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It is widely reported that the Omicron variant is much less dangerous than previous COVID-19 strains. Is there good scientific evidence to back this up or can the difference in hospitalization rates be explained by the Omicron strain appearing at a time when the majority of people have already been either vaccinated or previously exposed to COVID-19?


The link provided in the answer by Laurel can also serve as a source of the claim I was requested to provide.

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    If it's widely reported it should be easy for you to find an actual notable claim that you think is false
    – pipe
    Mar 15 at 14:46
  • Hello and welcome to the site! Skeptics is a site for challenging notable claims, and without a notable claim your question may be closed until one is edited in. You say it is widely reported, so could you add some news reports or notable people making the claim to your question?
    – Giter
    Mar 15 at 17:45
  • If it wasn't there wouldn't be so many countries seeing their hospitalisation rates and number of deaths plummeting despite the number of reported cases being stable or even going up... Heck, half my colleagues have come down with it over the last 2 months and none reported more than a bad headache and some mild fever for a few days, which is way less severe than the 2 months of bad headaches, fever, diarrhea, shortness of breath and other unpleasantness I had 2 years ago and that was a mild case back then.
    – jwenting
    Mar 18 at 9:04

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It does appear Omicron is really less severe (let's not use mild in this context). Studies comparing Omicron and Delta do this in the same environment, over the same time frame, so it's irrelevant that Omicron appeared later.

How does Omicron compare with Delta? Here’s what we know about infectiousness, symptoms, severity and vaccine protection:

Is it less severe? Yes, Omicron causes less severe disease than Delta. Part of this may be due to Omicron being less able to infect lungs as it does the upper airways.

The risk of hospitalisation and ICU admissions from Omicron are 40-80% lower than with Delta.

The risk of death is about 60% less with Omicron than with Delta.

Yet despite the reduced severity, this wave of Omicron has been associated with higher rates of hospitalisations in many countries because of the sheer numbers of those infected.

The only silver lining has been how the Omicron wave peaked within a few weeks in numerous countries, with hospitalisation and daily case numbers quickly coming down.

One of the linked papers, one example, Omicron SARS-CoV-2 Variant of Concern: A Review on its Transmissibility, Immune Evasion, Reinfection, and Severity:

A study conducted in England observed a reduction in the risk of hospitalization for Omicron infections when compared to Delta infections during the study period (Dec 1 to Dec 14, 2021). The magnitude of the reduction varied according to the inclusion criteria for cases and hospitalization, ranging from 20–25% when any hospitalization is used as the endpoint to 40–45% when hospitalizations lasting 1 day or more is used

So, no apples and oranges are being compared here, the same area, same time frame, and a clear difference is being shown.

And that England study has the following estimation regarding the unvaccinated cases you are mostly concerned with: https://www.imperial.ac.uk/media/imperial-college/medicine/mrc-gida/2021-12-22-COVID19-Report-50.pdf

Table 3:

Variant Vaccination or reinfection category Cases Hospitalisations % Reinfections (cases) % Reinfections (hosp) HR relative to primary Delta infection in unvaccinated pvalue
Delta Unvaccinated 109331 1466 1.9% 0.8% 1 (1-1) <1e-6
Omicron Unvaccinated (Uncorrected) 9585 56 18.6% 8.9% 0.59 (0.5-0.69) <1e-6
Omicron Unvaccinated (Corrected) 9585 56 56.4% 21.1% 0.76 -

Which shows that for unvaccinated, the Hopsitalization rate is ~24% lower for Omicron that for Delta.

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    It seems clear that Omicron is intrinsically less severe than Delta, but since Delta is more severe than most previous variants the comparison with the earlier variants is much less certain. For example, from Challenges in Inferring Intrinsic Severity of the SARS-CoV-2 Omicron Variant: "This meaningful but fairly small difference [vs Delta] implies that omicron, alpha, and wild-type SARS-CoV-2 have similar intrinsic severity." Mar 15 at 19:55
  • There are theories that exactly the differences that make omicron more infectious make a single infection less dangerous (more focus on the upper airways instead of the lungs). And of course there are many more people vaccinated now. And among vaccinated people, where delta might have caused no infection, omicron is more likely to create a harmless infection, changing the statistics.
    – gnasher729
    Mar 15 at 22:09
  • I have accepted this since the second linked paper is quite convincing. It still isn't completely clear that the difference is not due to differences in effectiveness of vaccines for different strains. It would be even more convincing if the same difference of severity was independently seen in unvaccinated populations.
    – Kvothe
    Mar 18 at 11:43
  • @Kvothe Actually I found that paper can be read online, so I can cite the data for the unvaccinated.
    – kutschkem
    Mar 18 at 11:54
  • Some of the preliminary analysis of the Hong Kong outbreak actually suggests the opposite - that Omicron is no less severe than Delta, but during the Omicron outbreak high-risk populations had much higher vaccine uptake and as a consequence the population-level risk was substantially lower. It may be that the apparent decrease in severity is just another example of Simpson's paradox. I wouldn't say that the science on this has settled yet.
    – CJR
    Mar 19 at 15:01

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