“Effectiveness of primary infection against severe, critical, or fatal COVID-19 reinfection was 97.3 percent … irrespective of the variant of primary infection or reinfection, and with no evidence for waning. Similar results were found in sub-group analyses for those ≥50 years of age,” Dr. Laith Abu-Raddad, with Weill Cornell Medicine-Qatar, and colleagues said after studying long-term natural immunity in unvaccinated people.
That percentage is higher than the protection from COVID-19 vaccines, according to other studies and real-world data.
Swedish researchers, for instance, found in May that two doses of a vaccine were just 54 percent effective against the Omicron variant of COVID-19.
South African scientists, meanwhile, found the effectiveness of the AstraZeneca and Pfizer vaccines peaked at 88 percent, and quickly dropped to 70 percent or lower.
The Qatar group found natural immunity for over 14 months after a person’s first infection “remains very strong, with no evidence for waning, irrespective of variant.”
The study was published ahead of peer review on the website medRxiv.
Few researchers have studied natural immunity long-term among unvaccinated persons, in part because many of the people have eventually received a COVID-19 vaccine.
The vaccines, meanwhile, have waned against both infection and severe illness over time, triggering recommendations for booster doses, with some Americans even getting five doses within 10 months.
Natural Immunity Performs Poorly Against Omicron Reinfection
The vaccines were once said to provide close to 100 percent protection against symptomatic infection. They now provide less than 50 percent protection against infection after a short period of time, even after booster doses, following the emergence of Omicron.
That strain and its subvariants are dominant in countries around the world, including the United States and Qatar.
Pre-Omicron primary infection against pre-Omicron reinfection was as high as 90.5 percent, and remained around 70 percent by the 16th month, according to the study. But pre-Omicron primary infection against Omicron reinfection was just 38 percent effective, although it was higher among people infected with the original Wuhan strain or with the Delta variant, and lower among those who got sick from the Alpha or Beta strains.
Modeling signaled a drop to zero percent protection by 18 months, but the shielding still appears to last longer than that of vaccines, researchers said.
“Vaccine immunity against Omicron subvariants lasts for <6 months, but pre-Omicron natural immunity, … may last for just over a year,” they wrote.
Limitations of the study included differences in testing frequency among the cohorts studied, and depletion of the groups who had a COVID-19 infection, due to their deaths.