In Portugal risk of hospitalization from Omicron is 75% lower than Delta

Researchers from Portugal reported a 75% risk reduction in hospitalizations for individuals infected with Omicron variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) compared to the Delta variant and a shorter length of stay and lower mortality if hospitalization does occur. Their paper is currently available on the medRxiv* preprint server while undergoing peer review.

Study: Omicron (BA.1) SARS-CoV-2 variant is associated with reduced risk of hospitalization and length of stay compared with Delta (B.1.617.2). ​​​​​​​Image Credit: NIAIDStudy: Omicron (BA.1) SARS-CoV-2 variant is associated with reduced risk of hospitalization and length of stay compared with Delta (B.1.617.2). ​​​​​​​Image Credit: NIAID

The Omicron variant (B.1.1.529) was initially reported to the World Health Organization (WHO) from South Africa on November 24, 2021. What was evident immediately was how easy this variant can spread and infect people – even among those vaccinated against coronavirus disease 2019 (COVID-19).

And indeed, the first real-world studies conducted in Denmark, England, and Scotland demonstrated reduced vaccine effectiveness for symptomatic Omicron infections following both complete vaccination schemes and booster doses.

But although studies regarding vaccine effectiveness for symptomatic infections are very informative, severity studies are also needed since these can inform about the potential impact on hospitalizations and COVID-19 deaths due to the Omicron SARS-CoV-2 variant.

Early reports showed that Omicron could be less severe, and this was corroborated in animal models, where Omicron showed less pronounced symptoms and lower viral load in the lungs. Moreover, in a real-world setting, reduced severity was confirmed in studies in the United States (US), Canada, England, and Scotland.

Recently, a research group led by Dr. André Peralta-Santos from the Universidade NOVA de Lisboa in Portugal aimed to appraise the risk of hospitalization and deaths from Omicron infections in a population with high vaccination coverage, but also to assess differences in viral loads and length of stay in hospitalizations.

A cohort study on infected patients

The research approach was envisioned as a cohort study in individuals with a positive polymerase chain reaction (PCR) test to SARS-CoV-2 infection during the month of December 2021. SARS-CoV-2 variants were initially classified by whole genomic sequencing (WGS) or by detecting the target failure in the gene for spike glycoprotein.

Consequently, this resulted in a total study sample of 15,978 participants older than 16 years of age, of which 9,397 were infected by Delta strain and 6,581 were infected with Omicron strain of the SARS-CoV-2.

The scientists considered a hospitalization for all the patients admitted within fourteen days after the infection. Various statistical methods have been used to compare the risk of hospitalization between Omicron and Delta variants of concern, as well as to assess the risk of death and the mean length of hospital stay.

Lower burden of disease for Omicron variant

In short, this study has found a 75% risk reduction of hospitalization in those study participants infected with the Omicron variant when compared to the Delta variant. More specifically, within the Delta group, a total of 148 infected individuals were hospitalized, while it was only 16 of them in the Omicron group.

The risk of death was also notably lower, as a total of 26 deaths were reported – all of them in participants infected with the Delta SARS-CoV-2 variant. Additional statistical analysis has shown a reduction in the odds of death of 86% when infected with Omicron compared to Delta infections.

It is also important to emphasize that those infected with the Omicron variant had a shorter length of hospitalization than those infected with Delta strain, which was approximately four days. This fact alone has significant ramifications for the management of hospital services.

Milder impact of Omicron waves

Taking everything into consideration, the impact of Omicron waves on health systems worldwide will be much milder compared with other variants of concern, especially the Delta variant.

“Our results also indicate a reduction in the risk of death of participants infected with Omicron, although with high uncertainty”, say study authors in this medRxiv paper. “Our study might be underpowered to study this risk reduction, but a reduction in the risk of death is plausible given the animal models showing less severe inflammation in the lungs, the preserved T cell function in vitro and a reduced risk in hospitalizations in humans”, they emphasize.

And although certain factors have not been taken into consideration (such as baseline differences and previous comorbidities in the patients included in the study), these findings corroborate many earlier observations regarding the Omicron variant of concern.

*Important notice

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

Journal reference:
Dr. Tomislav Meštrović

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.

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