Post-infection protection—known widely as natural immunity—was strong and remained significant over time, researchers found. Against the Wuhan, Alpha, and Delta variants, the protection against re-infection was 85 percent at four weeks, 78 percent at 40 weeks, and 55.5 percent at 80 weeks.
That protection dropped more quickly against the Omicron BA.1 subvariant, declining to 36 percent by 40 weeks, and protection against symptomatic disease also waned below 50 percent.
But shielding against severe disease was strong against all strains, including the BA.1 subvariant, researchers found. The naturally immune enjoyed 88.9 percent protection against BA.1 at 40 weeks, which was actually higher than against earlier strains.
"Our analysis found significantly reduced protection against re-infection from the omicron BA.1 variant but that levels of protection against severe disease remained high," Dr. Stephen Lim of the Institute for Health Metrics and Evaluation at the University of Washington's School of Medicine, and his co-authors, said in the study.
Dr. Vinay Prasad, an epidemiologist at the University of California, San Francisco who was also not involved in the study, said that the paper made a "compelling case that we can effectively stop boosting average risk individuals (most adults) who have had covid."
"Vaccine policy should have been different in people with prior illness," Prasad also said.
Comparison to Vaccination
The researchers performed a review and meta-analysis by looking for studies on natural immunity conducted through Sept. 31, 2022. Studies were included if a group of naturally immune, unvaccinated people were compared to unvaccinated people who had not been infected. Studies that also included vaccinated people were included if the research also included unvaccinated and naturally immune people. Studies that only had results for natural immunity in combination with vaccination, or hybrid immunity, were excluded.Researchers performed a modeling technique called Bayesian meta-regression to reach pooled estimates of protection by time since infection.
In total, 65 studies were included in the meta-analysis from 19 different countries. Just 30, though, included information on time since infection, and a subset of those included information on one or more of the outcomes—re-infection, symptomatic disease, and severe disease—during the BA.1 era.
One of the researchers' main conclusions was that the study showed that natural immunity "is at least equivalent if not greater than that provided by two-dose mRNA vaccines," or the Pfizer and Moderna messenger RNA vaccines.
The researchers also emphasized that COVID-19 can cause problems, including death, but did not mention side effects from vaccination that can also cause long-term issues, including mortality.
Limitations included the low number of studies that were analyzed for the analysis and the reliance on observational studies.
Previous Research
Previous studies primarily found that natural immunity is superior to vaccination, including a study published in January. The U.S. Centers for Disease Control and Prevention (CDC) reported in 2022 that natural immunity was better against Delta. Research has also shown that previous infection doesn't protect as well against infection over time since Omicron displaced Delta, but remains strong against severe disease.Researchers in the new study said that policymakers should take into account the protection from prior infection, saying it "supports the idea that those with a documented infection should be treated similarly to those who have been fully vaccinated with high-quality vaccine," they said.
The researchers were not able to provide protection estimates against newer strains, like the BA.5 subvariant, due to a limited number of studies. They said that further assessments of natural immunity should be conducted in the future.