How do we know that Person B was infected by Person A, unless Person B went into quarantine directly after interacting with Person A? If they don’t go into quarantine, how do we know that a random Person C didn’t interact with them and cause the infection?
And for the vast majority of infections that can’t be contact traced like the example above, how do we determine the date they were actually infected if the disease can be transmitted asymptomatically?
In: Biology
Because of contact tracing.
If they can determine who you got it from, and when you encountered that person, they just count the days until your symptoms started. Earlier on when there are fewer cases, it’s easier to do this. Do it for enough people and you can get a decent average.
Also, there have been at least two incidents now where one person (accidentally) infected a lot of people at once, so in those cases it was pretty obvious.
Sheer numbers and statistics. If someone comes to a hospital with symptoms of a novel virus, and then 7-10 days later 7 hospital workers report similar symptoms, then it’s a safe bet that the patient was the point of infection for all of them. **Statistics** is a field of study which helps us draw concrete conclusions from unreliable data. If only 1 hospital worker were to report symptoms, we wouldn’t have very strong confidence in that 7-10 day incubation period, but as the numbers go up, so does our confidence.
When there is an outbreak somewhere, epidemiologists (scientists who study epidemics) will trace back to the estimated first date of exposure, then watch the numbers of those reporting symptoms. First date of exposure is not very reliable data – it often relies on assumptions (like the hospital example above) or self-reporting (“Well, I was in the airport on Sunday so that was probably it”).
However, it takes far fewer patients than you might imagine to get a strong estimate. For instance, during the 2013 bird flu epidemic, it only took around 250 patients for epidemiologists to get a very strong sense of the incubation period – they were 95% confident that the average was between 3 and 4 days. There are a lot of specialized techniques used, too many to go into detail here – but we know, from experience, that people tend to underestimate exposure; that we should discount anybody who reports a very long or very short exposure time; that exposure times generally fit a bell curve; etc.
When possible, epidemiologists will look for volunteers to help them get better numbers. Known-healthy individuals living in an area with an outbreak can report for daily testing, which allows doctors to know the exact date of infection and the exact date that symptoms appear. This can give exact numbers, but the issue is that generally the number of volunteers / test kits available will be limited, and incubation periods can vary wildly, so often having a huge amount of unreliable data can actually be better than a small amount of very reliable data.
Further reading:
https://academic.oup.com/aje/article/182/8/723/207340
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4301984/
https://jamanetwork.com/journals/jama/fullarticle/2762028#jld200013r4
Well we don’t KNOW for sure that B wasn’t infected by C but we know A and B are both infected, and have both had contact with each other that could have lead to said infection.
Idk if this word math problem is about Corona or STDs, but chances are if A had it and “spent a lot of time” with B, then B has it and statistically got it from A.
As for the second part, with trace back and use timelines in which we know they had contact with those already infected. We know patient A and patient D were in contact 16 days ago, neither had symptoms, but patient A has had symptoms for the past 4 days, and patient D is starting to show symptoms. We can clearly derive from this hypothetical that the asymptomatic period is about two weeks. but Again it’s true that this could be incorrect, but based on probability, this is the strongest lead.
There are techniques using genetic sequencing that can be used as viruses mutates very fast. So if the genes of the virus in person A and B is very similar then they got it from each other. However this is not used for fast spreading infections as this is costly and time consuming. Instead what is being done is just a simple assumption after an investigation. We can not know for sure but we can come up with some good assumptions if we know who have been in contact with who and when the people involved developed symptoms.
Here’s the World Health Organization [basic explanation article](https://www.who.int/news-room/q-a-detail/q-a-coronaviruses). Covid19 is the same type of virus as SARS and MERS, so they have *some* experience with how it behaves and the typical incubation period.
Otherwise, we don’t know “for sure” that any individual person may or may not have been infected at a particular moment in time. It’s all statistical chances and probabilities, and the CDC / WHO and doctors in general have procedures that keep that in mind. They won’t test just persons A B and C, for example, but any others who have come into “close contact” with them, based on how the infection spreads (water droplets from coughing and sneezing, contact surfaces).
[The advice here](https://www.cdc.gov/coronavirus/2019-ncov/about/prevention.html?CDC_AA_refVal=https%3A%2F%2Fwww.cdc.gov%2Fcoronavirus%2F2019-ncov%2Fabout%2Fprevention-treatment.html) is actually what’s effective at stopping the spread and protecting you and everyone’s health. That page isn’t “opinions”, it’s *medical advice*. The doctors and health officials are taking it seriously, they’re not joking with it, they’re not cutting corners or holding back advice or instructions that could help.
In the US, depending upon where you are, the doctors are telling people to just stay home and if it gets worse to call them. They aren’t offering for people to get tested because there aren’t even test kits.
I have the flu, we think, and that is what happened when I called my doctor. They didn’t want me to leave the house until it got better or call them if it got significantly worse.
Based on that, I’m not sure how accurate the infection rate numbers are.
Thanks for asking this! I had my own assumptions on how it was done. But hoped I was wrong. Happy to find out I was indeed wrong.
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