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.
Protection waned after 4 or 5 months
A multivariable analysis showed an association between a third dose and reduced odds of COVID-19 hospitalization (34.7% of case-patients vs 49.3% of controls; adjusted odds ratio [aOR], 0.41, or a 59% reduction). The odds of hospitalization depended on time since the third dose (aOR at less than 50 days, 0.24; 50 to 100 days, 0.24; 101 to 150 days, 0.47; and after 150 days, 0.72).
The researchers noted that studies comparing COVID-19 rates among recipients of a booster dose and their unvaccinated counterparts have found 55% to 99% lower odds of COVID-19 among those boosted. "By matching cases with controls based on the date of second mRNA dose, this study was able to measure the added benefit of a booster dose to the primary series," they wrote.
Risk factors for COVID-19 hospitalization were age 70 years or older, male sex, cognitive impairment, chronic obstructive pulmonary disease, diabetes, impaired immune system, obesity, rheumatologic disease, history of organ transplant, and receipt of the Pfizer COVID-19 vaccine.
"This research shows us that even if you’re fully vaccinated, there's a real value to getting a booster," senior Ari Robicsek, MD, of Providence, said in a company news release. "Compared to people who only had their initial vaccinations, people with boosters were a lot less likely to have severe Covid for 4-5 months after the booster shot.”
The authors said that even if the added protection of a third dose wanes over time, the overall risk of hospitalization among vaccinated patients is still low.
