Healthcare worker tends to a patient in the COVID-19 Unit at United Memorial Medical Center in Houston, TX on July 2, 2020Share on Pinterest
A healthcare professional tends to a person in the COVID-19 unit at United Memorial Medical Center in Houston, TX, on July 2, 2020. MARK FELIX/Getty Images
  • A previous infection with SARS-CoV-2 and COVID-19 vaccination can provide immunity and protection against future illness.
  • A new study has compared the level of immunity afforded by a previous infection with the protection provided by a COVID-19 vaccine.
  • The results suggest that COVID-19 mRNA vaccines are around five times more effective at preventing hospitalization than a previous infection.

Numerous uncertainties and conflicting information continue to circulate around SARS-CoV-2 immunity. One commonly asked question is, “If I have had COVID-19, do I still need to receive a vaccine?”

A recent study from the Centers for Disease Control and Prevention (CDC) has attempted to address the issue of vaccine-induced versus infection-induced immunity.

Researchers from CDC’s VISION network gathered data from 187 hospitals across nine states. These were New York, Minnesota, Wisconsin, Utah, California, Oregon, Washington, Indiana, and Colorado.

There were over 200,000 admissions to these hospitals between January and September 2021 for COVID-19-like illnesses.

The study recruited people who were 18 years and older and who had been tested for SARS-CoV-2 infection previously, at least 2 weeks before their admission to the hospital and again around the time of admission. All participants had either been fully vaccinated 3–6 months earlier or had a previous SARS-CoV-2 infection 3–6 months prior.

An analysis of the data showed that 7,348 people, or a little over 3% of the participants, met these study criteria.

Among these people, 1,020 had previously had the infection and were unvaccinated, while 6,328 were fully vaccinated individuals who had not had the infection previously.

Next, the researchers looked for people within these groups who had a laboratory-confirmed SARS-CoV-2 infection through a COVID-19 polymerase chain reaction (PCR) test. This tool, also called a molecular test, detects the genetic material of the virus in the body.

Among the 6,328 people who were fully vaccinated, 324 (or 5.1%) had a positive COVID-19 PCR test. Among the 1,020 who were unvaccinated and who had previously had the infection, 89 (or 8.7%) had a positive COVID-19 PCR test.

According to the study authors, “These findings suggest that among hospitalized adults with COVID-19-like illness whose previous infection or vaccination occurred 90–179 days earlier, vaccine-induced immunity was more protective than infection-induced immunity against laboratory-confirmed COVID-19.”

Dr. William Schaffner, a professor of medicine in the Division of Infectious Diseases at Vanderbilt University School of Medicine in Nashville, TN, commented on the study during an interview with Medical News Today.

He said: “For a long time, we’ve known that people [who have the infection], if they are subsequently vaccinated, will have much higher levels of antibodies than people who [have the infection] but are not vaccinated. Natural infection plus vaccination is better than natural infection alone.”

The study authors refer to a recent Israeli study that reported conflicting results. The Israeli study found that “natural immunity confers longer lasting and stronger protection against infection, symptomatic disease, and hospitalization caused by the Delta variant of SARS-CoV-2 [than] the BNT162b2 two-dose vaccine-induced immunity.”

The VISION researchers say that this variation may be related to differences in the methods of the two studies and restrictions on the timing of vaccinations.

They also explain that the Israeli study “only examined vaccinations that had occurred 6 months earlier,” whereas the recent study recruited anyone who had received their vaccine 3–6 months previously.

The authors of the CDC study note several limitations.

For example, they explain, “If SARS-CoV-2 testing occurred outside of network partners’ medical facilities or if vaccinated [people] are less likely to seek testing, some positive SARS-CoV-2 test results might have been missed, and thus some [individuals] classified as vaccinated and previously [free from infection] might also have [had the infection].”

Also, the study assessed COVID-19 mRNA vaccines only. So, people should not generalize the results to non-mRNA vaccines, such as the Johnson & Johnson vaccine.

Elitza Theel, Ph.D. — the director of the Infectious Diseases Serology Laboratory at the Mayo Clinic in Rochester, MI — commented on the study’s limitations in an interview with MNT.

“There was a notably higher rate of index tests for COVID-19 performed during July–August among previously vaccinated individuals versus individuals [who previously had the infection] who had higher testing rates in March–April. The reason for this was not discussed in the study,” said Theel.

Understanding infection-induced and vaccine-induced immunity should be the focus of future studies, according to the authors. “The [study] focused on the early protection from infection-induced and vaccine-induced immunity, though it is possible that estimates could be affected by time.”

When asked about possible next areas of research, Theel said, “One question that I continue to be asked about is whether a single vaccine dose should be offered to [people who have previously had the infection], rather than two.”

“The CDC indicates that [individuals who have previously had the infection] should be vaccinated, but there is no specific discussion on the issue of one versus two mRNA vaccine doses. I think it would be beneficial to assess this further moving forward, especially given the vaccine hesitancy we continue to encounter, particularly among [people who have previously had the infection].”

The study concludes with a call to action: “All eligible persons should be vaccinated against COVID-19 as soon as possible, including unvaccinated [people who have previously had] SARS-CoV-2.”