SARS-CoV-2 Omicron infections were more common and linked to more severe outcomes than influenza and respiratory syncytial virus (RSV) in emergency department (ED) patients in Sweden, especially among those unvaccinated against COVID-19.

The findings were published yesterday in Clinical Infectious Diseases.

Karolinska Institutet researchers assessed rates of 30-day all-cause death, hospitalization, and intensive care unit (ICU) admission of adults seen in six EDs in Stockholm County for Omicron, flu, or RSV infection in 2021-2022 and 2015-2019. Of 6,385 patients in 2021-2022, 76% were infected with Omicron, 17% had flu, and 7% had RSV.

In total, 22.1% of Omicron patients were unvaccinated against COVID-19 before their ED visit, while 75% had received at least two doses.

Lower odds of death after COVID vaccination

Thirty-day death rates were 7.9% in the Omicron group, 2.5% in flu patients, and 6.0% in the RSV group. Omicron patients' adjusted death odds ratio (OR) was 2.36 compared with flu and 1.42 compared with RSV. Stronger links were seen in unvaccinated Omicron patients relative to flu (OR, 5.51) and RSV (OR, 3.29).

Death rates at 30 days were 15% among Omicron patients aged 75 years or older, compared with 8% in the 2021-2022 flu group, 12% in the 2021-2022 RSV cohort, 7% among 5,709 prepandemic flu patients, and 9% among 955 prepandemic RSV patients. Among Omicron patients who received at least two COVID-19 vaccine doses, the OR for death was 2.00 compared with flu and 1.20 compared with RSV.

This underscores the need for public health strategies for managing and mitigating the impact of Omicron and other viral respiratory infections, with continued assessments of their comparative severity.

Ninety-day mortality was 11% among Omicron patients, 3.7% in 2021-2022 flu patients, 8.7% in 2021-2022 RSV patients, 5.1% in the prepandemic flu group, and 9.6% in prepandemic RSV patients.

In 2021-2022, 30-day mortality was 2.6% for influenza A patients; none of the 17 influenza B patients died. From 2015 to 2019, 30-day mortality was 3.2% in influenza A patients and 3.8% in influenza B patients.

"This underscores the need for public health strategies for managing and mitigating the impact of Omicron and other viral respiratory infections, with continued assessments of their comparative severity," the authors wrote.

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