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Assuming the immune system is healthy,

How much SARS-CoV-2 virus (cause of COVID-19) is required to make a person sick?

This might also depend on time, if it was a high infectious dose but just some minutes, maybe not that bad outcomes.

Or if it's a very low infectious dose, but during a whole day, maybe a bad outcome.

In addition, Would it be possible that a person was exposed to SARS-CoV-2 virus, and so the virus got inside their system, and even still this person's immune system didn't let the virus to infect this person?

Note that it's not the case that this person will be totally asymptomatic, it's the case when the virus wasn't "strong enough" to infect this person even being inside their system.

  • MID - minimum infectious dose defined as"the smallest quantity of infectious material that regularly produces infection"
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    Your question is misphrased a bit: covid-19 stands for "coronavirus disease 2019". So by definition, you're sick when you suffer 'from the least bit' of covid-19.
    – Vergilius
    May 7, 2020 at 20:17

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Disclaimer: I am not entirely satisfied with this answer in as much as the author fails to source the claim that 1000 viral particles may be the threshold for infection. However the author has significant expertise and more importantly (to me as a non-expert) has written a piece that is entirely readable and plausible. I offer it for your consideration:

Dr Bromage The Risks - Know Them - Avoid Them posits that 1000 viral particles are sufficient to produce an inflection. He then discusses the risks of encountering that threshold and how to minimize one's risk.

Update:

The minimum infectious dose of SARS-CoV-2, the virus that causes Covid-19, is unknown so far, but researchers suspect it is low. “The virus is spread through very, very casual interpersonal contact,” W. David Hardy, a professor of infectious disease at Johns Hopkins University School of Medicine, told STAT. Blockquote STAT

For comparison purposes:

... it takes just 18 particles of norovirus to cause an infection. This can lead to the classic clinical signs of vomiting and diarrhoea

LINK, with citation to norovirus study for infectous dose

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  • A couple things that are probably worth pointing out are that Dr Bromage's explanations would be just as relevant if it took 100 or 5000 particles (the numbers are just used to give a general sense of the relative danger of different levels of contact), and does cite a bunch of experts making estimates in that range. Also that "minimum" is a bit of a misnomer here, as MID is defined as (in a controlled setting) the # of particles required to cause infection in 50% of exposed subjects, which would mean it's very possible to be exposed to less and still get infected (or MID and not).
    – Bryan Krause ♦
    May 11, 2020 at 20:15
  • @BryanKrause My intention is not to impugn Bromage's discussion (quite to the contrary), and yes you are correct that the explanations are equally valid for a broad range. However, I fear that some people may take "...some estimate that as few as 1000 SARS-CoV2 viral particles are needed for an infection to take hold..." as a literal truth, when in the full context of his discussions it it meant to be an example, as in IF 1000 were valid, consider the proportional risks from several environmental conditions.
    – BobE
    May 12, 2020 at 15:18
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    What I meant is that Bromage isn't relying on the 1000 number. If instead it was 100, you could read through the article and just divide everything by 10 (because we don't know how many particles are getting inhaled, either). Indeed, I have the same fear as you, that someone will take a number like 1000 too literally, and learn the actual number is more or less than that and think that means it's less or more infective than we (or, in this case, Bromage) thought.
    – Bryan Krause ♦
    May 12, 2020 at 15:35
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The amount of particles a person is exposed to can affect how likely they are to become infected and, once infected, how severe the symptoms become.

A high infectious dose may lead to a higher viral load, which can impact the severity of COVID-19 symptoms.

Viral load is a measure of virus particles. It is the amount of virus present once a person has been infected and the virus has had time to replicate in their cells. With most viruses, higher viral loads are associated with worse outcomes.

“The more viral particles that get into the lungs, the more damage to the lungs that is probably happening,” said W. David Hardy, a professor of infectious disease at Johns Hopkins University School of Medicine.

A report from China suggested that there is no difference between how much coronavirus a person is exposed to and how sick they get.

But another report showed that patients with milder disease had lower levels of the virus.

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  • This does not appear to be a very satisfying answer, as tends to confound infectious dose (initial exposure) with viral load (that may occur from an initial infectious dose and subsequent viral replication within the subject's body). Without being too simplistic about it , a similar question might be asked: How many times must a woman have sex in order to become pregnant ?
    – BobE
    May 5, 2020 at 18:01
  • @BobE Feel free to edit or post a new answer. I am also not that happy with this answer, is not as exact as I wanted (but at least helps somehow). I'll keep searching and see if something better is on google. Regarding your example, I don't think it applies here since a woman's body has certain weeks where the probability of getting pregnant is basically 0 or 1, so her body 'changes'. In this COVID-19 case, the body doesn't change, is constant the whole month
    – I likeThatMeow
    May 6, 2020 at 15:56
  • is constant in the sense that, there won't be a week where the probability of getting infected will be basically 0 or 1.
    – I likeThatMeow
    May 6, 2020 at 16:04
  • Ahh, perhaps you do see the parallel that I make: It's not simply the amount of virus that is inhaled, it is also the "fertility" of the host that is also determinative of viral replication and subsequent viral load development of the disease. There are just too many conflicting factors and too many unknowns specific to this virus to allow a generalization that a virus innoculation titer below "X" would not make a person sick.
    – BobE
    May 6, 2020 at 17:11
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It seems we are looking at the kinetics of viral growth in the body, the time to release of the cell containing the replicated virus, the amount of virus released per cell and the rate of adsorption of the virus into the cell. I don't speak with authority regarding the biochemistry of this process, but I do remember quite a lot about chemical kinetics. If the rate of replication of the virus in the body is faster than the rate of adsoprtion through the ACE-2 enzyme then you can launch all the virus you want at the cell, it is replicating the virus faster than the virus can penetrate cells. Similarly the time to release the virus from the infected cell and the amount of virus released per cell all contribute to the effect of any viral infection. See this for example https://doi.org/10.1016/j.virol.2011.12.005. This more recent article seemed to try to answer the question - but did not https://wwwnc.cdc.gov/eid/article/26/9/20-1495_article So it depends.. I guess.

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    Not sure how this answers the question asked
    – Bryan Krause ♦
    Oct 7, 2020 at 3:22

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