Statistical analysis reveals less severe COVID-19 outcome due to Omicron variant

By Tarun Sai LomteJan 5 2022Reviewed by Aimee Molineux

In a recent prospective cohort study, a team of researchers compared the severity of coronavirus disease 2019 (COVID-19) infection in patients infected after the emergence of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) Omicron variant with those patients who were infected during the Delta variant outbreak, and posted their findings to the medRxiv* preprint server.

The United States (US) is witnessing a rapid surge in COVID-19 infections due to the new SARS-CoV-2 Omicron variant, with an increase in prevalence to 58.6% between 19 and 25 December 2021 from 22.5% between 12 and 19 December 2021. Early reports from several countries suggest that although the Omicron variant is more contagious than the Delta variant, Omicron infections lead to lower rates of hospitalizations when compared to those caused by the Delta variant.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

The Study

In the present study, researchers classified the COVID-19-infected patients into three cohorts: 1) the emergent Omicron cohort which comprised of about 14,054 patients who were infected between 15 and 24 December 2021; 2) the Delta cohort with over half a million patients who were first infected with SARS-CoV-2 between 1 September 2021 and 15 November 2021; and 3) the Delta-2 cohort with 77,692 cases recorded between 16 and 30 November 2021 before the detection of the first Omicron cases in the US.

The authors were able to source all the COVID-19-related patient data based on their electronic health records (EHR), and none of the study population had reported previous SARS-CoV-2 infection as documented in their EHRs.

The researchers investigated whether patients in the emergent Omicron cohort showed any severe outcome within three days of infection when compared to those of the Delta cohort. They examined four clinical outcomes following a COVID-19 infection like the requirement of emergency services (ED visits), hospitalization, intensive care unit (ICU) admission, and mechanical ventilation within the first three days of a positive test result.

The propensity score of both the emergent Omicron and Delta cohorts were matched for demographics, adverse socioeconomic determinants (which include employment, housing, education, and economic circumstances), comorbidities (hypertension, dementia, renal and cardiac disorders, etc.), behavioral factors, COVID-19-related medications (remdesivir, dexamethasone, etc.) and vaccination (including booster dose). Similar analyses were done between the two Delta cohorts.

Results

The study found that patients in the Omicron and Delta cohort were similar in age but differed racially and ethnically. Fewer patients were vaccinated in the Omicron cohort compared to those in the Delta cohort, and COVID-19-related outcomes within the first three days were less pronounced in the Omicron cohort than the Delta cohort.

The findings further report that 4.55% of patients in the Omicron cohort required ED visits while 15.22% in the Delta cohort needed emergency services. Hospitalization in Omicron and Delta cohorts was observed in 1.75% and 3.95% cases, respectively. ICU admissions were noted for 0.26% (Omicron cohort) and 0.75% (Delta cohort) cases. Mechanical ventilation was required in 0.07% and 0.43% cases in the Omicron and Delta cohorts, respectively. The study observed no significant differences between the two Delta cohorts.

The results showed that only 3.89% of children under five years in the Omicron cohort received emergency services while more than 20% in the Delta cohort required ED visits. Hospitalization too was lower for kids in the Omicron cohort (0.96% against 2.62% in the Delta cohort). Lower hospitalization rates and ED visits were observed in other pediatric groups (5-11, 12-17 years), and similar to earlier observations, no marked changes were seen between the two Delta cohorts.

Conclusions

The findings of this study demonstrated that in the US, the COVID-19 cases arising after the emergence of the SARS-CoV-2 Omicron variant showed reduced severity of infection than in the cases recorded in previous time points (during the Delta variant outbreak). These findings, consistent with other studies conducted in England, Scotland, and South Africa, suggest that Omicron infections may cause milder disease with less pronounced symptoms when compared to Delta infections.

Although this reduced disease severity could be attributed to higher rates of vaccination were observed in November and December, the researchers believe that a substantially higher proportion of vaccination was needed to account for the significant differences observed in ED visits, hospitalization, ICU, and ventilation in this study. Moreover, no significant reduction was seen in cases immediately before the emergence of the Omicron variant, which suggests that the increased prevalence of the Omicron variant might be the likely reason for the reduced severity of COVID-19 cases.

The study did not report mortality risks due to COVID-19 as only a small number of patients reported death within the first three days, and to correctly ascertain the mortality risk, a longer follow-up time is needed. To conclude, the observations made in this study indicate that the emergence of the SARS-CoV-2 Omicron variant is associated with a less severe disease prognosis.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources