Omicron variant now represents 99.5% of COVID infections, but less severe

Jan. 26, 2022

According to a new report from the Centers for Disease Control and Prevention (CDC) the emergence of the Omicron variant in December 2021 led to a substantial increase in COVID-19 cases in the United States. Although the rapid rise in cases has resulted in the highest number of COVID-19–associated ED visits and hospital admissions since the beginning of the pandemic, straining the healthcare system, disease severity appears to be lower than compared with previous high disease-transmission periods.

The B.1.1.529 (Omicron) variant of SARS-CoV-2, the virus that causes COVID-19, was first clinically identified in the United States on December 1, 2021, and spread rapidly. By late December, it became the predominant strain, and by January 15, 2022, it represented 99.5% of sequenced specimens in the United States.

In addition to lower ratios of ED visits, hospitalizations, and deaths to cases observed during the Omicron period, disease severity indicators were also lower among hospitalized COVID-19 patients, including ICU admission, receipt of IMV, length of stay, and in-hospital death. This apparent decrease in disease severity is likely related to multiple factors, most notably increases in vaccination coverage among eligible persons, and the use of vaccine boosters among recommended subgroups.

For example, during the Omicron period, 207 million persons were fully vaccinated compared with 178 million persons and 1.5 million persons during the Delta and the winter 2020–21 periods, respectively. Further, during the Omicron period, 78 million persons had received vaccine boosters compared with 1.6 million persons during the Delta period; boosters were not available during winter 2020–21. Other key factors for lower disease severity include infection-acquired immunity, and potential lower virulence of the Omicron variant.

These findings are consistent with reports from South Africa, England, and Scotland, as well as from health systems in California and Texas, where the Omicron variant was not associated with an increase in hospital or disease severity indicators among patients with Omicron infections compared with those with Delta infections. Death and in-hospital severity indicators, including in the context of vaccination status, should continue to be monitored for changes or differential effects among subpopulations throughout the Omicron period.

Among children aged <18 years, in-hospital severity indicators, including length of stay and ICU admission, were similar to and lower, respectively, during the Omicron period compared with those during previous high-transmission periods.

However, high relative increases in ED visits and hospitalizations were observed among children during the Omicron period, which might be related to lower vaccination rates in children compared with those in adults, especially among children aged 0–4 years who are currently not eligible for vaccination.

Children’s susceptibility to the Omicron variant and the impact of changes in exposure on severity risk require additional study. Among adults aged ≥18 years, all in-hospital severity indicators assessed were lower during the Omicron period, which might be related to increased population immunity against SARS-CoV-2 because of higher vaccination coverage and booster rates and previous infection providing protection. Receipt of a third mRNA vaccine dose was found to be highly effective at preventing urgent care encounters, ED visits, and hospital admissions during both Delta and Omicron periods. Booster doses were also found to be effective at preventing infection during the early Omicron period, particularly among persons aged ≥50 years.

CDC report

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