What percentage of infections never develop symptoms?
Epistemic Status: Pretty confident in my uncertainty, expect more data to become available that eventually resolves the question
Tl;dr: Anything from zero to half of coronavirus infections are asymptomatic, but we currently can’t narrow it down much more than that.
One way to estimate the number of asymptomatic cases is to PCR test a bunch of people and see what percentage test negative. There’s three obvious problems with that:
- Unrepresentative samples- if you did this in the US the asymptomatic rate would be severely depressed because symptomatics are prioritized in testing.
- Some of those people might go on to develop symptoms later.
- The test could be wrong.
The most representative sample I know of is the Diamond Princess cruise liner, because everyone on board was tested, giving us some kind of reasonable denominator. It’s still not a random sample of the population, because cruise takers skew old-but-healthy-for-their-age, but it’s a start. Of that population, 3,063 tests returned 634 positive results, of which 306 were asymptomatic. But this only solves problem 1, a bit.
This pre-print uses statistical modeling to try to sort those into presymptomatic and true-asymptomatic cases, but has a weird math error I’m trying to get clarity on. I e-mailed the English-speaking author on 3/24 and will update if I hear back, but until then, here’s the problem: the model uses an incubation period of 6.4 to predict that 18% of C19 infections are true-asymptomatic (as opposed to merely pre-symptomatic). But when they let that vary from 5.5 to 9.5 days, they get a range of 20% to to 40%. That only works if percent-asymptomatic is a non-monotonic function of incubation period, and that would be extremely weird..
Even if that model is correct there’s still the problem of unreliable tests. This paper estimates that “almost half or more” of asymptomatic positives among people, tested in China, because they were in close contact with a known case, were false positives and the person never had the virus. Unfortunately the English abstract doesn’t specify how they came to this number, and the original article is in Chinese, so I can’t even attempt to translate this to other populations.
[EDIT 3/28: This paper has been retracted]
This news article based on “classified Chinese government data” says that ⅓ of people who tested positive asymptomatic, but doesn’t specify whether they were monitored to see if they stayed asymptomatic.
My best hope for better data on this is South Korea, thanks to its aggressive testing and monitoring strategy. Unfortunately I haven’t found any of their results, despite spending 30 minutes looking.
Early Appearance of Symptoms
Tl;dr COVID-19 might start with a cough, or a fever, or both, or occasionally maybe neither. It might start suddenly or slowly. It might remit and then come back worse.
This was hard to figure out. Most academic/medical papers start with the person’s first contact with the medical system, which is too late, so I looked at social media and news reports. These are obviously going to be biased towards people with symptoms severe enough to be interesting, but not so severe as to die. I also restricted myself to test-confirmed cases, which because I was also looking at mostly American sources biases things towards severe cases. And I’m counting on people to represent themselves honestly. So there’s a lot going against this sample.
In total I found 11 cases, plus two notes from doctors doing front line work. You can see my and Eli Tyre’s notes on every case here and the symptom tracking spreadsheet here. Both are messy and no work has been spent making them comprehensible to others.
From this very small and biased sample:
On the other hand, this pre-print found that only 44% of patients had a fever on admission to the hospital, and 88% ever had one. The fevers can be intermittent. Both that paper and the doctor I just linked to report very high occurrence (85%+) of lymphocytopenia (low white blood cell count), but that is not very useful to diagnosing yourself, and it’s not clear when it starts.
Known scientific source Business Insider says that most cases start with fever. They do not mention their source.
Commonly Repeated Things I Don’t Believe
You may have heard that 80% of cases are mild. Keep in mind that that paper defined mild to include mild pneumonia, which I would classify as at least moderately severe.
Similarly, the paper reporting 50% of patients appeared at the hospital with digestive symptoms counted “loss of appetite” as a digestive symptom. Only 19% presented with a more definitely gastroenteric complaint like nausea or vomiting.