Impact of SARS-CoV-2 Omicron and Delta variants on serious hospital events

In a recent study posted to the medRxiv* pre-print server, a team of researchers assessed the differences in risk of hospitalization in adults after infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron and Delta variants of concern (VOCs).

Study: Serious hospital events following symptomatic infection with Sars-CoV-2 Omicron and Delta variants: an exposed-unexposed cohort study in December 2021 from the COVID-19 surveillance databases in France. Image Credit: PHOTOCREO Michal Bednarek/Shutterstock

The emergence of the SARS-CoV-2 VOCs, Delta, and Omicron necessitated extensive research about the manifestations of the variants. The varied effects of the VOCs on adult patients, including the severity of disease and COVID-19-related hospitalizations, have been comprehensively studied. However, several studies suggest significant differences in the impact of the different variants on the occurrence of serious hospital events.    

About the study

The present retrospective cohort study investigated the differences and the various factors involved in COVID-19-related hospital events occurring in adults infected with SARS-CoV-2 Omicron and Delta VOCs.

COVID-19 cases in France between 6 December 2021 and 7 Jan 2022 were recorded and grouped into two arms – Delta and Omicron arms. The cases eligible for the study were of patients over 18 years of age, who had symptomatic manifestations of COVID-19 and were reverse transcription-polymerase chain reaction (RT-PCR) positive for SARS-CoV-2. Among the eligible cases, patients suspected to be infected with the Omicron VOC and the Delta VOC were known as exposed and non-exposed individuals, respectively.

The two groups were compared based on known risk factors like age, gender, the status of vaccination, and comorbidities. The study considered intensive care unit (ICU) admission of the COVID-19 patient, requiring critical care, or, death of the patient in the hospital as a serious hospital event.  

A multiplex quantitative RT-PCR (RT-qPCR) was used to characterize the variant present in the SARS-CoV-2-positive samples. The results of the mutation screening were evaluated based on the presence of spike proteins, characteristic to either Delta or Omicron VOC. The primary outcome of the study was the number of COVID-19-related serious hospital events among symptomatic patients who had tested positive for the SARS-CoV-2 Omicron or Delta VOC.        

Results

The study results showed that a total of 149,064 patients were included in the research. A total of 36% of patients in the SARS-CoV-2 Delta VOC cohort were unvaccinated as compared to the 17% of patients in the Omicron VOC arm. A slightly higher number of comorbidities was observed in the Delta arm than in the Omicron arm.

During the follow-up period of the study, 497 serious hospital events were observed, of which the Delta arm accounted for 447 cases while the Omicron arm had 50 cases. ICU admission was the most common serious hospital event.

Within the Delta VOC arm, 92% and 97% of the serious hospital events were observed within 14 and 21 days of SARS-CoV-2 detection while in the Omicron cohort, 84% and 100% of the serious events were observed within 14 and 21 days of the positive COVID-19 test, respectively. Notably, patients who were vaccinated with the booster dose had a higher number of serious hospital events as compared to the patients vaccinated with the primary dose.

Patients belonging to the age group of 18 to 79 years were at 7.7 times lower risk of serious hospital events in the case of Omicron symptomatic patients as compared to the Delta symptomatic patients. In patients 80 years and older, Omicron-infected patients had a 3.3-fold lower risk of serious events than patients infected with Delta. It was also noted that the odds of occurrence of a serious event increased with the increasing age of the patient. Also, male patients were at a significantly higher risk of serious events than women; this risk was lesser in vaccinated individuals.

The risk of serious hospital events was lowered by 6.7 times in vaccinated patients of ages between 18 and 79 years, irrespective of whether they received the booster vaccine or not, while the same risk was lowered by 2.2 times in patients of ages over 80 years who had received the primary vaccine dose; the risk was further reduced by 4.3 times when the patients received the booster dose.  

Conclusion

The study findings showed that the SARS-CoV-2 Omicron VOC has lower disease severity and a lower incidence of serious hospital events as compared to the Delta VOC. Despite the lower vaccine efficacy reported against the Omicron VOC as compared to the Delta VOC, no significant difference was observed in the protection provided by the vaccines against severe hospital events in this study.

According to the authors, the higher risk of occurrence of serious events in the elderly can be lowered with vaccination and booster dose administration.

*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.

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.