The effectiveness of previous COVID-19 infection in preventing reinfection was estimated at 56% against the Omicron variant, compared with 90% against the Alpha variant, 86% against the Beta variant, and 92% against the Delta variant, reported Laith Abu-Raddad 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 letter to the New England Journal of Medicine.
The authors noted that because Omicron “harbours 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 used 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, or 1:3 according to the same criteria for patients with Omicron.
They excluded vaccinated patients and found that protection against reinfection with Omicron was 62%.
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 hospitalisation or death caused by reinfection appeared to be robust, regardless of variant”, while protection against reinfection with Omicron was “lower” but “still considerable”.
See more from MedicalBrief archives:
Infection + vaccination delivers better protection than immunisation only — Large US study
Vaccination 5x more effective against hospitalisation than prior COVID infection — CDC analysis
Previous COVID prevents Delta infection better than Pfizer shot — Israel study
Increased risk of SARS-CoV-2 reinfection associated with Omicron — South African study
Previous infection + vaccines deliver ’stronger than basic' Omicron defence — China in-vitro study