‘Natural Immunity’ Holds Up Against Severe Omicron Cases in Qatar Study

Prior SARS-CoV-2 infection was estimated to be less effective at preventing reinfection with the Omicron variant compared to prior variants, though it did appear to hold up against severe disease, researchers in Qatar found.

The effectiveness of previous infection in preventing reinfection was estimated at 56.0% (95% CI 50.6-60.9) against the Omicron variant, as compared to 90.2% (95% CI 60.2-97.6) against the Alpha variant, 85.7% (95% CI 75.8-91.7) against the Beta variant, and 92.0% (95% CI 87.9-94.7) against the Delta variant, reported Laith Abu-Raddad, PhD, of Weill Cornell Medicine Qatar in Doha, and colleagues.

However, no reinfections resulted in death, and effectiveness of natural infection against “severe, critical or fatal” COVID from the Omicron variant was 87.8% (95% CI 47.5-97.1), they wrote in a correspondence in the New England Journal of Medicine.

The authors noted that because Omicron “harbors multiple mutations that can mediate immune invasion,” they wanted to examine how effective prior infection was at preventing symptomatic new COVID cases.

Abu-Raddad’s group utilized a test-negative case-control study from Qatar’s national databases to determine “the proportional reduction in susceptibility to infection” among those who recovered from infection versus those who were uninfected. Only cases with a cycle threshold (Ct) of 30 or less were included, to ensure that only “epidemiologically relevant reinfections were considered.” Median age was 31-35, which was representative of the population of Qatar.

“Reinfection often occurs with negligible symptoms and high Ct values, indicating reduced epidemiologic significance,” the authors explained, based on their previous research.

They also conducted separate analyses where cases and controls were matched 1:5 according to sex, 10-year age group, nationality, and calendar week of PCR testing for patients with Alpha, Beta, and Delta (March 23 to Nov. 18, 2021) or 1:3 according to the same criteria for patients with Omicron (Dec. 23, 2021-Jan. 2, 2022).

They also excluded vaccinated patients and found that protection against reinfection with Omicron was 61.9% (95% CI 48.2-72.0).

One patient reinfected with Alpha progressed to severe COVID, as did two patients apiece who were reinfected with the Beta or Omicron variants , while no patients with the Delta variant did.

Abu-Raddad’s group noted that the young age of their population was a study limitation, but concluded nonetheless that “protection of previous infection against hospitalization or death caused by reinfection appeared to be robust, regardless of variant,” while protection against reinfection with Omicron was “lower” but “still considerable.”