An estimation of the proportion of asymptomatic Omicron infection (Maccallini, 2022)

[Pyrrhus]

An estimation of the proportion of asymptomatic Omicron infection (Maccallini, 2022)
https://paolomaccallini.com/2022/01/03/happy-new-variant/

A new (self-published) article from @paolo. This article is currently relevant as some people have suggested that the Omicron variant of SARS-CoV-2 might result in less serious disease than previous variants.

Abstract
In this document, I compare data from two COVID vaccine trials carried out in South Africa (the Ubunto trial and the Sisonke sub-study). Both trials were conducted on unvaccinated subjects, asymptomatic at the time of enrollment. Among them, some were found positive to SARS-CoV-2, but while the variant was identified as Delta in the Sisonke sub-study, the Omicron variant was reported in asymptomatic subjects of the Ubunto trial.

By considering the proportion of SARS-CoV-2 positive subjects in these two populations, the average number of daily confirmed cases, and the percentage of unvaccinated South African citizens at the respective times of enrollment of the two trials, I calculated the proportion of asymptomatic infections for Omicron, in function of the same parameter for the Delta variant.

I found that if we accept a proportion of asymptomatic infections of 17% for Delta, then we conclude that about 60% of all Omicron cases are asymptomatic, in unvaccinated subjects. This points towards significantly lower pathogenicity for the Omicron variant when compared to the Delta variant.

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