Three doses of an mRNA COVID-19 vaccine during a time of Omicron variant predominance were tied to a 59% lower odds of hospitalization than two doses, although protection waned over time, finds a US case-control study published late last week in JAMA.

A team led by a University of Chicago researcher used electronic health record data to estimate the odds of COVID-related hospitalization after the receipt of only the primary vaccination series (two doses) or a third (booster) dose of an mRNA vaccine among adults admitted to hospitals in the Providence Health & Services network in one of six Western states from Oct 1, 2021, to Jul 26, 2022. During the study period, 81% of cases were attributed to the Omicron variant.

Each of the 3,062 COVID-19 patients were matched 1:4 with 12,248 control patients admitted to the hospital for non-COVID indications within 3 days of the case-patient in the same geographic region and received a second vaccine dose (Pfizer/BioNTech or Moderna) within 7 days of the case-patient. Average age was 70.8 years in case-patients and 67.1 in controls, and proportions of men were 52.6% and 46.7%, respectively.