Omicron appears to cause lower disease severity but higher number of cases
The U.S. has experienced the pandemic’s highest numbers of COVID-19 cases and hospitalizations during the Omicron surge, but disease severity indicators — including length of stay, ICU admission, and death — were lower than during previous pandemic peaks, the Centers for Disease Control and Prevention (CDC) reported in a new study.
The CDC added a cautionary note to the positive results. “Although disease severity appears lower with the Omicron variant, the high volume of hospitalizations can strain local healthcare systems and the average daily number of deaths remains substantial. This underscores the importance of national emergency preparedness, specifically, hospital surge capacity and the ability to adequately staff local healthcare systems,” CDC researchers wrote in an early-release study for Morbidity and Mortality Weekly Report.
The CDC assessed multiple indicators across three high-COVID-19 transmission periods: December 1, 2020-February 28, 2021 (winter 2020-21); July 15-October 31, 2021 (Delta predominance); and December 19, 2021-January 15, 2022 (Omicron predominance).
The highest daily seven-day moving average of daily cases occurred during January 9-15, 2022, with 798,976 daily cases, 48,238 emergency department visits, and 21,586 hospital admissions. However, the highest daily seven-day moving average of deaths (1,854) was lower than during previous periods.
During the Omicron period, a maximum of 20.6% of staffed inpatient beds were in use for COVID-19 patients, which was 3.4 and 7.2 percentage points higher than during the winter 2020-21 and Delta period, respectively. However, intensive-care-unit (ICU) bed use did not increase to the same degree: 30.4% of staffed ICU beds were in use for COVID-19 patients during the Omicron period, 0.5 percentage points lower than during the winter 2020-21 period and 1.2 percentage points higher than during the Delta period.
Among hospitalized COVID-19 patients from 199 U.S. hospitals, the mean length of stay and percentages of patients admitted to an ICU, received invasive mechanical ventilation (IMV), and died while in the hospital were lower during the Omicron period than during previous periods.
In the study, CDC researchers attribute the decrease in disease severity to increases in vaccine coverage, use of vaccine boosters, infection-acquired immunity, and the possibility that Omicron is less virulent than earlier variants.
