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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)

by André Peralta-Santos, Eduardo Freire Rodrigues, Joana Moreno, Vasco Ricoca, Pedro Casaca, Eugenia Fernandes, João Paulo Gomes, Rita Ferreira, Joana Isidro, Miguel Pinto, Vítor Borges, Luís Vieira, Sílvia Duarte, Carlos Sousa, José Pedro Almeida, Luís Menezes, Bibiana I. Ferreira, Ana Matias, Ana Pelerito, Samanta Freire, Teresa Grilo, Cláudia Medeiros Borges, Vera Moutinho, Andreia Leite, Irina Kislaya, Ana Paula Rodrigues, Pedro Pinto Leite, and Baltazar Nunes

Posted: January 23, 2022
Server: medRxiv
DOI: 10.1101/2022.01.20.22269406

Introduction

Early reports showed that Omicron (BA.1) SARS-CoV-2 could be less severe. However, the magnitude of risk reduction of hospitalization and mortality of Omicron (BA.1) infections compared with Delta (B.1.617.2) is not yet clear. This study compares the risk of severe disease among patients infected with the Omicron (BA.1) variant with patients infected with Delta (B.1.617.2) variant in Portugal.

Methods

We conducted a cohort study in individuals diagnosed with SARS-CoV-2 infection between 1^stand 29^thDecember 2021. Cases were individuals with a positive PCR test notified to the national surveillance system. SARS-CoV-2 variants were classified first by whole genomic sequencing (WGS) and, if this information was unavailable, by detecting the S gene target failure. We considered a hospitalization for all the patients admitted within the 14 days after the SARS-CoV-2 infection; after that period, they were censored.

The comparison of the risk of hospitalization between Omicron (BA.1) and Delta (B.1.617.2) VOC was estimated using a Cox proportional hazards model. The mean length of stay was compared using linear regression, and the risk of death between Omicron and Delta patients was estimated with a penalized logistic regression. All models were adjusted for sex, age, previous infection, and vaccination status.

Results

We included 15 978 participants aged 16 or more years old, 9 397 infected by Delta (B.1.617.2) and 6 581 infected with Omicron (BA.1). Within the Delta (B.1.617.2) group, 148 (1.6%) were hospitalized, and 16 (0.2%) were with the Omicron (BA.1). A total of 26 deaths were reported, all in participants with Delta (B.1.617.2) infection. Adjusted HR for hospitalization for the Omicron (BA.1) variant compared with Delta (B.1.617.2) was 0.25 (95%CI 0.15 to 0.43). The length of stay in hospital for Omicron (BA.1) patients was significantly shorter than for Delta (confounding-adjusted difference -4.0 days (95%CI -7.2 to -0.8). The odds of death were 0.14 (95% CI 0.0011 to 1.12), representing a reduction in the risk of death of 86% when infected with Omicron (BA.1) compared with Delta (B.1.617.2).

Conclusion

Omicron (BA.1) was associated with a 75% risk reduction of hospitalization compared with Delta (B.1.617.2) and reduced length of hospital stay.

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