Care provider talking to a senior woman during home visit

A study of nearly 1,000 nursing home residents has nailed down the top factors that provide the best relative protection against infection with the SARS-CoV-2 omicron variant.

Investigators analyzed infection prevention data, such as vaccine type and number of shots in 17 Canadian nursing homes and eight retirement homes between March and November 2021. All study participants had received at least three mRNA vaccinations.

Data show strong protection

In total, 17% of residents had a presumed omicron-related SARS-CoV-2 infection between the local start of the first omicron wave in December 2021 and May 2022. Infection risk decreased by 47% with three doses of Moderna’s vaccine compared with Pfizer’s vaccine. In addition, infection risk fell by 81% with any fourth mRNA vaccine dose.

Hybrid immunity, defined as a prior SARS-CoV-2 infection plus vaccination, was also highly protective. An infection sustained within three months before the omicron wave began lowered subsequent infection risk by 48% in the vaccinated study subjects.

The risk of becoming infected with omicron did not differ by age or sex.

The results underscore the importance of vaccine type and the number of vaccine doses in maintaining protective immunity, lead author Jessica A. Breznik, PhD, of McMaster University in Canada reported. These factors are also key to reducing the risk of breakthrough infections, she and her colleagues added. 

“These findings promote continued public health efforts to support vaccination programs and monitor vaccine immunogenicity in older adults,” the researchers concluded.

Full findings were published in JAMDA.

Related articles: 

Study: Some 56% of people don’t know they are contagious, omicron study finds

CDC: Seniors receive 80% protection against severe omicron illness with fourth vaccine dose

CDC: LTC residents account for many omicron hospitalizations among the vaccinated (2022)

Best bet against omicron? Prior infection or a booster shot, early research suggests (2021)