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Statistically, from 100 people exposed to the coronavirus that causes COVID-19, how many get infected?

Say for example in the USA.

Since they talk about exposed people, then such cases should be identifiable https://arstechnica.com/science/2020/08/cdc-loses-its-mind-says-people-exposed-to-covid-19-do-not-need-testing/

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    I doubt if anyone has good data on this, but even if they do, how do you define "exposed?" Being in a closed room with coughing COVID patients and no distancing or masks will probably produce very different numbers from being outdoors with everyone wearing masks and distancing.
    – Carey Gregory
    Aug 26 '20 at 18:55
  • This question lacks prior research.This site requires questions to show some degree of prior research.
    – Thomas
    Aug 26 '20 at 19:14
  • I don't know, how does CDC know if a person was exposed or not? Since they talk about exposed people, then such cases should be identifiable arstechnica.com/science/2020/08/…
    – Joe Jobs
    Aug 26 '20 at 19:54
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The definition for "exposed" the CDC is using that is referred to in the article you link is "close contact with an infected person (that is, be within six feet for 15 or more minutes)". This is a standard used in contact tracing, where once you identify someone who is infected you go back and try to identify people they had contact with that are at highest risk. This is not the only reasonable definition of "exposure", and does not include many people who are nonetheless exposed to some level of contact with an infected person or surfaces/rooms containing virus.

Many people get infected yet are not known to have close contact with an infected person. Among those who do have "close contact", they do not all have the same level of close contact. Longer contact in a more enclosed space is worse than shorter contact in a less enclosed space. Other details matter like whether the infected person is doing things like coughing, singing, talking, and whether either person is wearing a mask.

In the end, an answer to your question "how many of those exposed get infected?" is not particularly meaningful. A related question that might be answerable is "what percentage of people identified via contact tracing go on to test positive?" but only limited data on contact-tracing is available and I am not aware of any published data that report results for people who have been marked as contacts. I don't believe there is typically follow-up (or resources for follow-up) with contacts once they have been informed.

New Jersey's database suggests that 12% of contacts who are reached have symptoms, but that isn't the same as the number who test positive. It's also not clear if those people got symptoms because of exposure to the index case or someone else, whether they were actually infected before the index case, or how many of the other 88% are either infected but asymptomatic or infected and will later develop symptoms. Additionally, most of the potential contacts are not traced due to limited cooperation from cases or lack of contact information.

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  • Is there any other kind of flu where such numbers were studied?
    – Joe Jobs
    Aug 26 '20 at 21:23
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    @JoeJobs COVID-19 is not a "flu", it's a disease caused by a different virus called a coronavirus. Aug 26 '20 at 21:25
  • Ok then similar diseases.
    – Joe Jobs
    Aug 27 '20 at 21:52
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    @JoeJobs For every infectious disease it will also depend on the same things, depending on transmission route. Aug 27 '20 at 22:04

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