Numerous uncertainties and conflicting information continue to circulate regarding immunity to SARS-CoV-2. One frequently asked question is: "If I've had COVID-19, do I still need a vaccine?"
A recent study by the Centers for Disease Control and Prevention (CDC) has attempted to address the problem of vaccine-induced immunity versus infection-induced immunity.
Researchers from the CDC's VISION network collected data from 187 hospitals in nine states: New York, Minnesota, Wisconsin, Utah, California, Oregon, Washington, Indiana, and Colorado.
The study method
There were more than 200,000 admissions to these hospitals between January and September 2021 for COVID-19 similar illnesses.
The study recruited individuals aged 18 years or older who had previously been tested for SARS-CoV-2 infection at least two weeks before hospital admission and again around the time of admission. All participants had either been fully vaccinated three to six months prior or had a previous SARS-CoV-2 infection three to six months prior.
An analysis of the data showed that 7,348 people, or slightly more than 3% of the participants, met these study criteria.
Among these people, 1,020 had had the infection previously and were not vaccinated, while 6,328 were fully vaccinated people 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 virus's genetic material in the body.
What the team found
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 not vaccinated and 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 a COVID-19-like illness whose previous infection or vaccination occurred 90-179 days prior, vaccine-induced immunity was more protective than infection-induced immunity against laboratory-confirmed COVID."
Dr. William Schaffner, professor of medicine in the Division of Infectious Diseases at Vanderbilt University School of Medicine in Nashville, commented on the study during an interview with Medical News Today .
He said: “For a long time, we have 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 illness, and hospitalization caused by the SARS-CoV-2 Delta variant than immunity induced by the two-dose BNT162b2 vaccine.”
VISION researchers say this variation may be related to differences in the methods of the two studies and restrictions on the timing of the vaccines.
They also explain that the Israeli study “only examined vaccines that had been produced 6 months prior,” while the recent study recruited anyone who had received the vaccine between 3 and 6 months prior.
The need for more research
The authors of the CDC study point out several limitations.
For example, they explain: “If SARS-CoV-2 testing was carried out outside of network partners’ medical facilities or if vaccinated people are less likely to seek testing, some positive SARS-CoV-2 test results may have been missed, and therefore some people classified as vaccinated and previously free of infection could also have had the infection.”
Furthermore, the study only evaluated mRNA COVID-19 vaccines. Therefore, 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 Disease Serology Laboratory at the Mayo Clinic in Rochester, commented on the limitations of the study 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 compared to individuals who previously had the infection, who had higher testing rates in March-April. The reason for this was not discussed in the study,” Theel said.
Understanding immunity induced by infections and vaccines should be the focus of future studies, according to the authors. "The study focused on early protection from infection- and vaccine-induced immunity, although estimates may be affected by time."
When asked about possible next areas of research, Theel said, "One question I keep getting asked is whether a single dose of vaccine should be offered to [people who have had the infection previously], rather than two."
“The CDC indicates that people who have had the infection previously should get vaccinated, but there is no specific discussion on the issue of one or two doses of the mRNA vaccine. I think it would be beneficial to continue evaluating this in the future, especially given the vaccine hesitancy we continue to encounter, particularly among [people who have had the infection previously].”
The study concludes with a call to action: "All eligible people should get vaccinated against COVID-19 as soon as possible, including people who have not previously been vaccinated against SARS-CoV-2."