Submitted by AcJc_0604 t3_z4tozd in askscience
I know that you are most contagious in the couple of days before symptoms show and for the couple of days after. But how does that apply to exposure. For example, a person is exposed in the afternoon, are they automatically contagious after exposure or does that take time to "build up" in the body?
May I know, in terms of detection (which I assume works by viral load), does that mean if I’ve just been exposed to a COVID-positive person, even if I take the ART test right away, it’s likely the results will not show I’m infected? If so, how long should I wait (not in terms of beginning to quarantine) before taking an ART test assuming I’m suspicious I’ve infected, before the test is accurate/have reliable results?
24-48 hours post suspected exposure is the generally accepted minimum wait time to have a sufficient viral load for a rapid test to be accurate. 72 hours post exposure is considered ideal for testing. I appreciate you clarifying/implying that you would obviously be quarantining from the time of suspected exposure, for the health and safety of others, even if a rapid test couldn't give you an accurate result in the first day or so.
According to the CDC, you need to wait 5 days unless symptomatic. Earlier tests are likely to come back negative even if the person is carrying the virus.
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since when? I am an RN and did one of the mass COVID testing sites in my town (back in 2021) and later one of the mass COVID vaccine clinics.
I mean, it’s in their guidelines: https://www.cdc.gov/coronavirus/2019-ncov/your-health/if-you-were-exposed.html
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...and I work for the Department of Health. Waiting has been the standard for awhile now.
https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/testing.html#when-to-get-tested
Ah, yes I see that page was updated September 2022. That would be after 2021, which I specified was the time I was doing the swabs and later the vaccines.
That was a page update; the recommendations to wait before testing have been in place for quite some time now. Though they were somewhat recently changed from waiting 4 days to the current standard.
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Rapid antigen tests have high rates of false negative results. Repeat testing is needed to confirm.
While true, they’re quite effective for point in time “am I contagious at this second”
If you’ve got antigens in your nose, you are shedding.
Rapid antigen tests commonly give false negatives. They are better thought of as tests to prove you have COVID, not tests to prove you don't. You need multiple tests if you're sick and want to be sure it's not COVID. Time is only part of it.
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Actually, your experience is spot on. It's recommended to wait 5 days to test if not symptomatic sooner. Earlier tests are not expected to accurately detect the virus.
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If you get to day 5 with no symptoms and with negative rapid tests on days 4 and 5, you probably aren't infected. Two negative tests after the onset of symptoms means the symptoms probably aren't from COVID.
A positive test at any point means you probably are infected.
A single negative test, or any number of negative tests before day 5 in the absence of symptoms, is inconclusive.
Typically if you have a fever you’re considered contagious. Once the fever ends you’re still possibly contagious for up to 72-96 hours, depending on how much virus you’re shedding and how much mucus you’re shedding.
However the best way to be sure to not spread it should you have to go out is a mask. The goal is droplet containment more than anything. Remember say it don’t spray it is truth.
Is this in reference to Covid-19, or general respiratory infection/cold?
Typically your non-covid viruses. Flu A/B, rhinovirus, adenovirus, parainfluenza, etc.
Covid has been shown to be shed with high variability between individuals and so the best way is a covid PCR test that actually checks for the virus. That said if you no longer have a fever that’s a good indicator that your body has fought off the offending agent (in most cases).
In immune competent individuals this means you have circulating antibodies that should inactivate the virus as it exists the cells and target it for destruction.
PCR’s are incredibly sensitive, so you want to be sure you can see the cycle count. PCR doesn’t check for the virus, It checks for viral RNA fragments, which can be shed for months after you are contagious. That RATs actually are looking for certain surface proteins, binding to them, then that is what is deposited on the positive line. The BinaxNOW tests will flip positive if there’s the N (Neuclocapsid) protein in your nose, which is a certain sign COVID is actively reproducing in your airways.
Does the protein BinaxNOW tests for differ from what Flowflex tests for? I don't trust BinaxNOW because I've never gotten a positive on them despite having had Covid twice now as confirmed by several days in a row of positive Flowflex tests. If they test for different proteins then that would explain it...
They test for the same protein.
I’m hesitant to recommend the Flowflex just because they had a massive counterfeit problem: https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/counterfeit-home-otc-covid-19-diagnostic-tests#diagnostic
I do want to thank everybody here for listing the information about covid, but remind that the question was asking about the flu.
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Correct. The virus needs time to multiply before it will show up on an antigen test. ART tests are a reasonable gauge of how infectious you are. If you were exposed mere hours before, you are not yet infectious and so will not register as infected. An exception being if you were just exposed to a massive dose of the virus, such that there is a substantial amount of the virus trapped in the mucous in your nose.
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Could we make a big difference by encouraging people to reduce contact with others for a couple days after being part of a get together or large, packed event?
Even if it doesn't mean a quarantine, just not going right from one social event to another and saving the next shopping trip for a couple days after you get back should have an impact on virus spread, right?
Yes that would help.
But do you really see people listening to reason like that?
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I don't see people doing that without a quarantine, especially since the difference between that and a quarantine is very subtle.
Plus, I imagine this wouldn't work well. Okay, after a packed event you're supposed to avoid crowds. So you individually meet several friends and infect them. None of them are inclined to quarantine, since they haven't been to any packed events like you, so they're free to go to any packed events they choose. They go and spread the virus just the same as if you'd go. To make a dent to overall infection rates you need to mask up and limit packed events altogether.
You were able to logic out the need for the mandates and restrictions in a single short paragraph, yet here we are nearly 3 years in and people still don't understand.
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>meet several friends and infect them. None of them are inclined to quarantine
I don't know if it would be helpful or not, but this argument is a little questionable. You have seen several friends rather than a second gathering, reducing the number of people you infect. Plus you implied that someone cannot opt to see nobody in a social manner for the two days.
Who knows, it could be a waste of time still, but I would give it a little more credit.
... which is why we need mask mandates.
Thank you for coming to our joint presentation on the matter.
Follow up to this:
It takes about 2 weeks for the flu vaccine to take effect. It’s too late for thanksgiving, but folks might consider getting the flu shot by December 11th to have full vaccine protection by Christmas.
So, if you visit two thanksgiving gatherings, and you’ve seen none of the people involved in weeks, your odds of transmitting a contagious disease from one gathering to another is very low to nonexistent?
I suppose that would depend on fomites on clothing and such, wouldn’t it.
That depends on the virus, but generally speaking, COVID and other respiratory viruses don't seem to survive longer than 3 days on surfaces in ideal conditions and degrade much, much faster in the natural environment. There is even some evidence your skin naturally serves somewhat of a disinfectant and kills flu virus pretty rapidly ( https://academic.oup.com/cid/article/48/3/285/304169).
That being said, you should still wash your hands.
So if people caught influenza during Thanksgiving on Thursday, would it be a bad or typically higher risk move to go out to eat Saturday (today) night after Thanksgiving because most of the infected are now at peak concentration, timeline wise, but not feeling their symptoms yet? Or was yesterday the worst day to go out, because today they’ll wake up feeling ill? Asking cuz have dinner date tonight and concerned about influenza
There is a statistical distribution for all of these time periods and a lot could depend on things like the infectious dose you received, your immune system's abilities, the particular strain of the virus, how susceptible your cells are to infection with the virus (e.g. do your cells have a high concentration of the receptor the virus is looking for to gain access to your cells?), etc.
I said "Shortly after reaching peak concentration, you begin feeling symptoms", but this is not correct.
Typically peak concentration would hit shortly *after* feeling symptoms rather than before. However, you will be highly infectious a up to several hours before feeling symptoms for influenza.
The high probability zone of being being contagious before symptoms is somewhere around 36-48 hours after exposure for influenza, but someone below mentioned that it could be much shorter for influenza B.
This is just where most people will be contagious. It's possible you would be contagious 24 hours after exposure or not until 72 hours or more.
Why do people go to the emergency for the flu? Older people get vaxxed, and younger people are expected to just deal with it for a few days where I'm from.
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I started showing symptoms last Sunday (coughing, fever, congestion). What makes this turn into pneumonia? Why do I still have a fever off and on? (I'm on plenty of medicine from the doctor).
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https://pubmed.ncbi.nlm.nih.gov/13263486/ this study goes into a bit more detail how long it takes for a virus getting absorbed by a cell and how long it takes to create copies. Your guess with an afternoon isn't that far off. However, there's also a chance that a virus can be exhaled immediately after getting breathed in.
> However, there's also a chance that a virus can be exhaled immediately after getting breathed in.
Technically that doesn't mean you're contagious; you just bounced a contagion from someone else back into the room for someone else to be a potential new host for instead.
>However, there's also a chance that a virus can be exhaled immediately after getting breathed in.
Right, If exhaled immediately after breathing it in, it never deposited in your respiratory system to begin with.
Particles in the 0.3 micron diameter range are very evasive. They can follow stream lines around obstacles and they are too big to be influenced by diffusion (random motion of air molecules) so air molecules cannot "bump" them into a surface.
A droplet nuclei carrying virions in this size range could be inhaled and exhaled right back out. This size range is what an N95 is designed to filter out 95% of the time. For particles larger or smaller than 0.3 microns, an N95 filters more than 95%.
They would not automatically be contagious right away. The incubation period is 1-4 days for influenza. Depending on when a person develops symptoms, you are able to establish the exposure period (when the person is considered contagious). So say you are exposed on day zero, develop symptoms on day three, the exposure period would start on day two. Anyone the person encountered on day two and on may have been exposed. In a hospital setting, we would isolate a patient for seven days after symptom onset, or 24 hours after resolution of fever and symptoms -whichever is longest. Source: CDC
1-4 days but the median is under 2. Significantly under 2 for Influenza B where the median incubation time is something like 14 hours! It always amazes me how quickly influenza can work its evil magic.
The "good" news, I guess, is if you think you've been exposed you'll know one way or the other pretty darn quick instead of worrying about it for a week.
Generally the difficulty I have in questions like this is that often the answer is couched in Boolean terms which are useless to me.
It's not like its "At 24 hours you were not contagious at all and couldn't spread it to anybody, then at 25 hours all of a sudden you were 100% contagious and will spread it to everyone you ever talk to, then 5 days later you will be 0% contagious again and cannot spread it at all." It begs the question, how contagious do you have to be before a virologist will call you "contagious"? What's the actual cutoff they're using? It's not Boolean so what is it? The answers are often couched in terms as if it was, which frustrates me as a layman.
As someone who was vaxxed but got Covid-19 a couple of weeks ago, I still have a slightly drippy nose making me have to clear my throat of mucous about once an hour or so. All other symptoms but that one only lasted a few days and I feel absolutely fine. I can't tell when it's going to start being okay to visit other people again. The isolation and cancelled plans are frustrating me, and home antigen tests are still positive even though I feel just fine. How contagious am I really? It's probably not completely impossible for me to spread it, but it's probably nowhere near as likely as it was back when I was 4 days into it and was having high fever, headache, and constant snot everywhere.
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The problem with this question is that you are contagious after certain period of exposure, but may not have any idea that you've been exposed because you may be asymptomatic with things like influenza. Hypothetically after the first 24 to 48 hours you would be at risk of contagion, but may not show symptoms until later. Also depending upon the strain type, the amount of time you are sick and the time you will recover can vary greatly thus making the contagion time different for different strains of the flu.
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For the flu (Covid is different - all viruses are generally speaking) Symptoms of the flu can manifest anywhere from 1 hour to 4 days after initial exposure, however most symptoms are presented around two days after initial exposure. You can be contagious anywhere from one day before symptoms (so that would probably assume as quickly as you have a transmittable viral load) and up to 7 days after symptoms resolve. “Immediate” would be like you touched flu, touched your nose to infect yourself and then touched an object that you squish the flu on to and then someone else then touches that item - immediately transmitting the virus - but if we are talking from moment it touches a cell that it infects to the viral load being large enough for say - a cough or bodily fluid to infect someone else - that would be highly dependent on the viral load you received, the rate at which this particular strain multiplied and how quickly your immune system kills them. I always assume immediately after exposure- I let people know I have it and if they around anyone vulnerable, similar to my Covid protocols. And I wear a mask for about a week after. For testing, once symptoms are present, you can test.
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To be contagious the virus has to enter a cell and hijack the cell to replicate the virus, once this has happened to many cells the virus can now travel around the body and into the respiratory system where it can then spread to others.
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KauaiCat t1_ixsuar8 wrote
No, they are not immediately contagious. It could take days to become contagious.
Initially the virus multiplies unchecked and it reaches a peak concentration. This is when you are most contagious. Shortly after reaching peak concentration, you begin feeling symptoms as the immune system has realized it is under attack. Normally by the time you feel symptoms the concentration of the virus in your body has already begun to decrease thanks to the immune response.
This is true of influenza and many other viruses. The time it takes between you getting infected and symptom onset is called the incubation period and it's normally more than 24 hours for influenza.
For Covid-19 the average incubation period is several days, but you could be spreading the virus for 24 hours or a little longer before you first feel symptoms.
You feel fine, but you are highly contagious. Because you feel fine you are likely to be near people and talking to them in close proximity. This is where the virus shines. As you speak, microscopic spit balls filled with virions are floating through the air and those nearby are breathing them in.
Emergency rooms will normally see an spike in influenza this time of year in the US. A few days after friends and family congregated closely together for Thanksgiving.
Edit: Peak concentration is typically reached *shortly after feeling symptoms* rather than shortly before as stated above.