When should people who've already gotten COVID-19 get the vaccine? Doctors say it depends on whether they still have symptoms.

• Doctors say it's generally alright for people who had a prior coronavirus infection to get vaccinated, assuming they don't have symptoms or an active infection.
• A CDC advisory committee suggested that individuals wait 90 days after their infection, since reinfection is unlikely during that period.
• But for patients with long-term coronavirus symptoms, it's unclear whether the vaccine could aggravate an existing inflammatory response.
• For that reason, doctors say it's best for these individuals to hold off.

Individuals who previously had COVID-19 are at the bottom of the priority list when it comes to US vaccinations.

Emerging research suggests that immunity to the virus could last anywhere from several months to several years, so US officials remain focused on getting shots to those who could get sick for the first time.

"We want to look at vaccinating patients who have not been infected with COVID who are susceptible," Todd Ellerin, the director of infectious diseases at South Shore Health in Massachusetts, previously told Business Insider. "The post-COVID patients are not going to be your first, second, third, or fourth tier of groups that you're going to look at to want to vaccinate."

Still, people who've had prior infections aren't prohibited from getting the shots if they're in a priority group like healthcare workers or nursing-home residents.

Both Pfizer's and Moderna's late-stage clinical trials suggest the vaccines are safe for individuals with a history of coronavirus infections - and are likely just as effective in this group as among healthy individuals.

There are some exceptions, however. The Centers for Disease Control and Prevention recommends that people with an active infection wait until their symptoms have resolved - and the standard 10-day isolation period has passed - before getting vaccinated. That includes people who've already received the first dose in the vaccine's two-dose regimen.

"The recommendations for receiving any dose of the vaccine are not to get it if you're frankly ill at the time," Dr. Sandra Sulsky, an epidemiologist and principal at Ramboll, a global health sciences consulting firm, told Business Insider.

In December, a CDC advisory committee said individuals with access to a vaccine can wait 90 days after their initial infection for the first shot if they want to, since reinfection is unlikely during this period.

"In terms of whether the vaccine is needed to prevent reinfection, in general I'm of the mindset that it can't hurt and it might help," Dr. Steven Deeks, a professor of medicine at the University of California, San Francisco, told Business Insider. "So for the general population who've done well post-COVID, [if] it has been three months, get a vaccine."

But for people who continue to experience long-term symptoms, the CDC hasn't offered a recommendation yet. That's because researchers still aren't sure what's causing these lingering ailments.

For now, doctors suggest that these patients hold off on getting vaccinated.

What clinical trials tell us so far

Pfizer didn't screen participants for evidence of a prior coronavirus infection during its late-stage clinical trial. So it turned out that 3% of participants had been infected before. The data indicated the vaccine was just as effective in this group, but a Food and Drug Administration review said there wasn't enough evidence to know whether the vaccine prevented reinfection.

In Moderna's trial, 2.2% of participants had been infected before.

"These were small numbers so their statistics are not particularly robust and you can't really rely on them very much," Sulsky said.

Still, if a person is no longer symptomatic, doctors say there's little risk of an adverse reaction based on a history of coronavirus infection alone.

Read more: What's coming next for COVID-19 vaccines? Here's the latest on 11 leading programs.

Instead, it's the "long-haulers" - coronavirus patients whose symptoms last three weeks or more - who continue to mystify doctors.

"It'd be hard to engage a long-hauler in [a vaccine] study if they're having ongoing health problems," Natalie Lambert, an associate professor of medicine at Indiana University, previously told Business Insider. "Ethically there'd be big problems with having them take a vaccine."

Deeks said it's likely that some long-haulers will get vaccinated anyway, whether in clinical trials or as part of general vaccinations in the US. So eventually, scientists may acquire enough data to know whether the shots are safe for this group.

Long-haulers should wait

The FDA has said there's "insufficient data" to assess whether coronavirus vaccines are safe for people with weakened immune systems. It's possible that long-haulers may fall into this category.

That makes it hard for doctors to give individual advice about whether long-haul patients should get vaccinated.

"It would certainly make sense to consult with your primary care provider, but since no one knows what to do, you're not really going to get an informed expert's advice," Deek said.

At the moment, he said, there are two prevailing theories about why some people develop long-term symptoms.

The most plausible explanation, Deeks said, is that long-term coronavirus symptoms are related to an inflammatory response initiated by the virus. In that case, he added, the vaccine could make the inflammatory response even worse.

"It's easier to argue that it would cause more harm than good," Deeks said. "In the absence of data and no urgency to get a vaccine, I would wait."

The other idea, he said is "a theoretical argument that there is a persistent infection that's causing the symptoms and that if you enhance your immune response to the infection by a vaccine, you'll get rid of the infection and you'll get better."

But the theory, he added, "seems highly unlikely."