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Imagine someone attends a relatively large social gathering during the day and evening. That morning they had a negative COVID test. There was substantial close contact, such as hugging.

The next day, that individual tests themselves and finds that they are positive for COVID.

I am not asking for medical advice, but what is the current medical information available with regards to risk of transmission to other partygoers?

Online sites and UK NHS info tend to focus on testing and test results, rather than how false negative and later positive results influence risks and incubation times.

What do we know about the risk of infection for other partygoers? What are the infection/incubation windows? What actions should they take?

I understand that partygoers probably wouldn't be contagious for a day (maybe 2 or 3), if they had caught it.

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On this site we can not provide medical advice to anyone. However, as with any disease, you should monitor for symptoms and if concerned visit a health professional for diagnosis and treatment.

However, I don't think your core question is personal medical advice - you are asking about the sensitivity of the tests and whether someone could test negative on a test but still be infectious.

I'm assuming that you and the others in your group were using Rapid Antigen Tests (RATs, AKA Lateral Flow Assays [LFA]).

If so, this infographic from Dankova et al. might be of some use to you:

Timeline

In this picture, you can see the dotted line for the viral infection going though 3 main phases of infection, incubation (grey), infectious (pink/peach) and post-infectious (green). Note that the timing of the infection is an idealized model, and not necessarily what is experienced in real-life. Most people will have an infectious process with roughly the same timing, but for some the phases may be longer or shorter. It is entirely possible to have a very short incubation, long infectious and long post-infectious phases.

On the X-axis there is a box showing the relative sensitivities for the RAT/LFA (red) and PCR tests (cyan). As you can see the RAT/LFA have a fairly short window in which they will detect the virus, and that this window does not completely overlap the infectious phase, missing the early part of this phase.

So, it is possible that transmission can occur from a person who has tested negative by a RAT/LFA, but this is less likely from a qPCR test.

References:

  • Dankova, Z.; Novakova, E.; Skerenova, M.; Holubekova, V.; Lucansky, V.; Dvorska, D.; Brany, D.; Kolkova, Z.; Strnadel, J.; Mersakova, S.; Janikova, K.; Samec, M.; Pokusa, M.; Petras, M.; Sarlinova, M.; Kasubova, I.; Loderer, D.; Sadlonova, V.; Kompanikova, J.; Kotlebova, N.; Kompanikova, A.; Hrnciarova, M.; Stanclova, A.; Antosova, M.; Dzian, A.; Nosal, V.; Kocan, I.; Murgas, D.; Krkoska, D.; Calkovska, A.; Halasova, E. Comparison of SARS-CoV-2 Detection by Rapid Antigen and by Three Commercial RT-qPCR Tests: A Study from Martin University Hospital in Slovakia. Int. J. Environ. Res. Public Health 2021, 18, 7037. https://doi.org/10.3390/ijerph18137037

Other articles you might be interested in:

  • Johansson MA, Quandelacy TM, Kada S, et al. SARS-CoV-2 Transmission From People Without COVID-19 Symptoms. JAMA Netw Open. 2021;4(1):e2035057. doi:10.1001/jamanetworkopen.2020.35057
  • Krumkamp R, Kreuels B, Jaeger VK, May J, Mikolajczyk R, Karch A. Negative SARS-CoV-2 PCR or rapid antigen test result and the subsequent risk of being infectious: a mathematical simulation study. BMC Med Res Methodol. 2021 Aug 10;21(1):165. doi: 10.1186/s12874-021-01361-3. PMID: 34376152; PMCID: PMC8353420.
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  • Exactly the information wanted. How test sensitivity and infectiousness track each other, or lead/lag, during the infection cycle. Thank you so much for understanding the intent - not personal medical advice, but as you say, sensitivity of tests and data related to the rapid antigen/lateral flow test outcomes vs actual infectiousness, during the cycle. Thank you again!
    – Stilez
    Jun 28, 2022 at 5:14
  • The yellow section is the period of interest. But 2 questions: (1) Can you confirm that in the graph, the yellow bar is intended to be part of the infectious phase, not the incubation phase. The use of dotted lines for both makes it ambiguous. (2) If so, it makes sense, and the yellow bar is drawn as around 1.1 days duration (typical/idealised). Do any of the papers have statistics on how that duration is distributed? Thanks!
    – Stilez
    Jun 28, 2022 at 5:34
  • @Stilez from the graphic, the yellow bar is in the non-infectious phase, but basically as soon as virus is being generated and escaping the cells there is some potential for transmission, so the rise in the line for viral load in the "early phase" would still have some potential for transmission. It certainly isn't cut-and-dried like the background suggests, more of a blending of one phase into another. Post infectious virus tends to be wrapped in antibodies and/or defective in some manner, so no longer infectious.
    – bob1
    Jun 28, 2022 at 20:59
  • @Stilez re 2 - I'm sure that the stats are out there and the point at which the viral load increases will be from ~1 day to roughly 14 days, though I think I saw early pandemic reports of people testing positive for the first time at longer time frames too.
    – bob1
    Jun 28, 2022 at 21:02

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