Prior COVID Infection or Vaccine: Herd Immunity

Researchers have potentially good news about how long immunity lasts after COVID-19 infection as well as the impact of combined natural plus vaccine immunity.

There’s no guarantee you’ll never contract a serious case of COVID-19. But developing a strong immune response that stops the coronavirus in its tracks can go far to keep you free of the infection.

There are three ways that immunity can develop.

First, there’s natural immunity. If you’ve tested positive for the coronavirus, even if the infection didn’t make you sick, your body typically made protective antibodies to fight the infection. Getting vaccinated is a proactive way to get protected; All of the COVID vaccines create antibodies to keep the coronavirus at bay. If you were infected with COVID previously and get vaccinated, you may have a powerful combined immunity.

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The good news: While more research is needed, evidence is mounting immunity may last longer than previously assumed.

Of course, even if most of the population has some immunity to COVID-19 eventually, that doesn’t mean the virus will completely disappear. In fact, it has mutated several times and may always linger and reemerge from time to time.

If immunity is widespread and fairly robust, however, future outbreaks will likely be manageable, potentially preventing widespread lockdowns and other serious societal and economic repercussions.

Immunity to the coronavirus is clearly building in the U.S. Cases of COVID-19 have dropped dramatically, and deaths from the virus have decreased.

Natural immunity plus vaccinations are helping beat COVID in the U.S.

Also known as “population immunity,” herd immunity describes the indirect protection from an infectious disease. It occurs when most of a population becomes immune either through vaccination or immunity they developed from previous infection, the World Health Organization (WHO) explains.

The U.S. is moving in the right direction when it comes to widespread immunity. More than 80 percent of Americans have received at least one dose of a COVID vaccine, and nearly 70 percent are fully vaccinated.

There’s no way to know how many people in a state who are not vaccinated have natural immunity to the virus from a prior infection, which could have been so mild or asymptomatic it was not documented. Everyone would have to be tested for antibodies to the coronavirus regularly to come up with a statistic showing who has ongoing natural immunity and who does not.

There’s little doubt natural immunity, acquired by folks who already had COVID-19 and recovered, is playing an important role in slowing the pandemic and helping the U.S. move toward an appreciable amount of herd immunity.

Scott Gottlieb, MD, a medical policy and public health expert who led the Food and Drug Administration during the Trump administration and now serves on Pfizer’s board of directors, says declining infection levels of COVID-19 infections are certainly the result of more Americans receiving vaccinations to protect from the coronavirus, but vaccinations don’t tell the whole story.

He believes fewer cases are also due to many unvaccinated people already having immunity to COVID from previous infections. Official U.S. statistics list more than 100 million confirmed COVID cases.

Evidence COVID-19 infection can produce strong protection

People who have contracted the virus and recovered develop antibodies against the infection. Although there are very rare reports of people having COVID more than once, the antibodies from past infection likely protect most people from catching it again and transmitting it to others. How long the protection lasts isn’t known exactly, but there’s reason to think it may last longer than previously assumed.

Some of the strongest evidence about the lasting power of post-infection antibodies, and other immune system factors revved up to keep people from having another bout of the infection, comes from a National Institutes of Health-funded study conducted by scientists from the Center for Infectious Disease and Vaccine Research at the La Jolla Institute for Immunology, the University of California’s Division of Infectious Diseases and Global Public Health, and infectious disease experts at the Icahn School of Medicine

The researchers analyzed immune cells and antibodies from almost 200 people who had been exposed to COVID and recovered from the infection. The results, published in Science, found the protection was strong, lasted for at least eight months, and declined only slightly six to eight months after infection.

Some virus fighting B cells increased over time, with the levels reaching a plateau after a few months and then not declining for the entire length of the study. T cells, another part of the immune system’s arsenal to fight off all viruses, also remained high for months after the infection.

Both the number of antibodies, as well as the amount of different immune cell types, varied substantially among the people tested. Eight months after their initial infection, the vast majority of research subjects — 95 percent — still had, at a minimum, three of five immune system components that protect against the COVID-19 causing virus.

“Several months ago, our studies showed that natural infection induced a strong response, and this study now shows that the responses last,” one of the leaders of the study, La Jolla Institute for Immunology scientist Daniela Weiskopf, PhD, said. “We are hopeful that a similar pattern of responses lasting over time will also emerge for the vaccine-induced responses.”

More good news about natural immunity

Scientists at Washington University School of Medicine have found people who’ve had a mild case of COVID-19 and recovered may have long-term protection against the disease because, post-infection, their immune cells are still pumping out antibodies against the virus. The study, published the journal Nature, even suggests mild cases of COVID-19 leave those infected with immune cell activations that may protect from new COVID-19 infections for a lifetime.

When a person is infected with a virus, antibody-producing immune cells immediately multiply and circulate in the blood, so antibodies are at first extremely high. Once the person is over the illness, most of these cells die off and antibody levels in the bloodstream decrease dramatically.

Some antibody producing cells, known as long-lived plasma cells, remain. According to the Washington University researchers, these cells travel to the bone marrow where they remain and keep secreting low levels of antibodies into the bloodstream to help protect against another infection with the virus. So, the Washington researchers decided to see if people who have recovered from mild COVID-19 cases might still have plasma cells in their bone marrow producing antibodies to fight the coronavirus (technically known as SARS-COV-2).

Working with blood samples taken over the course of several months from 77 research volunteers who had recovered from mild cases of COVID-19, and bone marrow samples from 18 of the research subjects seven or eight months after their initial sickness, the scientists compared their findings to bone marrow of 11 people who were never infected with COVID-19.

The results showed that almost all of the bone marrow samples from people who had been infected with COVID-19 contained antibody producing cells specifically targeting the coronavirus — but none of the people who had never been sick with COVID had those antibody producing cells in their bone marrow.

Although it’s a small study and more research is needed, it does raise the possibility that natural immunity may last far longer than was originally assumed. Next, Ellebedy and colleagues plan to study whether vaccinations for COVID-19 also result in long-lived antibody producing cells in the bone marrow.

A more recent large study, a meta-analysis of 65 studies, showed that protection from reinfection for most variants of the coronavirus (except for Omnicron) may last for nine to 10 months, but then antibodies will drop.

Combined immunity may be the strongest protection against COVID

What if you’ve had a documented bout of COVID-19? Does that mean you should skip vaccination? The CDC and other medical experts urge those who have had the virus to get vaccinated. (The only exception are people whose coronavirus infection was treated with monoclonal antibodies; they need to wait about 90 days before receiving the vaccine because the antibodies may neutralize the vaccine if given sooner.)

Scientists at Fred Hutchinson Cancer Research Center have produced laboratory evidence that patients who were infected with the coronavirus and recovered — and who are subsequently vaccinated against COVID-19 — have a significantly stronger combined immunity than people who were never infected with the coronavirus but received the vaccination.

The research team reported just one shot of the Pfizer or Moderna vaccines boosted the immune response against COVID-19 by as much as 1,000 times in people who had previously been infected with the coronavirus. The immune response also reacted strongly to the variants of concern first identified in South Africa.

The scientists found a second dose of COVD-19 vaccine in the previously infected didn’t boost the already strong immunity the first dose elicited. That suggests only one shot of a COVID-19 vaccine may be all that most people who already had the coronavirus may need to avoid another infection.

“I think it is encouraging that you can harness immune memory to SARS-CoV-2 and potentially neutralize forthcoming variants,” said Fred Hutchinson Cancer Research Center molecular and cellular biologist Andrew McGuire, PhD.

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