Natural immunity from previous infection remains strong against Omicron, particularly with respect to severe disease and death, a study from Qatar has found.
The study, published in the NEJM, included all reported infections in the country, excluding only those with Ct value over 30 to avoid non-infectious and false positives. The study is a follow-up to one we reported in December, with the new study including results for Omicron (we have previously linked to the pre-print version).
The researchers used a test-negative case-control design and matched cases to controls according to sex, 10-year age group, nationality, and calendar time of PCR testing to avoid biases and confounders. The limitations of a test-negative case-control design need therefore to be borne in mind.
The topline finding was that previous infection was 92% protective against Delta and 56% against Omicron – so reinfection is significantly more likely with Omicron but protection is still strong. Two Omicron reinfections became severe, but none became critical or fatal. Here are the result in full, with the table below.
The effectiveness of previous infection in preventing reinfection was estimated to be 90.2% (95% confidence interval [CI], 60.2 to 97.6) against the Alpha variant, 85.7% (95% CI, 75.8 to 91.7) against the Beta variant, 92.0% (95% CI, 87.9 to 94.7) against the Delta variant, and 56.0% (95% CI, 50.6 to 60.9) against the Omicron variant. Sensitivity analyses confirmed the study results, as expected for this study design, which is robust regardless of the approach that is used to control for vaccine-induced immunity. An additional analysis that was adjusted for the interval since previous infection also confirmed the study results.
Among the patients with reinfection, progression to severe COVID-19 occurred in one patient with the Alpha variant, in two patients with the Beta variant, in no patients with the Delta variant, and in two patients with the Omicron variant. None of the reinfections progressed to critical or fatal COVID-19. The effectiveness with respect to severe, critical, or fatal COVID-19 was estimated to be 69.4% (95% CI, −143.6 to 96.2) against the Alpha variant, 88.0% (95% CI, 50.7 to 97.1) against the Beta variant, 100% (95% CI, 43.3 to 100) against the Delta variant, and 87.8% (95% CI, 47.5 to 97.1) against the Omicron variant. Limitations of the estimations (e.g., the relatively young population of Qatar) are discussed in Section S1.
Overall, in a national database study in Qatar, we found that the effectiveness of previous infection in preventing reinfection with the Alpha, Beta, and Delta variants of SARS-CoV-2 was robust (at approximately 90%), findings that confirmed earlier estimates. Such protection against reinfection with the Omicron variant was lower (approximately 60%) but still considerable. In addition, the protection of previous infection against hospitalisation or death caused by reinfection appeared to be robust, regardless of variant.
Other studies, including one from Qatar, found vaccine immunity waned much more quickly than natural immunity. Natural immunity is topped up by repeated exposure to the same or similar viruses, which sometimes results in mild illness but, as the results from Qatar show, rarely progresses to anything serious.
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