This Week in the Pandemic, December 4, 2020
It’s a week past Thanksgiving and that means we’re going to talk about incubation periods! Why? Because many people are probably incubating right now because they were exposed 5–6 days ago. Don’t be fooled if case counts don’t jump in the next few days. I think it’ll be at least another week before we start seeing the effects of Thanksgiving travel and gatherings, partly because of…
INCUBATION PERIODS! Yay! Boo! So divisive.
In honor of the occasion, because incubation periods are such important, fickle, frustrating things, I’ve written a poem.
Today’s agenda:
- Why is the COVID-19 (covid) incubation period so important?
- What’s the incubation period for covid?
- Why is the covid incubation period so confusing?
Why is the covid incubation period so important?
An incubation period, just so there’s no confusion, is the “interval from receipt of infection to the time of onset of clinical illness.” That’s literally the textbook definition, according to Leon Gordis in Epidemiology (Gordis, 2004). I knew keeping those grad school books would pay off eventually. In normal-people terms:
The incubation period is the amount of time between when you’re exposed to a germ and when you start to get sick.
For covid, the incubation period has been a topic of much interest pretty much since the first case was diagnosed over a year ago. At that point, no one knew that this virus would affect the lives of all 7.8 billion people on Earth, but they still would have wondered how soon someone else might get sick.
If someone else did get sick, then it’s a contagious disease and precautions need to be taken. If no one else gets sick then medical staff need only concern themselves with that single patient. But how long do you wait to see if someone else gets sick? A week? 2 weeks? A month? That’s what the incubation period tells us early in an outbreak.
Later in an outbreak or epidemic or pandemic, the incubation period tells us how long someone needs to be in quarantine after they’re exposed to a pathogen (in this case, a virus). In theory, if everyone who was exposed stayed home for the full incubation period that would stop the pandemic. In theory.
So if you’ve been exposed to covid and your health department told you “Stay home for 14 days” that’s because the incubation period for covid is 14 days. Or is it?
What’s the incubation period for covid?
Lately there’s been some reconsideration of the quarantine time and, thus, the incubation period, for covid. Is it 14 days? Is it 10? What you really want to know is how long you need to be in quarantine, right? So let’s revise the question.
How long do I need to be in quarantine?
Like everything else in epidemiology the answer isn’t black and white, instead relying on probabilities. So let’s revise that once again.
What’s the likelihood that I will be contagious after each day of quarantine, assuming I have no symptoms?
The best answer we have right now is depicted below.
Do you see where the blue bars start to get so small you can’t see them? That’s where most people are super unlikely to be contagious and where the quarantine period for most people ends. To be more specific, look at the black lines in each of the blue bars. On day 11 that black bar goes below 0%. This means that 11 days after someone is exposed their risk of transmitting the virus to anyone else is statistically indistinguishable from 0%. This is why the CDC is now saying people can, at the discretion of local public health authorities, be done with quarantine.
Why is the covid incubation period so confusing?
Did you notice the italicized print above? “At the discretion of local public health authorities” Why would this be the case? Isn’t the incubation period the same for everyone? Is human biology in Scranton that much different than human biology in Panama? Probably not, because they’re both in Iowa. HA!
But, no, human biology doesn’t change that much with geography and I’d be willing to bet that the chart above would look much the same if a study was conducted in any place in the world.
The reason local public health authorities may have differing quarantine periods, and may change their quarantine period is based on the answer to the following question: How much risk can the community handle?
In some parts of the U.S., hospitals are almost full and/or understaffed. Places like that have very little margin for adding covid cases. That means they’ll advise longer quarantine periods because longer quarantine reduces the likelihood of being contagious and causing more cases.
So if your rules for quarantine don’t make any sense to you, there’s probably a reason that you just don’t know about yet. Ask questions. Get answers. But for all of our sake, stay in quarantine as long as you’re asked to, probably up to 14 days.
References
Gordis, L. (2004). Epidemiology (3rd ed, p. 21). Elsevier Saunders.