Vaccinated adults have a lower risk of severe illness due to the Omicron variant of coronavirus, and are less likely to land in intensive care compared with unvaccinated patients, a US study suggests. 

The study, published in a weekly report by the US Centers for Disease Control and Prevention (CDC), also found that during the Omicron period fewer patients died while hospitalised, compared with those admitted when the Delta variant was dominant.

"Overall, the Omicron-period group had a lower likelihood of being admitted to the intensive care unit (ICU) and were also less likely to require invasive mechanical ventilation compared with the Delta-period group," said Matthew Modes, a pulmonologist at Cedars-Sinai Medical Centre in the US, and co-first author of the study.

The researchers compared that group with 737 patients admitted with COVID-19 during December 2021-January 2022, when the Omicron variant was most prevalent. Clinical information was gathered from the electronic health records of the patients in the study.

The analysis shows that a greater portion of the patients hospitalised during Omicron was vaccinated as compared to patients hospitalised during the summer of 2021 when the Delta variant predominated. "In addition to the protection that vaccination offered people admitted to the hospital when Omicron dominated, we saw that the addition of a booster dose appeared to be particularly important in reducing the severity of illness, especially among older adults," said senior author of the study Peter Chen, from Cedars-Sinai.

"Unvaccinated patients hospitalised with COVID-19 during the Omicron variant dominance still had a higher chance of being admitted with serious complications and appeared to be at higher risk for the development of respiratory failure, compared with vaccinated patients," said Chen. The researchers noted that vaccination, including a booster dose for those who are fully vaccinated, remains critical for mitigating the risk of severe illness associated with SARS-CoV-2 infection.

"A clear pattern emerges if you take just the Omicron-period patients and compare their vaccination status against the percentage of them who ended up in the ICU," said Michael Melgar, a co-first author of the study and a medical officer with the CDC. "The more vaccinated someone is -- from unvaccinated, partially vaccinated, fully vaccinated without a booster dose to fully vaccinated with a booster dose -- the better the outcome for the patient," Melgar added.