With Covid-19, are people infectious in the first days after infection before they test positive on a Covid test?

I have learned that Covid-19 tests are ineffective in the first few days after being infected: in those first few days, you'll likely test negative even though you have the virus. As I understand it, it takes time for the viral load to grow high enough for the test to detect it, so someone who is infected will be likely to test negative in the first few days after being infected. For instance, one study reports a 67% false negative rate 4 days after infection and 20% after 8 days [1].

Could people be infectious during those days before testing positive? I know that people can be infectious before they show any symptoms; does this mean that people might be infectious 4-8 days before they tested positive? Or does it mean that the viral load in the first few days is so low that during the time window when Covid tests are negative, people are unlikely to infect others precisely because their viral load is low? I know the CDC recommends quarantining for 14 days after you might have been exposed, even if you test negative, but I don't know if this recommendation is primarily out of an abundance of caution [2].

I'm wondering about the implications for very frequent testing: e.g., daily Covid-19 testing. Does the delay in testing positive mean that daily Covid-19 testing is insufficient to prevent outbreaks, because people who get infected can still spread the virus for several days before they test positive? Or is it that in the first few days after infection, people are likely to test negative but are also unlikely to infect others?

[1] Variation in False-Negative Rate of Reverse Transcriptase Polymerase Chain Reaction–Based SARS-CoV-2 Tests by Time Since Exposure.
Lauren M. Kucirka, Stephen A. Lauer, Oliver Laeyendecker, Denali Boon, Justin Lessler. Annals of Internal Medicine, 18 August 2020. https://www.acpjournals.org/doi/10.7326/M20-1495#s1-M201495

[2] https://www.cdc.gov/coronavirus/2019-ncov/if-you-are-sick/quarantine.html

  • No one will know they are "positive" until at least a day or two (or more) afterwards, even if they are tested at the earliest possible time.
    – Bryan Krause
    Commented Sep 23, 2020 at 14:50
  • @BryanKrause, ok, but my question is: will they be infectious in that time period?
    – D.W.
    Commented Sep 23, 2020 at 15:53
  • Unlikely to be known or knowable. If you can't test whether someone is positive you can't know whether they've transmitted it.
    – Bryan Krause
    Commented Sep 23, 2020 at 15:58
  • @BryanKrause, I don't understand the connection between those two; can you help me understand why that would be? I would imagine you might be able to determine whether they've transmitted it, retroactively; what am I misunderstanding?
    – D.W.
    Commented Sep 23, 2020 at 16:04
  • Pretty difficult to know, there is too much variability in incubation times. If someone they had contact with is infected, how do we know they didn't get it from somewhere else? Few people are being tested so regularly as to know when exactly is the first day they tested positive, and those that are must be in high risk circumstances. It's also possible to test positive, then negative, then positive again. You could possibly do a controlled study by intentionally infecting people and following them very closely, but this is unethical.
    – Bryan Krause
    Commented Sep 23, 2020 at 16:07


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