COVID-19 vaccines up protection from previous infection against OmicronPrevious non-Omicron SARS-CoV-2 infection was tied to a 44% lower risk of Omicron reinfection and 81% lower odds of related hospitalization, which was further improved by one, two, or three mRNA COVID-19 vaccine doses (86%, 94%, and 97%, respectively), according to a study published late last week in JAMA Network Open.Canadian researchers conducted a test-negative case-control study of 224,007 infected participants and 472,432 uninfected controls aged 12 years and older tested for COVID-19 in Quebec from Dec 26, 2021, to Mar 12, 2022, before the emergence of the more highly contagious Omicron BA.4 and BA.5 subvariants.In the respective cohorts, 62.2% and 63.9% of participants were female, 87.4% and 75.5% were aged 18 to 69 years, and 4.2% of infected participants and 6.3% of controls had been previously infected with a non-Omicron strain.In unvaccinated participants, previous non-Omicron infection was linked to a 44% lower risk of Omicron infection; protection was 66% 3 to 5 months post-infection, falling to 35% at 9 to 11 months and to under 30% thereafter.
Severe previous infection conferred the greatest protection.Among previously infected participants, mRNA COVID-19 vaccination was associated with risk reductions in Omicron infection of 65% after one dose (vs 20% among the unvaccinated), 68% after two doses (vs 42%), and 83% after three doses (vs 73%).Estimated protection against Omicron-related hospitalization among previously infected participants was 81%, rising to 86% after one vaccine dose, 94% after two doses, and 97% after three doses, with no evidence of waning.The study authors noted that Omicron is the most highly transmissible and