People previously infected with SARS-CoV-2 are substantially protected against COVID-19 for several months, even in the absence of vaccination, a study found.
Beyond that, however, their natural immunity wanes but may be boosted with vaccination, researchers reported in findings from a study conducted among employees of the Cleveland Clinic between Dec. 16, 2020, and Dec. 27, 2021.
The study period included the emergence of the delta and omicron variants of SARS-CoV-2. The researchers found that natural immunity that is several months old does not protect against the omicron variant.
“The arrival of the omicron variant greatly changed the risk of COVID-19 for all individuals regardless of whether they were previously infected or not, and regardless of whether they were previously vaccinated or not,” Nabin K. Shrestha, MD, MPH, a staff physician in the Cleveland Clinic department of infectious diseases, told Healio. “Protection against COVID-19 from prior infection or vaccination may be of shorter duration than before the arrival of the omicron variant.”
Shrestha and colleagues included employees of Cleveland Clinic who were working on Dec. 16, 2020 the day vaccinations started. According to the study, anyone who tested positive for COVID-19 at least once before then was considered previously infected. The researchers examined the cumulative incidence of COVID-19, symptomatic COVID-19 and hospitalizations for COVID-19 over the next year.
“It’s important to note that the study was conducted in an active, health care employee population,” Shrestha said. “It included no children, very few elderly individuals, and likely few immunocompromised individuals.”
Among the 52,238 employees who were included, 9% were previously infected and 71% were vaccinated by the end of the study. The cumulative incidence of COVID-19 was “substantially higher” throughout the study for people previously uninfected with COVID-19 who remained unvaccinated than it was for all other groups although it was lower for the vaccinated than the unvaccinated and lower for those previously infected than those not, the researchers said.
Study data also showed that the incidence of COVID-19 increased “dramatically” in all groups after the emergence of omicron, although vaccination was associated with a significantly lower risk for symptomatic COVID-19 in both the pre-omicron (HR = 0.6; 95% CI, 0.4-0.9) and omicron (HR = 0.36; 95% CI, 0.23-0.57) phases.
“The omicron variant is substantially different from previous variants in the rapidity with which it spread and its ability to infect previously infected or vaccinated persons, something that was unusual with prior variants,” Shrestha said. “What applied to prior variants in terms of immunity and vaccine effectiveness should not be assumed to apply to the omicron variant without appropriate investigation.”