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From The Telegraph:

Face masks should be mandatory on planes and trains as it reduces coronavirus spread by 98.5 per cent, the Chief Executive Officer of Ryanair has said.

Michael O'Leary told BBC Radio 4's The Today Programme: "If you really want to do something that's effective wear masks.

"If everybody is wearing masks you eliminate the risk of spread of Covid-19 by 98.5 per cent. So let's take some effective measures."

It comes as a considerable amount of Britons return to their morning commute today following a slight easing of lockdown.

Is this figure true and if so, from what scientific study is it taken?

  • If everybody is wearing a mask (100% adherence) then the problem reduces to the effectiveness of the mask (which I guess is assumed to be N99 in that claim you quoted), assuming no other transmission routes (e.g. unwashed hands). See 2nd half of my answer in the "duplicate" question for a graph on the duality. – Fizz May 18 at 17:27
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Reuters did a fact-check on this claim and rates it as "partly false".

The context for the 98.5% figure is:

There is a 70% contagion probability between a COVID-19 carrier not wearing a mask and a non-carrier wearing a mask; a 5% contagion probability between a COVID-19 carrier wearing a mask and a non-carrier not wearing a mask; and a 1.5% contagion probability between a COVID-19 carrier and a non-carrier both wearing masks.

Reuters conclusion:

This claim is not substantiated. Although some health authorities recommend the use of masks to help limit the spread of COVID-19, Reuters could not find any evidence to back up these percentages.

They go on stating that the CDC recommends masks but has no specific data, the ECDC states that masks should be considered "complementary" and also has no data, and the WHO points out the risks of masks and only recommends usage under specific circumstances (correct usage in combination with frequent hand washing for symptomatic people).

The AP and snopes.com also investigated the claim with similar results.

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    I thought when I closed this question I would be given an opportunity to move your answer to the original, but I didn't see that option. Would you please consider posting this answer to the original? – Oddthinking May 18 at 13:51
  • @Oddthinking Oh, I didn't see that question, thanks for pointing it out! I'm not sure if my answer is a good fit for it though. The infographic fits, but my answer to the title question "To what degree do facemasks protect against COVID-19?" would basically be "I don't know, but reliable sources say that there isn't any data supporting 98.5%". – tim May 18 at 14:04
  • @Oddthinking: I recall from Electronics SE (where this feature was used IMHO too often) that mods can merge questions. I think that discards one of the questions but merges all the answers to one list. (I have no idea how to do that though, since I'm not a mod anywhere, but see meta.stackexchange.com/questions/158066/…) – Fizz May 18 at 16:59
  • @Fizz: You are absolutely right. I have used this feature before, which is why I recall it was possible, but I don't think I have used it for years, so I forgot where it was. – Oddthinking May 18 at 18:36

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