This "natural immunity" even provides a longer window of protection compared to mRNA vaccines, according to the study, which was led by Dr. Ari Robicseck, chief medical analytics officer at Providence, a health system in the Seattle area. Researchers from the University of Chicago also contributed to the study.

The study included 121,615 patients that tested for COVID-19 at 1,300 sites in six western U.S. states in the Providence healthcare system between October 1, 2020, and November 21, 2021. After 90 days of testing positive on an initial SARS-CoV-2 nucleic acid amplification test (NAAT), patients were monitored for subsequent COVID-19 infection, as determined by a positive SARS-CoV-2 NAAT result in the presence of symptoms.

Risk of SARS-CoV-2 reinfection among unvaccinated people with prior COVID-19 is a subject of debate. This study revealed that prior COVID-19 was associated with protection of 85% against any recurrent COVID-19, 88% against hospitalization for COVID-19, and 83% against COVID-19 not requiring hospitalization. Up to nine months from initial infection, protection remained stable over the study period with no attenuation.

Strengths of the study include its large sample size, long duration of follow up, and inclusion of only unvaccinated individuals with symptomatic COVID-19. Possible COVID-19 testing or vaccination at outside healthcare facilities is a possible limitation.

"We found that, before the emergence of the omicron variant, natural immunity provided a similar degree of protection against COVID-19 infection as mRNA vaccination," said Robicsek.