A new study of COVID-19 patients who had the omicron variant of the disease shows that vaccinated adults had less severe illness than unvaccinated adults and were less likely to land in intensive care. (photo courtesy of Getty/Cedars-Sinai)

The highly contagious omicron variant became the dominant strain in the U.S. in mid-December 2021, coinciding with a rise in hospitalizations of patients with COVID-19.

Among those admitted during the surge, vaccinated adults had less severe illness compared with unvaccinated adults and were less likely to land in intensive care, according to a new study by Cedars-Sinai and the Centers for Disease Control and Prevention.

“Overall, the omicron-period group had a lower likelihood of being admitted to the intensive care unit and were also less likely to require invasive mechanical ventilation compared with the delta-period group,” said Dr. Matthew Modes, a pulmonologist at Cedars-Sinai and co-author of the paper.

Investigators also found that, during the omicron period, fewer patients died while hospitalized (4%) compared with those admitted when the delta variant was dominant (8.3%).

In a single-hospital study published in the CDC’s Morbidity and Mortality Weekly Report, scientists looked at the characteristics of 339 patients hospitalized with COVID-19 at Cedars-Sinai Medical Center from July to September 2021, when the delta variant was dominant. They compared that group with 737 patients admitted with COVID-19 from December to January 2022, when the omicron variant was most prevalent.

Clinical information was gathered from the electronic health records of the patients in the study and then analyzed by a team of investigators led by Dr. Sharon Isonaka, chief value officer and vice president for clinical efficiency and value at Cedars-Sinai.

The analysis revealed that a greater portion of the patients hospitalized during omicron were vaccinated as compared to patients hospitalized during summer 2021, when the delta variant predominated, likely reflecting the higher percentage of the populations that were vaccinated during omicron.

“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 Dr. Peter Chen, senior author of the study and director of pulmonary and critical care medicine at Cedars-Sinai. “Unvaccinated patients hospitalized 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.”

Large numbers of hospitalizations during the pandemic have strained health systems throughout the country. Vaccination, including a booster dose for those who are fully vaccinated, remains critical for mitigating the risk of severe illness associated with COVID-19 infection.