A dentist receives the Moderna COVID-19 vaccine in Anaheim on January 8, 2020.
  • People who had Covid-19 developed at least 10 times more antibodies after their first vaccine dose than the average uninfected person who received two doses, new research shows.
  • Another preliminary study similarly found that healthcare workers who had Covid-19 responded to their first shot the way most people respond to their second.
  • The researchers both suggested that post-Covid patients may only need one shot to sufficiently protect them from the disease again.
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People commonly report more side effects after their second coronavirus shot than their first.

But researchers at the Icahn School of Medicine at Mount Sinai discovered a slightly different response among patients who already had Covid-19.

These patients not only had stronger side effects - including fatigue, headache, chills, fever, and muscle/joint pain - after their first injection, but they also had more antibodies compared to those who had never been sick before.  

In a study out Monday that's still awaiting peer review, the researchers found that people who already had COVID-19 developed at least 10 times more antibodies after their first dose than the average uninfected person who received two doses.

That could mean that previously-infected individuals only need one shot to sufficiently protect them from getting sick again. A single shot might also help them avoid more uncomfortable side effects after a second dose.

Indeed, the researchers wrote, "changing the policy to give these individuals only one dose of vaccine" could "spare them from unnecessary pain and free up many urgently needed vaccine doses."

The findings are supported by another preliminary study, also out Monday, which found that healthcare workers who previously had Covid-19 displayed higher antibody levels after their first shot compared to vaccinated individuals who had never been infected. 

"It was a very large difference. It was something that we could easily see," Dr. Mohammad Sajadi, an associate professor at the Institute of Human Virology at the University of Maryland School of Medicine, told Insider.

Typically, Sajadi said, Covid-19 patients develop antibodies around two to three weeks after their initial infection. But it didn't take nearly as long for post-Covid patients to develop antibodies in response to a vaccine: The individuals showed high antibody levels a week after their first shot, with antibody levels peaking around days 10 to 14 post-vaccination.

But both studies only examined a small group of vaccinated individuals - a few hundred in total. For this reason, many scientists are wary of prescribing anything other than the two-dose regimen tested in clinical trials.

"I'm a big proponent of the right dosing and right schedule, because that's how the studies were performed," Maria Elena Bottazzi, an immunologist at Baylor College of Medicine, told The New York Times.

Post-Covid patients have a 'memory response'

Healthcare worker Elizabeth Cameros, right, administers a COVID-19 test to traveler Wade Hopkins in Los Angeles, California on November 23, 2020 .

Both Pfizer's and Moderna's late-stage clinical trials suggest the vaccines are safe for individuals with a history of coronavirus infections. 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 their first dose.

"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, previously told Insider.

But scientists still aren't sure when vaccines are necessary for previously-infected individuals. Emerging research suggests that antibodies to the coronavirus could last anywhere from several months to several years - and even then, antibody levels don't always correlate to immunity.

All the participants in Sajadi's study, for instance, tested positive for coronavirus antibodies back in July and August. By the time they got their shot, he said, some of them had "very, very low levels" of antibodies - but they still seemed to mount a strong response to the vaccine.

"What that shows you is that individuals who had a prior COVID infection have what we call a recall response or a memory response," Sajadi said. "For most infections, the second time you see that microorganism, you should get a faster response."

He cautioned, however, that the findings only apply to people with a "run of the mill Covid infection" - patients that developed antibodies and have since recovered from their illness.

An interim solution to limited doses

People wait in line in a Disneyland parking lot to receive COVID-19 vaccines in Anaheim, California.

Sajadi said giving only one dose to post-Covid patients is a short-term strategy to address critical vaccine shortages.

With only 32 million Americans vaccinated so far, many states continue to report that they don't have enough doses to meet demand. In recent weeks, some local health departments have even been forced to cancel vaccine appointments or shut down scheduling websites.

The CDC has now said that vaccination sites can delay administering a second dose for up to six weeks - instead of the recommended three to four weeks - in "exceptional circumstances" where it isn't feasible to give the second dose on time.

"In times of vaccine shortage, where every vaccine dose counts, we think the data shows if you had previous Covid infection, you may only need one dose for the booster," Sajadi said. That "may even be the ideal scenario" for post-Covid patients when vaccines are widely available, he added. 

So far, however, researchers have only examined how post-Covid patients respond to the Moderna and Pfizer vaccines - both of which use the same mRNA technology to trigger an immune response.

Sajadi said it's hard to know whether post-Covid patients will mount a similar antibody response to the single-dose viral-vector vaccine from Johnson & Johnson, which plans to apply for emergency use this week. 

"There's no reason to think it would act differently," he said. "But you just never know until you test." 

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