Although CDC and various health experts recommend vaccination regardless of prior Covid-19 infection, some say the combined immunity from infection and an initial vaccination may be protective enough that booster doses are not needed among that population.

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Vaccination is still necessary after Covid-19 infection

While it has been argued that natural immunity is protective against coronavirus reinfection, CDC and many immunologists recommend vaccination regardless of prior infection, the New York Times reports.

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So-called "natural immunity," which is the result of having Covid-19, varies from patient to patient. According to the Times, 85% to 90% of people who recover from Covid-19 have detectable antibodies, but the strength and durability of natural immunity can vary depending on a person's age, health, and the severity of their initial infection.

For example, research from Akiko Iwasaki, an immunologist at Yale University, and her team found a stepwise increase in antibody levels as a patient's Covid-19 severity increases. Another study found that around 43% of Covid-19 survivors had no detectable neutralizing antibodies—the specific type of antibodies needed to prevent reinfection. And a CDC study found that antibodies from natural immunity decrease to undetectable levels in around 30% of Covid-19 survivors after approximately two months.

In addition, most studies of natural immunity against Covid-19 have tracked people for only about a year, said Nabin Shrestha, an infectious disease physician at Cleveland Clinic. "The important question is, how long does it protect, because we're not under any illusions that this will be a lifelong protection," he said.

Although individuals with powerful natural immunity may be protected from reinfection, many health experts recommend vaccination to further boost their immune response and gain longer-lasting protection. According to the Wall Street Journal, the immunity developed after infection and vaccination is called "hybrid immunity," which some preliminary research has suggested offers stronger protection—even against variants—than infection or vaccination alone.

"If you've gotten the infection and then you've been vaccinated, you've got superpowers," Jennifer Gommerman, an immunologist at the University of Toronto, said.

Boosters aren't currently needed for fully vaccinated people who have had Covid-19, experts say

CDC's and FDA's booster shot recommendations do not include distinctions between patients who have and have not been previously infected. However, because of the hybrid immunity that results from vaccination and natural infection, some health experts, including Paul Offit, a member of FDA's advisory panel on vaccines, and various immunologists, currently do not recommend booster shots for people who have had Covid-19 and are fully vaccinated, the Journal reports.

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According to scientists who spoke with the Journal, studies on patients with hybrid immunity suggest that a coronavirus infection functions similarly to a dose of a vaccine. Like a vaccine dose, an infection causes the immune system to generate antibodies, B cells, and T cells that could help fight off future infections.

In addition, Iwasaki said a real-world infection may provide a significant protective boost because it exposes the immune system to more of the coronavirus than just the spike protein targeted by vaccines. According to the Times, an infection stimulates immune defenses in the nose and throat—where they're most need to prevent reinfection—while vaccines primarily produce antibodies in the blood.

In fact, researchers affiliated with the ZOE Covid Study analyzed 650,000 self-reported Covid-19 test results and found that an infection and two doses of the Pfizer-BioNTech Covid-19 vaccine was 94% effective against infection up to six months after vaccination. In comparison, vaccination alone was 80% effective, and infection alone was 65% effective.

In addition, a CDC study published in September found that unvaccinated people with prior Covid-19 infection were more than twice as likely to be reinfected by the coronavirus than fully vaccinated people with prior Covid-19 infections.

People who have had Covid-19 and been vaccinated "just won the game," Offit said. "I wouldn't ask them to get a booster dose. I think they just got it [through a coronavirus infection]."

Iwasaki agreed, saying people with hybrid immunity "are likely to be the last group [who] really needs the booster because they really had three exposures."

However, the ZOE Covid Study researchers noted that the strong protection provided by the combination of real-world infection and vaccination does not mean these people may not eventually need a booster in the future. In addition, they said some people in this group with weakened immune systems or underlying medical conditions may want to receive a booster dose regardless of their potentially increased immunity. (Mandavilli, New York Times, 10/12; Schwartz, Wall Street Journal, 10/10)