• People may need to get booster shots after the initial coronavirus vaccine doses in order to stay protected, experts say.
  • Other vaccines, like the tetanus shot, also require periodic boosters. 
  • That adds further complication to the impending challenge of distributing coronavirus vaccines, which are likely to require two doses.
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We may still not know precisely how long immunity to the new coronavirus lasts, but researchers don't think it's forever.

"With human coronaviruses,  you can get repeatedly infected — you're not immune for life, you're immune for some time," Florian Krammer, a vaccinologist at the Icahn School of Medicine at Mount Sinai, told Business Insider.

"There's no reason to think this coronavirus will behave differently," he added.

That means that even after coronavirus vaccines become available and get widely distributed, we'll likely need booster shots to stay protected over time.

"If immunity does turn out to be fleeting, we'll need a plan of a vaccination plus a booster, or revaccination at periodic intervals," Marm Kilpatrick, a disease ecologist, previously told Business Insider.

Two shots to start, then perhaps boosters, too

The two leading coronavirus vaccine candidates so far, from Moderna and Pfizer, both require two shots. Moderna's two doses are administered a month apart, while Pfizer's are given three weeks apart.

The more shots we need, however, the harder it is to ensure everyone gets them.

Plus, a two-dose vaccine regimen comes with supply-chain challenges: it requires twice as many vials, syringes, refrigerators, and clinic visits at a time when such resources are already limited.

But even after all those problems are mostly solved, a new challenge will arise: the need to figure out when our immunity fades, and if — or when — a booster shot is needed. 

"Once we start seeing vaccine failures increasing, then we can consider booster doses," Walt Orenstein, the former director of the US National Immunization Program, previously told Business Insider. He added, though, that "we don't know at this stage whether that will be necessary."

Some viruses, like hepatitis A or measles, are a one-and-done deal: Once you're infected (or inoculated), you're immune for life. But with coronaviruses, reinfection is possible after a period of months or years, according to the Mayo Clinic. That only happens to a fraction of people, though, and their second illness is usually mild.

Indeed, limited evidence suggests people could get reinfected with the new coronavirus. Some research has found that coronavirus antibodies decline after a period of months, which could mean our immunity might be similarly transient. Given that the efficacy of a vaccine hinges on its ability to prompt the body to generate antibodies, it's therefore unlikely coronavirus shots will be a one-time affair. 

But our immune systems do have more than just antibodies defending us from future infection, and a recent study suggests those other defenses stick around for at least six to eight months.

Time will tell if we'll need boosters

It's not a deal-breaker if people become susceptible to reinfection.

"This happens for a lot of vaccines," Krammer said. "It's not a problem. You can get revaccinated."

That's what booster shots are for. Your tetanus vaccine, for example, requires a booster every decade. The question is how frequently follow-up coronavirus shots might be needed, but experts won't be able to answer that until vaccines are widely distributed.

For now, Orenstein said, public-health officials should start making a plan now to keep track of how well the shots work long-term:

"It's critical, as the vaccine is rolled-out, to have continued evaluation," he said, adding, "we need to measure vaccine effectiveness in observational studies to see whether it declines or not."

That work, he said, should begin alongside the earliest vaccine rollouts.

"We don't want to wait two, three, four years before doing so," Orenstein added.

Read More: Meet Dr. Vivek Murthy, Biden's front-runner for Health secretary and COVID wingman. His resume and past comments on guns are raising bipartisan concerns if he's the right pick.

Monitoring will also be crucial to determine whether the vaccine efficacy that Moderna and Pfizer have found in their trials — 94.5% and 95%, respectively — hold up outside of clinical conditions in the first place.

For example, researchers could implement a surveillance system that looks at new COVID-19 cases and notes how many infections are diagnosed in vaccinated versus unvaccinated people, Orenstein said.

"If the rates are comparable, that's a sign of waning immunity, he said. "That's when we'd need to determine whether boosters are necessary, and if so, how often."

But if it turns out that people need to be revaccinated regularly, that decreases the likelihood that people will get the shots they need to stay protected.

"The more complicated the schedule, the more difficult it is to get people to come in," Orenstein said.

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