Why Doesn't the U.S. Account for Natural Immunity?

From the early days of the COVID-19 vaccine rollout, the question of whether a prior infection should count as equivalent to a dose of vaccine has emerged and re-emerged in both scientific and political circles.

Countries like Israel, Canada, and the United Kingdom consider a past infection to count towards an individual’s immunity status. Some experts have also pointed to mounting evidence that immune protection from natural infection is at least as protective as vaccination.

But in the United States, immunity status solely hinges on vaccination. There’s no spot on the vaccine card to indicate recovery from a prior infection. Many institutions and employers require workers, students, or visitors to be fully vaccinated and sometimes boosted. In its attempts to bolster vaccination rates, the White House has repeatedly expressed frustration with those who refuse the shot.

Still, many people in the U.S. are on their way to developing hybrid immunity, or the combined immunity from vaccination and infection.

As of September 2021, more than 146 million people in the U.S.—more than 44% of the country’s population—had contracted COVID-19 since the start of the pandemic, according to a CDC estimate. During the nationwide surge in cases fueled by the Omicron variant, the proportion is likely even higher.

As the number of recovered people increases and data continues to emerge on the durability of natural immunity, should the U.S. consider people to be immune or non-immune, rather than vaccinated or unvaccinated?

Peter Chin-Hong, MD, professor of infectious diseases at University of California, San Francisco, said that vaccines are standardized and more reliable than natural immunity.

“We know what’s going to happen based on the experience of now millions of vaccinated people and thousands in the trials, which are really rigorously controlled,” Chin-Hong told Verywell.

The Case for Considering Natural Immunity

The CDC maintains that people should get a COVID-19 vaccine regardless of prior infection, saying that the level of protection from natural immunity may vary based on the severity of their illness, age, and the time since their infection. The CDC says, “No currently available test can reliably determine if a person is protected from infection.”

In discussions about immunity, the focus tends to fall on the importance of antibodies. Meanwhile, memory T and B cells are often overlooked. Once these virus-fighting cells are primed to recognize an invader—either through vaccination or infection—they can rouse a defense against subsequent viral infections.

After the body has successfully fought off an infection, it is natural for antibodies to clear from the blood. T and B cells linger for months, and sometimes years, longer, ready to make new antibodies and attack future antigens.

A study published in the journal Science in 2021 indicated that antibodies stayed relatively stable for at least six months after COVID-19 infections, and participants had more virus-specific B cells at six months post-infection than after one month.

In an investigative piece published in The BMJ, some experts argued that natural immunity should be considered equally protective as vaccine-based immunity.

A preprint study in Israel found that those who recovered from COVID-19 fared better than never-infected, vaccinated people in terms of both symptomatic and severe COVID-19. A preprint study from another set of researchers in Israel found that people who had recovered from COVID-19 for three months were as protected against reinfection and severe outcomes as vaccinated people. (Keep in mind that preprint studies have not been peer-reviewed by outside experts.)

While such data emphasizes the capability of the human immune system, experts warn that COVID-19 infections come with great risk.

Surging Omicron cases prompted many to question whether it’s best to just “get it over with.” Paul Goepfert, MD, professor of medicine and microbiology and director of the Alabama Vaccine Research Clinic, said this is a dangerous approach that could lead to more hospitalizations and fatalities.

“You don’t want people to go out and get infected thinking that’s the way they’re going to get protected,” Goepfert told Verywell. “You’re going to get a lot of people who are going to get really sick that way and some will die.”

Most of the data showing the durability of antibodies were collected before the Omicron variant became dominant in the U.S. According to a December preprint of a study in South Africa, the risk of reinfection by Omicron is more than three times greater than that of prior variants.

Plus, there’s little data on how well infection with one variant, like Delta, protects against other variants.

Immune Response Varies Among Naturally Infected Individuals

The caveat with natural immunity is that not all infections stimulate the immune system equally.

In general, the severity of an illness determines someone’s antibody levels. For instance, a person who has a high-grade fever for several days likely produces more antibodies to the virus than someone who only experiences a scratchy throat.

But that’s not always the case, Goepfert said. In a recent study, Goepfert’s team found that 36% of the participants who tested positive for COVID-19 developed little to no antibodies, despite having symptoms during their course of infection.

“I know a lot of people are testing their antibodies after they become infected and say, ‘well, I’ve got these antibodies, therefore I’m fine.’ But we know that that’s not necessarily the case,” Goepfert said.

A large number of antibodies is key for a good immune response, but scientists don’t yet have an easy way to judge the level of antibodies a person requires to be protected from severe outcomes of infection.

Plus, antibody levels are just one piece of the puzzle. Knowing an individual’s T and B cell levels could provide key information about their immune protection against reinfection. One study of symptomatic and asymptomatic COVID-19 cases found similar levels of T cells in both groups, even when antibodies waned.

However, testing for T cells can be difficult—there’s currently only one commercially available test.

Since immune responses to infection can be so disparate and antibody tests are relatively inaccessible, health officials continue to tout vaccination as the most sure-fire way to strengthen one’s immune system against COVID-19.

The vaccines have been proven again and again to be highly effective at preventing severe outcomes of COVID-19 and greatly minimize the risk of infection and transmission.

Timing Is Important

For a vaccine or infection to best stimulate the immune system, there must be a period of rest where the antibodies can clear from the bloodstream. When a new dose of antigen is introduced, immune cells are roused again and get another practice run at defending the body against the attacker.

Chin-Hong, who specializes in infectious diseases, said that “the more times you remind the immune system of the enemy, the better it’s going to perform in the future.”

The CDC said people are eligible to receive a booster shot five or more months after completing their primary regimen. The vaccination and booster schedule was designed to bolster the immune system around the time when most people’s antibody levels decline.

Natural immunity, on the other hand, doesn’t subscribe to a set schedule. It can be impossible to know when an individual will be infected, so the immune boost granted by an infection may not be at the most opportune time.

For instance, a person who received a two-dose regimen per the normal schedule may become infected a month after their second dose. In that case, their antibody levels may not have had a chance to wane and reactivate, meaning that individual doesn’t get much of a boost.

If an individual is infected later—say six months after the second shot—Goepfert said they might wait until the next surge to get a boost.

While it’s not unsafe to get a booster shot soon after recovering from COVID-19, some experience more severe side effects and the vaccine’s effectiveness may be diminished.

“You can’t continue to be activated to the same thing over and over again—your body has sort of defense mechanisms or regulatory mechanisms that go against that,” Goepfert said.

But Chin-Hong recommends not to wait for more than three months after recovery to get a booster dose. Recovery from COVID-19 could only be considered equivalent to one dose of vaccine, or possibly less. Someone who had a natural infection, but no vaccination, isn’t likely protected against reinfection or from transmitting the virus to their community, he added.

An Already Complicated Rollout

U.S. health officials made vaccine-focused recommendations and mandates, opting for vetted inoculations over antibody testing or accepting PCR test results as proof of immunity.

“Are we equipped as a system to understand and to verify documented infections? We probably could if we wanted to. It may just be logistically easier to look at a card and see a lot number, date, and vaccine to verify exposure,” Chin-Hong said.

With the spread of Omicron and possible future emergence of other variants, easy-to-edit mRNA vaccines may be key to a more targeted immunization approach than natural immunity can provide. Pfizer, Moderna, and Johnson & Johnson are developing Omicron-specific vaccines. Pfizer said it began phase one clinical trials last week.

“The vaccine can sort of keep up with variants moving forward, but natural immunity cannot,” Goepfert said.

Hybrid Immunity May Help in the Long Term

Both at a personal level and within a larger population, hybrid immunity—the combination of natural and vaccine-induced immunity—is the “holy grail” of an immune response, Chin-Hong said.

With the high case rates in the U.S., a majority of the population likely has some degree of natural immunity to COVID-19. Coupled with relatively large vaccination coverage, the country is well on its way to developing a hybrid immune system.

In the future, when the disease becomes endemic, Chin-Hong said showing some documentation of prior infection or antibody status might be enough to demonstrate immunity.

“My prediction is, we’re going to descend very rapidly from Omicron and the country and hopefully the world will have a lot more immunity. The dust will settle and then people will feel more relaxed about loosening some of what immunity means,” Chin-Hong said.

Until then, he said, “we want to maximally protect people in the best way we know how.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.