Finnish Epidemiologist: The IFR of the Omicron Variant is 0.04%, About Half That of Seasonal Flu

The Danish Government report released on January 3rd 2022 states: 

Until Sunday December 19th 2021 nearly all positive SARS-CoV-2 samples were tested with variant PCR to identify Omicron cases. As of Monday December 20th 2021 only a representative sample of the positive SARS-CoV-2 samples are being tested with variant PCR at TCDK, hence, the number of identified Omicron cases does not represent the total number of Omicron cases in Denmark. In the Clinical Microbiological Departments, the PCR samples are from hospitalised patients prioritised to be variant PCR tested.

The report gives a very conservative estimate of the cumulative number of omicron cases by January 2nd, which is 55,673, because the Danish authorities stopped sequencing all COVID-19 positive cases on December 20th. Since then, as stated above, only a daily random sample has been sequenced to see the proportion of Omicron cases in the population. When I use this daily proportion starting from December 20th and multiply it by the daily number of all SARS-CoV-2 positive test results and subtract the number of the daily established Omicron cases I can get a reliable estimate of the true number of cumulative Omicron cases on January 2nd, which is 143,237. From December 29th onwards until January 2nd I assumed that 90% of COVID-19 diagnoses were the Omicron variant. 

The median length of time from COVID-19 diagnosis to hospitalisation and death is about six and 13 days respectively. Until Dec 19th all COVID-19 PCR diagnoses were sequenced and the cumulative number of Omicron cases can be summed since November 21st. On December 20th the number of Omicron cases can be calculated by multiplying total number of COVID-19 cases by the pro-portion of Omicron cases in the representative sample. By December 20th, i.e., 13 days prior to January 2nd, the cumulative number of omicron cases was 43,200. As median means that 50% of deaths have accumulated by January the 2nd among the 43,200 I further assume that in this 43,200 fixed cohort another 18 will die after January the 2nd.

Thus I can calculate an estimate Omicron case fatality rate (CFR) in this fixed cohort, which is 36/43,200 = 0.08%, which corresponds roughly to that of seasonal flu’s mortality rate. When further I assume that only half of the true COVID-19 infections are diagnosed, which is a reasonable assumption for many reasons, I get an Omicron infection fatality rate (IFR) of 0.04 % indicating very low lethality. My calculations are conservative.

Since January 2nd until January the 11th there have been an additional 140,270 infections, assuming conservatively that 90% of new cases are Omicron variant cases. Fewer people are classified into the severe category of whom 44 (three fewer than on January the 2nd) were on ventilators as of today. The expected number of additional deaths among the accumulated Omicron cases 289,507 by January the 11th is 214. It is interesting to note that less than five of ICU patients, i.e., one in twenty in Denmark on January the 2nd, were Omicron patients.

It is possible that my CFR and IFR estimates are affected by the fact that Omicron cases have been especially prevalent among younger people. However, a recent Danish paper indicates that boosted patients, i.e., thrice vaccinated individuals, prevent secondary Omicron transmission effectively. Older age groups in Denmark are well protected with booster shots, which also protect them against severe disease. Thus, the observed low Omicron mortality in Denmark is attributable to very likely lower pathogenicity of Omicron variant itself and favourable effects of mass vaccination.

We will get an accurate final CFR estimates from Denmark very soon as the Omicron status is known for all hospitalised patients. Already by now we can with great confidence deduce that the Omicron variant is at least 16 times less lethal in a highly vaccinated population than the SARS-COV-2 variant circulating in spring 2020 in Europe and the world.

The most important issue related to very fast spreading Omicron is how it affects the labour force in short term. In South Africa, the Omicron wave peaked rapidly and lasted around two months. 

Mikko Paunio is an adjunct professor in epidemiology at the University of Helsinki, Department of Public Health, and an advisor to the Finnish Government. 

Stop Press: The Danish Government has just announced the easing of restrictions at the end of this week, having concluded that Omicron is a lot milder than previous variants.

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October 2022
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