An Early Assessment of the Omicron Covid-19 Variant: Case-Fatality Rates

By Craig Getz and Dr Gary Kantor

Executive Summary

• The fourth wave of the Covid-19 pandemic has been dominated by the Omicron variant. The Omicron case-fatality rate (CFR)[1] has been between 89.7% and 95.2% lower than the case fatality rate of the preceding variants (the original strain, Beta and Delta).
• Persons testing positive for the Omicron variant are significantly younger than in previous waves[2]. After adjusting for differences in age, gender and chronic health conditions, the Omicron case-fatality rate is between 83.4% and 90.0% lower than the case-fatality rate of the preceding variants. Risk-adjusted case-fatality rates are a proxy for disease severity. The lower case-fatality rate suggests that omicron has coincided with less severe disease than previous variants.
• The lower case-fatality rate may be due to the inherent characteristics of Omicron or to other factors such as vaccination and previous infections. Our analysis does not seek to disentangle these factors.
• Nonetheless, our data, along with other research findings, suggests that Omicron may cause less severe disease than the Delta variant. Amongst fully vaccinated persons, Omicron is associated with a risk-adjusted case-fatality rate which is 77.5% lower than the case-fatality rate of the Delta variant.
• Should Omicron continue to be the dominant variant and should no further and more dangerous variants arise, it could signal a significant, more benign change in the trajectory of the pandemic. This is not to suggest that Covid-19 and the Omicron variant should be taken lightly in view of its ability to spread extremely rapidly, the immune escape it has shown, with infections occurring in both previously vaccinated and previously infected persons, and the possible long-term health effects of infection.

[1] The case fatality rate (CFR) is defined as the proportion of cases (diagnosed infections) which result in death within 28 days. Case-fatality rates are considered rather than infection-fatality rates. This is because the number of infections cannot accurately be determined [2] In the original period, 8.9% of infections were in persons under the age of 18. 19.4% of infections related to persons under the age of 30. The average age was 42.1. In the Beta period, 8.4% of infections were in persons under the age of 18 and 18.6% of infections were in persons under the age of 30. The average age was 44.8. In the Delta period, 15.7% of infections were in persons under the age of 18 and 28.4% of infections were in persons under the age of 30. The average age was 40.6. In the Omicron period, 13.1% of infections were in persons under the age of 18 and 28.8% of infections were in persons under the age of 30. The average age was 38.4.

Introduction

Insight Actuaries and Consultants (Insight) provides actuarial services to a broad spectrum of medical schemes. Medical scheme members, like the overall South African population, have been severely impacted by the Covid-19 pandemic. Schemes collect extensive data on the clinical and demographic characteristics of their members and the medical care they receive. These data offer important insights into the effects and possible future outcomes of the pandemic. Covid-19 has caused the deaths of as many as 250,000 South Africans[3]. There is hope that the Omicron variant causes much less severe disease than previously demonian variants. In this report we use scheme data to identify changes in case-fatality rates from the beginning of the pandemic to now. Case fatality rates are a reasonable proxy for disease severity. The analysis builds on our previous report which showed that Omicron is associated with lower hospital admission rates than the previous variants of Covid-19. [3] https://www.samrc.ac.za/reports/report-weekly-deaths-south-africa

Methodology

Demarcating the Pandemic

Genomic studies[4] show that the Omicron variant became the dominant variant in South Africa from the middle of November 2021. To assess the impact which the Omicron variant has had, experience in the medical scheme setting before this time (when other variants were dominant) is contrasted with the subsequent period. The Covid-19 pandemic can be demarcated in time. The original variant was dominant before the 30th of September 2021 (original period). The Beta variant was dominant between the 1st of October 2020 and the 30th of April 2021 (Beta period). The Delta variant was dominant between the 1st of May 2021 and the 14th of November 2021 (Delta period). [4] https://www.krisp.org.za/manuscripts/25Nov2021_B.1.1.529_Media.pdf

Measuring Case-Fatality rates

Case-fatality rates are influenced by factors other than the characteristics of the virus itself. The principal drivers of severe outcomes (hospital admission or death) are age and chronic health conditions (comorbidities). Higher aged cohorts with comorbidities, have had much higher case-fatality rates than those composed of young and healthy persons. Such variations in cohort composition can be adjusted for using actuarial risk adjustment techniques to arrive at comparisons of the effect of changes in the virus. This risk adjustment uses data on age, gender and chronic health conditions with known associations to outcome (asthma, diabetes, HIV, hypertension and chronic renal failure). Only Covid-19 cases prior to the 2nd of December 2021 are considered. Subsequent cases are not considered to allow for the lag between diagnosis and death. The case-fatality rate covers deaths within 28 days of diagnosis. This analysis was run on the 2nd of January 2022. Case-fatality rates are a reasonable proxy for disease severity. However, case-fatality rates may be influenced by other important factors. For example, increased testing capacity may lead to the detection of many more mildly ill persons. This will deflate the case-fatality rate (as a lower proportion of cases will result in death). Variations in case-fatality rates must therefore be carefully interpreted.

Case-Fatality Rates

Omicron is associated with a case-fatality rate of 0.3%. This is far lower than the case-fatality rates associated with previous variants. The Omicron case-fatality rate is 89.7% lower than during the original period, 95.2% lower than during the Beta period and 91.9% lower than during the Delta period. These results are influenced by the profile of persons testing positive. After adjusting for the differing profile of persons in each period, the Omicron variant is associated with a case-fatality rate of 0.5%. This is 83.4% lower than during the original period, 90.0% lower than in the Beta period and 86.4% lower than in the Delta period. This suggests that Omicron has produced less severe disease then previous variants. Whether this is due to the inherent characteristics of the Omicron variant or other factors such as vaccination status or prior infection remains undetermined and requires additional research.

Case-Fatality Rates by age

Lower risk-adjusted case-fatality rates are evident across age groups. This suggests that Omicron has produced less severe disease[5] in adults. Most notably, the Omicron case-fatality rate for persons aged 50 to 59 is 98.1% lower than that of Delta on a risk-adjusted basis (from 4.9% to just 0.1%). For persons aged 60 to 69, the risk-adjusted case fatality rate decreased from 9.7% to 2.2%. For persons aged 70 to 79, the risk-adjusted case fatality rate decreased from 16.8% to 4.8%. For persons aged 80 and above, the risk-adjusted case fatality rate decreased from 31.0% to 4.5%. [5] Our previous article demonstrated that children are associated with higher case-admission rates in the Omicron period than in Delta

Case-Fatality Rates of Fully Vaccinated[6] Persons

Analyses are repeated after limiting to data on fully vaccinated persons (as defined below). This provides insights into the impact of the Omicron variant relative to the Delta variant. Other variants are not considered as they preceded the availability of vaccination. In this population, on a risk adjusted basis, Omicron is associated with a 0.3% case-fatality rate. This is 77.5% lower than the 1.2% case-fatality rate associated with the Delta variant. This indicates that disease caused by the Omicron variant is less severe than disease caused by the Delta variant amongst vaccinated persons. [6] Fuller vaccinated persons are here defined as persons who received one Johnson and Johnson dose or two Pfizer doses between 28- and 180-days prior. The 180-day limitation is applied given reduced vaccine efficacy over time and the recommendation that persons receive a booster vaccine after 6 months. A comparison of the case-fatality rates across the periods specifically amongst unvaccinated persons is not yet possible because of data availability.

Conclusions

Based on this medical scheme data, the Omicron case-fatality rate is far lower than that of preceding variants. This is only partly due to the younger age of persons in this cohort. After adjusting for age and other relevant characteristics of the cohort, the Omicron risk adjusted case-fatality rate remains much reduced, between 83.4% and 90.0% lower than the case-fatality rate of the preceding variants. The lower risk-adjusted case-fatality rate indicates that Omicron has coincided with less severe disease than previous variants. More research is needed to determine whether the reduced case-fatality rate is attributable to the inherent characteristics of the Omicron variant that might cause less severe disease or to other factors such as vaccination status and previous infection. Our analysis does not seek to disentangle these effects. Amongst fully vaccinated persons, Omicron is associated with a risk adjusted case-fatality rate which is 77.5% lower than during the Delta period. This may suggest that reduced severity is not simply a function of vaccination status as the vaccinated cohort has seen a decline in its case-fatality rate. Whilst the need for further research is clear, this early analysis suggests that Omicron could alter the course of the pandemic to one that is more benign, assuming Omicron continues to be the single prevailing Covid-19 variant. This is not to suggest that the Omicron variant should be taken lightly in view of its ability to spread extremely rapidly, the immune escape demonstrated by infections occurring in both previously vaccinated and infected individuals, the as yet undetermined effect on persons not previously exposed to the virus or to vaccination, and the similarly uncertain long term health effects of Covid-19 on those who recover from mild or severe illness.