Researchers from the University of California, Los Angeles, (USC) University of Southern California (USC), and the Cleveland Clinic conducted a meta-analysis of studies probing the risk associated with SARS-CoV-2 reinfection among those previously infected with SARS-CoV-2. Searching scientific publications on PubMed and the preprint server medRxiv from the pandemic start until August 18, 2021, the investigators pinpointed studies on a range of established criteria helping to identify for a comprehensive overview of findings. Using broad enough terms to include all relevant studies, the researchers excluded studies that failed to describe cohorts with estimates involving the risk of COVID-19 infection among previously infected persons. With a number of acknowledged limitations, the authors declared that based on the findings, natural immunity associated with SARS-CoV-2 infection is relevant and significant. They declared, “The protective effect of prior SARS-CoV-2 infection on re-infection is high and similar to the protective effect on vaccination.” Obviously more evidence—meaning additional studies—is necessary to better understand, systematically, the duration of protection and the impact of various variant strains of COVID-19.

While federal authorities such as U.S. Health and Human Services in America have all but ignored natural immunity, mounting data indicate that previous SARS-CoV-2 infection either leads to comparable or even superior immunity to the type afforded by the mRNA COVID-19 vaccination. 

In this study led by UCLA’s Noah Kojima, MD, prominent infectious disease researcher Dr. Jeffrey Klausner from USC, and Cleveland Clinic’s Nabin Shrestha, MD, the trio sought to identify the most relevant studies coupled with applicable control groups. In doing so, they established important criteria for relevant studies, including the following:

1Baseline polymerase chain reaction (PCR) testing
2A uninfected comparison group
3Longitudinal follow-up
4Cohort of human participants (e.g. not a case report or case series)
5Outcome determined by PCR

Following PRISMA guidelines, the esteemed physician-researchers identified 1,392 reports, including ultimately only 10 that met their stringent eligibility criteria for systematic review.


The authors write that there was a weighted risk reduction against reinfection of 90.4%, including a standard deviation of 7.7% (p-value: <0.01). The authors discovered that natural immunity appears to last at least ten (10) months.  However, the studies they did base their findings on could include selection, information, and analysis bias. But they confidently conclude, “The Protective effect of prior SARS-CoV-2 infection on re-infection is high and similar to the protective effect of vaccination.”

Lead Research/Investigator

Noah Kojima, MD

Jeffrey Klausner, USC

Nabin Shrestha, MD, the Cleveland Clinic

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