Important context left out of COVID-19 natural immunity claims after CDC study release

Claims about natural immunity from COVID-19 and its effectiveness in preventing infection compared to the protection people receive from vaccinations have been circulating for months.

In a video posted on Jan. 24, 2022, YouTube personality John Campbell, Ph.D. – who identifies himself as a retired nurse and instructor – claimed that “if someone’s had the natural [COVID-19] infection and they’re subsequently vaccinated, it really doesn’t improve their level of protection.” Other social media users, referencing a study recently published by the CDC, have also made claims about natural immunity being “more protective than the COVID vaccine” (here and here).

Viewers Gail and Carrie also reached out to VERIFY via email with questions about protection for those with natural immunity from a prior infection compared to people who have received COVID-19 vaccines.

THE QUESTION

Does a study published by the CDC show that natural immunity is more protective against infection than the COVID-19 vaccine? 

THE SOURCES

• A study published in Centers for Disease Control and Prevention (CDC) Morbidity and Mortality Weekly Report (MMWR)
• Dr. Erica Pan, state epidemiologist for California Department of Public Health 
• Israeli study comparing natural-immunity to vaccine-induced immunity
• Johns Hopkins Medicine study 
• Dr. Marion Pepper, associate professor of immunology at the University of Washington School of Medicine

THE ANSWER

This claim needs context. The study published by the CDC shows natural immunity from a prior COVID-19 infection among those who survived is more effective against reinfection than vaccines only, but prior infection combined with vaccines offered even more protection. 

The study was also conducted during the delta variant wave – when vaccine-induced immunity was waning for many people and the highly contagious omicron variant had not yet taken hold in the U.S. It does not take booster shots into account. 

WHAT WE FOUND

Natural immunity is the antibody protection your body creates against a germ or virus once you’ve been infected with it, making it easier for your body to fight off future infection of the same virus, Johns Hopkins Medicine says. Natural immunity strength varies depending on the person and the virus. For viruses that have vaccines, such as COVID-19, people can also gain protection through vaccine-induced immunity.

Natural immunity and vaccine-induced immunity from COVID-19 both wane over time, Johns Hopkins Medicine says.

CDC study shows natural immunity from COVID-19 is effective against reinfection, but protection increases with vaccination

In January 2022, the CDC published findings from a study that took place from May 30 to Nov. 20, 2021, in its Morbidity and Mortality Weekly Report (MMWR). The study analyzed testing, surveillance, and COVID-19 immunization data from California and New York, which account for 18% of the U.S. population. 

It’s important to note that the data were collected before the widespread circulation of the highly transmissible omicron variant and before most people had received additional or booster COVID-19 shots to protect against waning immunity.

The study looked at four groups of adults: unvaccinated with no previous COVID-19 diagnosis; vaccinated (14 days after completing the two-dose COVID-19 vaccine series); unvaccinated with a previous COVID-19 diagnosis, and vaccinated with a previous COVID-19 diagnosis. It was conducted before most U.S. adults were eligible to receive booster shots.

During the week of May 30, 2021, COVID-19 incidence in both states was highest among people who were unvaccinated without a previous infection. Compared to this group, case rates were nearly 20 times (California) and nearly 18 times (New York) lower among vaccinated people without a previous COVID-19 infection; nearly 7 times (California) and nearly 10 times (New York) lower among unvaccinated people with a previous COVID-19 infection; and nearly 10 times (California) and nearly 9 times (New York) lower among vaccinated people with a previous COVID-19 infection. 

That means case rates were lowest at this time for people who were vaccinated and not previously infected with COVID-19. 

According to the study, these trends changed after the delta variant of COVD-19 became predominant in late June and July 2021. By early October, case rates were substantially lower among both unvaccinated and vaccinated people with previous infections. 

Compared to COVID-19 case rates among unvaccinated people without a previous COVID-19 infection, case rates were nearly 6 times (California) and nearly 5 times (New York) lower among vaccinated people without a previous COVID-19 infection; 29 times (California) and nearly 15 times (New York) lower among unvaccinated people with a previous infection; and nearly 33 times (California) and nearly 20 times (New York) lower among vaccinated people with a previous COVID-19 infection. 

This second set of case numbers collected in October – after delta spread – found that people who were vaccinated and not previously infected had higher rates of infection than those who were unvaccinated and previously infected.

But it’s important to take into account the timing of this study. The CDC notes that “infection-derived protection,” or natural immunity, was higher after the delta variant became predominant, a time when “vaccine-induced immunity” declined for many people. The CDC did not expand booster shot eligibility for all adults until Nov. 19, more than one month after the second set of data was collected.

Experts highlight one constant throughout both data sets: Case rates were lowest among vaccinated groups. 

Dr. Erica Pan, state epidemiologist for the California Department of Public Health, told VERIFY data in the report “clearly shows that vaccination provides the safest protection against COVID-19 and additional protection for individuals who have had a prior infection.”

“In addition, it shows that people who remain unvaccinated are at the greatest risk of hospitalization and death,” Pan wrote in a statement.

Other studies show similar results on natural immunity and vaccines

Other studies have also shown that natural immunity from a prior COVID-19 infection is effective at preventing reinfection, but natural immunity combined with the COVID-19 vaccine can offer even more protection against the virus. 

A study out of Israel published on Aug. 25, 2021, found that natural immunity gave people stronger and longer lasting protection against infection caused by the delta variant. The study is a preprint and has not yet been peer-reviewed. 

A team of researchers studied 2.5 million charts of patients from one of Israel’s largest health systems, Maccabi Healthcare Services. According to the data, people who were fully vaccinated with Pfizer between Jan. 1 and Feb. 28 were more at risk of a breakthrough infection with the delta variant, compared to those who had been previously infected and not vaccinated. Vaccinated individuals were “also at a greater risk for COVID-19-related hospitalizations compared to those that were previously infected,” the study said.

Of the more than 32,000 people studied who had been previously infected or were fully vaccinated, there were 238 breakthrough infections in fully vaccinated individuals and only 19 reinfections among unvaccinated people who had been previously infected.  

The findings did not include information about people who previously had COVID-19 and received both doses of the Pfizer vaccine.

RELATED: Yes, an Israeli study did find natural immunity is effective in fighting COVID-19, but health experts still recommend vaccination

A study from Johns Hopkins Medicine also looked at antibody measurements in people who were vaccinated versus those who were both vaccinated and previously infected with COVID-19. A research letter was published in the Journal of American Medicine in November 2021. 

Researchers enrolled 3,500 health care workers from the Johns Hopkins Health System starting June 2020 and followed up with them through Sept. 3, 2021. 

The study says 1,960 health care workers provided serum blood samples at least 14 days after receiving their second COVID-19 vaccine. Seventy-three of them had evidence of previous COVID-19 infection. 

Those with a prior COVID-19 infection maintained higher antibody measurements at one, three and six months after vaccination than those who were not previously infected, the study says. Researchers found that the relative differences were 14% higher at one month following the second vaccine dose, 19% at three months and 56% at six months. 

Vaccines improve immune response in those with previous COVID-19 infection, University of Washington professor’s research suggests

Dr. Marion Pepper, an associate professor of immunology at the University of Washington School of Medicine, helped to author a study that suggests vaccines improve immune response in those with a prior infection. The study found those who receive two Pfizer or Moderna mRNA vaccines followed by a booster shot can reach almost the same level of immunity as those who were previously infected with COVID-19 and then vaccinated. The study is a preprint and has not yet been peer-reviewed. 

During this study, researchers collected blood and plasma samples from people before vaccination, and one week, three months and six months after two mRNA vaccines. About eight months after initial vaccination, some participants received a third vaccine dose and researchers collected samples an average of two weeks later. Researchers have been tracking participants for more than a year. 

The study suggests that both COVID-19 infection and vaccination “induce multifaceted, functional immune memory,” but together they produce improved protection from disease, termed “hybrid immunity.” The study also shows that antibody neutralization of omicron increased after a person received their booster shot. 

“There have been a lot of studies that have shown that people who have had a prior infection and then been vaccinated have better immunity than those who are either infected or vaccinated alone,” Pepper said. “My lab has investigated the underlying principles of why that enhanced immunity might result from an infection plus vaccination. And we can see higher antibody titers, we can see better T cell responses, and we can see better maintained immune responses if you've had an infection followed by a vaccination.”

So we can VERIFY that studies show natural immunity to COVID-19 is effective at preventing reinfection, but vaccination after a prior COVID-19 infection gives you even more protection against the virus. Additionally, experts say vaccines are a safer way to build immunity than infection, as they help to prevent severe outcomes such as hospitalization and death.

More from VERIFY: Yes, vaccinated people can spread COVID-19 to others