticking. It’s only a matter of time before another pandemic strikes. In that case, a universal vaccine is our best defense.

Christie Aschwanden is FiveThirtyEight’s lead writer for science.

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