Here’s why experts say people who had COVID-19 should be vaccinated

As the U.S. tries to increase COVID-19 vaccination rates, skeptics on social media are challenging the efficacy of vaccines for people who were previously infected by the virus.

Dr. Joseph Mercola, an osteopathic physician identified as a top spreader of anti-vaccine misinformation on Facebook, dismissed the need for vaccination in people who have had COVID-19.

"It makes no sense to require vaccinations for the previously infected," Mercola wrote in a post on his Facebook page.

The post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

Mercola’s post responded to a tweet criticizing universities that are requiring vaccination, even for people who have recovered from COVID-19. The American College Health Association has recommended COVID-19 vaccination requirements for all on-campus college students this fall. According to the Chronicle of Higher Education, 602 institutions had some type of vaccination requirement, as of July 26.

For this fact check, we wanted to know if medical experts agreed with Mercola’s claim that a prior COVID-19 diagnosis means you shouldn’t need to get vaccinated.

We should note that the science regarding how much protection a natural infection offers compared with a vaccine is still developing. But the U.S. Centers for Disease Control and Prevention recommends COVID-19 vaccines for people 12 and older, even if they have been infected before.

"That’s because experts do not yet know how long you are protected from getting sick again after recovering from COVID-19," the CDC website says. 

In an emailed statement to PolitiFact, Mercola said, "Natural immunity induces lasting antibody protection, and individuals who have had an SARS-CoV-2 infection are unlikely to benefit from a COVID-19 vaccination."

His response included a link to a Cleveland Clinic study that examined 2,579 people previously infected with COVID-19 — including 1,359 who were not vaccinated — and found that none were re-infected over a period of five months. The study concluded that individuals who were previously infected were unlikely to benefit from COVID-19 vaccination and that the vaccine could be prioritized for people who have been uninfected. This study, which PolitiFact explored in June, has not yet been peer-reviewed or published in a scientific journal.

In a June 9 statement about the study, the Cleveland Clinic said that their results "could help guide vaccination efforts should there be a shortage of vaccine supply and in countries where vaccine supply is limited."

"We do not know how long the immune system will protect itself against re-infection after COVID-19," the statement said. "It is safe to receive the COVID-19 vaccine even if you have previously tested positive, and we recommend all those who are eligible receive it."

Mercola also cited two other studies to back his claim that vaccination is unnecessary for someone who has already had COVID-19. We reached out to an author of each of these studies to see what they thought about Mercola’s use of their respective findings.

Dr. Ali Ellebedy’s study measured the decline of bone marrow plasma cells, an important source of antibodies, in people previously infected with COVID-19. The study found that the levels of anti-SARS-CoV-2 antibodies "declined rapidly in the first 4 months after infection and then more gradually over the following 7 months, remaining detectable at least 11 months after infection."

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Ellebedy said he encourages vaccination even if someone has been previously infected, and that his study should not be cited as a reason for a previously infected person to not get vaccinated.

"Our study shows that mild SARS-CoV-2 infection induces persistent immune molecules that are directed against the virus but does not address whether these levels of immunity can actually protect against infection," Ellebedy said. "Also, it is important to remember that not all infected people mount a robust immune response, so it is wrong to generalize and say ‘if you are infected, you are protected.’"

The other study looked at whether the immune systems of people previously infected with the virus remembered contracting COVID-19. In a March 25 YouTube video sent to PolitiFact, co-author Dr. Shane Crotty said that with the laboratory measurements his team took, scientists can make some useful inferences, "but they don’t directly show protection."

"The vaccines are eliciting even more immunity than natural infection," Crotty said. He also reiterated Ellebedy’s point that the immune response to a natural COVID-19 infection can vary widely from person to person. Crotty said that these two factors have contributed to the public health recommendation of still getting vaccinated if you’ve been infected, and that he would get vaccinated if had COVID-19.

A natural infection could probably protect against hospitalization and death from a subsequent infection, possibly up to a few years, but it might not protect someone from a newer strain of the virus, said Dr. Paul Offit, chair of vaccinology at the University of Pennsylvania’s Perelman School of Medicine.

"Your natural infection will provide protection against whatever (strain) you were naturally infected with, but now there's a different virus circulating," Offit said. "So by getting a vaccine, you'll have a broader immune response to a more diverse population of SARS-CoV-2 viruses."

Offit also pointed out that three peer-reviewed and published studies suggest that a single dose of an mRNA vaccine (such as ones from Pfizer or Moderna) gives comparable protection against COVID-19 in a previously infected person as it does in someone who got both doses.

Virologist Dr. Sabra Klein, a professor at Johns Hopkins Bloomberg School of Public Health, said in a May interview published by Johns Hopkins University that vaccines provide better protection for people who have previously been infected.

"It gives them a strong, lasting immunity boost," Klein said. "After receiving the first dose of the Pfizer or Moderna vaccine, they have immunity levels comparable to those of uninfected people who have received their second dose."

Offit said that in light of these studies, one mRNA shot could suffice in people who have been previously infected.

"There is no downside to vaccinating. All vaccination does is broaden and lengthen your immunity," Offit said.

Additionally, the CDC has evidence that people who are fully vaccinated with an mRNA vaccine are less likely to transmit SARS-CoV-2 to others than unvaccinated people. This means that in the rare chance a person does get reinfected, they would likely have a lower chance of spreading the virus.

Mercola wrote, "It makes no sense to require vaccinations for the previously infected."

Experts are unsure exactly how long someone is protected against COVID-19 after they have been infected. The CDC says vaccination is a more effective way of building protection against the virus, including variants.

The authors of the studies Mercola cited recommended that people who were previously infected with COVID-19 still get vaccinated. They say vaccination does make sense, because everyone can have varying degrees of immunity and it is unclear how long natural immunity lasts.

We rate Mercola’s claim False.