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The SARS-CoV-2 Omicron wave began when most populations had already experienced previous SARS-CoV-2 infection, vaccination, or both. To assess the effectiveness of these various immunologic backgrounds on incidence of symptomatic Omicron infection, investigators in Qatar performed a national test-negative, case-control study. Cases were categorized based on previous infection and vaccination histories (homologous BNT162b2 [Pfizer-BioNTech] or mRNA-1273 [Moderna]).
Effectiveness against symptomatic Omicron BA.1 infection was as follows (BNT162b2):
Previous infection without vaccination, 50.2%.
Two-dose vaccination without previous infection, 4.9% (i.e., negligible).
Three-dose vaccination without previous infection, 59.6%.
Two-dose vaccination with previous infection, 51.7%.
Three-dose vaccination with previous infection, 74.4%.
In all 5 groups:
Effectiveness was >90% against severe COVID-19, critical COVID-19, or death (collectively 0.3% of cases) caused by BA.1.
Effectiveness against symptomatic BA.2 infection was comparable to BA.1.
Effectiveness was >70% against severe, critical, or fatal disease caused by BA.2.
Results were comparable between BNT162b2 and mRNA-1273.
Altarawneh HN et al. Effects of previous infection and vaccination on symptomatic omicron infections N Engl J Med 2022 Jun 15; [e-pub]. (https://doi.org/10.1056/NEJMoa2203965)
Comment
The study adds to the evidence that, for Omicron-associated mild disease, hybrid immunity is more effective than that conferred by infection or vaccination (especially with longer intervals after vaccination), and that immunity gained from infection is relatively durable. Study limitations include observational design, short observation interval for the three-dose vaccination groups, and that only 9% of the Qatari population is older than 50. Importantly, against severe COVID-19 outcomes, all forms of immunity were highly effective — the silver lining of successive pandemic waves.