Study shows Omicron less severe than Delta in patients hospitalized with COVID-19

New research published on the medRxiv* The preprint server suggests that the Omicron variant produces less severe COVID-19 symptoms than earlier severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants.

There was a 74% reduction in the risk of ICU admission, a 91% reduction in the risk of death, and no patient infected with Omicron required mechanical ventilation. Additionally, Omicron-infected patients recovered faster and were discharged earlier than Delta-infected patients.

Although Omicron may be less severe than other variants, the spike in COVID-19 cases is devastating hospitals and causing severe burnout among frontline workers. An overload of COVID-19-related hospitalizations can lead to delays in medical care and potentially preventable deaths. The research team advises practicing several COVID strategies — wearing masks, isolating yourself if infected, and getting vaccinated — to limit community transmission of Omicron and relieve an overwhelmed healthcare system.

Study: Clinical outcomes among patients infected with the SARS-CoV-2 variant of Omicron (B.1.1.529) in Southern California. Image Credit: anushkaniroshan/Shutterstock

Increase in the rate of Omicron infections at the end of 2021

The researchers collected and analyzed data on positive cases of COVID-19 in the Kaiser Permanente Southern California health system from November 30, 2021 to January 1, 2022.

A total of 88,576 positive cases of COVID-19 were reported in outpatients from November 30, 2021 to January 1, 2022. Among hospitalized patients, approximately 26.6% tested positive.

People with Omicron cases were more likely to be in their 20s or 30s than Delta cases. Additionally, Delta infections were primarily from low-income neighborhoods, people who identify as white, and at high risk for comorbidities.

Omicron cases accounted for 75.5% of COVID-19 cases, including an additional 51.4% who were tested in a hospital setting.

Trend-wise, the number of COVID-19 cases linked to Omicron has been increasing every week. The only exception was Christmas week, when the holidays delayed the processing of tests.

Analyzes showed that repeat infections were more likely with Omicron than with Delta. People who had a COVID-19 infection 90 days or more since their current infection were 4.45 times higher among Omicron cases than Delta cases. In addition, vaccinated people were more likely to be infected with Omicron than with Delta.

When researchers collected samples from 1,477 infected patients for sequencing, 100% of SARS-CoV-2 isolates detected by S gene target failure belonged to Omicron.

Of the 288,534 people who were outpatients and followed up and then tested positive, 88 were infected with Omicron and required hospitalization. There have been 189 hospitalizations for Delta-related infections.

Attributes of cases with SGTF and non-SGTF samples detected.  Panels include (A) test dates for all cases analyzed (truncated to December 29, 2022 to account for jitter of ±1 day);  (B) age distribution of all analyzed cases;  (C) history of exposure (documented previous infection and vaccination) among all analyzed cases;  (D) symptomatic hospital admission dates (truncated to December 29, 2022 to account for jitter of ±1 day);  (E) age distribution of cases with symptomatic hospitalizations;  and (F) history of exposure (documented prior infection and vaccination) among symptomatic hospitalizations.  Pink and gray bars correspond to detections with and without SGTF (interpreted as a proxy for SARS-CoV-2 Omicron variant infection; Table S1), respectively.  Totals represent samples processed on the TaqPath COVID-19 High-Throughput RT-PCR Combo Kit and do not reflect all cases at KPSC.

Attributes of cases with SGTF and non-SGTF samples detected. Panels include (A) test dates for all cases analyzed (truncated to December 29, 2022 to account for jitter of ±1 day); (B) age distribution of all analyzed cases; (C) history of exposure (documented previous infection and vaccination) among all analyzed cases; (D) symptomatic hospital admission dates (truncated to December 29, 2022 to account for jitter of ±1 day); (E) age distribution of cases with symptomatic hospitalizations; and (F) history of exposure (documented prior infection and vaccination) among symptomatic hospitalizations. Pink and gray bars correspond to detections with and without SGTF (interpreted as a proxy for SARS-CoV-2 Omicron variant infection; Table S1), respectively. Totals represent samples processed on the TaqPath COVID-19 High-Throughput RT-PCR Combo Kit and do not reflect all cases at KPSC.

Disease severity in hospitalized patients infected with Omicron versus Delta

Of the 88 Omicron-infected patients admitted, 95.5% had symptoms. Similarly, 98.9% of patients infected with Delta were admitted with symptomatic infection.

About 7 patients infected with the Omicron variant required intensive care, including five who were first identified on an outpatient basis. One person died. None received mechanical ventilation.

Of those admitted with Delta variant infections, 23 were admitted to intensive care, 14 patients died, and 11 patients required ventilation.

The daily risk of needing mechanical ventilation was significantly higher in patients with Delta infections than in patients with Omicron infections.

Approximately 79.7% of Omicron infections reported acute respiratory symptoms prior to hospitalization, compared to 80% reported for Delta infections. Once hospitalized, approximately 9.4% of Omicron cases developed breathing problems. Nearly 16% of Delta infections developed respiratory symptoms during hospitalization.

Delays in severe progression in cases of SGTF and non-SGTF infections detected for the first time in outpatients.  Panels include (A) symptomatic hospital admissions;  (B) ICU admissions;  (C) initiations of mechanical ventilation;  and (D) mortality.  Inset charts in each panel show cumulative probabilities on the same y-axis scale as panel A. Shaded regions indicate 95% confidence intervals.  Green and purple correspond to detections with and without SGTF (interpreted as a proxy for SARS-CoV-2 Omicron variant infection), respectively.

Delays in severe progression in cases of SGTF and non-SGTF infections detected for the first time in outpatients. Panels include (A) symptomatic hospital admissions; (B) ICU admissions; (C) initiations of mechanical ventilation; and (D) mortality. Inset charts in each panel show cumulative probabilities on the same y-axis scale as panel A. Shaded regions indicate 95% confidence intervals. Green and purple correspond to detections with and without SGTF (interpreted as a proxy for SARS-CoV-2 Omicron variant infection), respectively.

Omicron cases showed higher hospitalization recovery rates than Delta

Approximately 83.5% of hospitalizations for Omicron cases were completed as of January 1, 2022, compared to 77.8% of hospitalizations for Delta.

All patients hospitalized with Omicron infection have been discharged home or to home care. Only 92.9% of Delta cases are discharged from hospital.

People with Omicron infections had nearly 70% shorter hospital stays than Delta infections. The average hospital stay for symptomatic Omicron infections was 1.5 days and 90% of patients were expected to recover within 3 days.

The average hospital stay for Delta infections was almost 5 days.

Study limitations

Researchers had a limited time to track all COVID-19 cases and their hospital discharge rate. Discharge data was available for 84% of Omicron cases and 78% of Delta cases. Also, the follow-up data for Omicron infections was shorter since they occurred towards the end of the study period.

Because COVID-19 testing in the Southern California Kaiser Permanente Health System may differ from testing in the general population, the rate of Omicron infections and hospitalizations compared to non-Omicron may differ at the population level.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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