A CDC study published Tuesday found that, while the omicron wave has led to record numbers of cases, a smaller proportion of these cases has led to severe outcomes and deaths. However, even with omicron's reduced severity, high patient volumes continue to stress hospitals across the country.

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Study details and key findings

For the study, researchers used data from three surveillance systems and a health care facility database to analyze data related to Covid-19 during three periods of high viral transmission:

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• The current omicron period (Dec. 19, 2021 to Jan. 15)
• The delta period (July 15, 2021, to Oct. 31, 2021)
• The 2020-2021 winter surge (Dec. 1, 2020, to Feb. 28, 2021)

Overall, the researchers found that the maximum daily seven-day average of cases, ED visits, and hospital admissions during the current omicron surge outpaced both the delta and prior winter surges. In particular, Covid-19 patients in the omicron period used a maximum of 21% of staffed inpatient beds—seven and three percentage points higher than patients in the delta and winter surge periods, respectively.

However, when comparing event-to-case ratios for all three periods, the omicron period has resulted in fewer ED visits, hospitalizations, and deaths compared with both the delta and previous winter surges, respectively:

• ED visits: 87 per 1,000 cases (vs. 167 and 92 per 1,000 cases)
• Hospitalizations: 27 per 1,000 cases (vs. 78 and 68 per 1,000 cases)
• Deaths: 9 per 1,000 cases (vs. 13 and 16 per 1,000 cases)

In addition, the percentage of patients admitted to the ICU during omicron (13%) was lower than during the delta (17.5%) and previous winter (18.2%) surges, and fewer hospitalized patients have required mechanical ventilation during omicron than the two other surge periods. Covid-19 patients during the omicron period also experienced shorter hospital stays (5.5 days) compared with delta (7.6 days) and the previous winter (8.0 days).

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According to the Washington Post, these findings are consistent with those from other recent studies from health systems in California and Texas, as well as from other countries, such as South Africa, England, and Scotland. For example, a preprint study from Kaiser Permanente Southern California that examined nearly 70,000 Covid-19 cases found that omicron infections were associated with significantly lower risks of hospitalization, ICU admission, and death than delta infections.

The reduced disease severity seen with omicron is likely due to several factors, the authors of the CDC study wrote, with the most notable being increased vaccination coverage. Vaccination rates increased from 1.5 million people during the previous winter surge to 178 million during delta and 207 million during omicron. Booster uptake also increased between the delta and omicron periods from 1.6 million people to 78 million people.

Increased immunity through infection and potentially lower virulence from the variant itself many also have influenced the severity of outcomes. However, the authors noted that some groups, including the unvaccinated, remain at an increased risk for severe Covid-19.

"People with underlying conditions, people with advanced age, people who are unvaccinated, can have a severe form of Covid-19, following infection from omicron,” said Maria Van Kerkhove, an epidemiologist at the World Health Organization. "People are still being hospitalized with this variant, as well as dying."

In addition, the authors noted that, despite omicron potentially being less severe, "the high volume of ED visits and hospitalizations can strain local health care systems in the U.S., and the average daily number of deaths remains substantial."

Overall, the authors emphasized the importance of "national emergency preparedness, specifically, hospital surge capacity and the ability to adequately staff local health care systems when critical care needs arise and before the system is overwhelmed." (Sun, Washington Post, 1/25; Carbajal, Becker's Hospital Review, 1/25; Walker, MedPage Today, 1/25; Iuliano et al., Morbidity and Mortality Weekly Report, 1/25)