CNN - Infected people without symptoms might be driving the spread of coronavirus more than we realized

Additional evidence:

The earliest swabs were taken on day 1 of symptoms, with symptoms often being very mild or prodromal.

[...]

In all patients except one, throat swab RNA concentrations seemed to be already on the decline at the time of first presentation.

BTW, my speculation of how presymptomatic transmission happens is that, in addition to droplets caused by normal speaking, during that time the virus also causes very mild itching/tingling in the throat, which causes very mild coughs (throat clearings) that don't even get noticed as symptoms. (This is based on a family member having a cold recently, and me noticing such very mild symptoms in myself that I probably wouldn't have noticed if I hadn't been looking for them.)

ETA: The above linked paper is also interesting for these reasons:

Together, these data indicate active replication of SARS-CoV-2 in the throat during the first 5 days after symptoms onset.

When aligned to viral load courses, it seems there is no abrupt virus elimination at the time of seroconversion. Rather, seroconversion early in week 2 coincides with a slow but steady decline of sputum viral load. Whether certain properties such as glycosylation pattern at critical sites of the glycoprotein play a role in the attenuation of neutralizing antibody response needs further clarification. In any case, vaccine approaches targeting mainly the induction of antibody responses should aim to induce particularly strong antibody responses in order to be effective.

COVID-19 is successfully spreading in countries which have taken these measures ["tell people to stay home if they have those symptoms"] and other more extreme measures

How true is this? I haven't delved in that closely, but my impression is that a big part of what's been successful in containing the spread in places like Hong Kong and mainland China has involved identifying & isolating people as soon as they show symptoms.

It's funny how counterintuitive this post was at the time, and yet how obvious it feels now. Thanks again for writing it, if I hadn't learned this I would've been much worse off in my qualitative thinking.

Promoted to curated: I think this is the kind of literature review and analysis that is really useful at this point, in particular given that lots and lots of papers are skipping peer-review, and so it's often hard to judge the quality of things. 

Asked on Facebook crosspost: If it's transmitted by cough, how do asymptomatic people transmit it? Is it dependent on other diseases or allergies to provoke coughing?

None of the studies cited address the question of how presymptomatic transmission occurs, but my guess is that it's the same: respiratory droplets. People also emit droplets when they talk and breathe. There aren't as many and they don't travel as far as when they cough, but they're there.

A Significant Portion of COVID-19 Transmission is Presymptomatic [LW · GW] argued for something that is blindingly obvious now, but a real surprise to me at the time. Covid has an incubation period of up to 2 weeks at the extreme, where you can have no symptoms but still give it to people. This totally changed my threat model, where I didn't need to know if someone was symptomatic, but instead I had to calculate how much risk they took in the last 7-14 days. The author got this point out fast (March 14th) which I really appreciated. I give this +4.

(This review is taken from my post Ben Pace's Controversial Picks for the 2020 Review [LW · GW].)

Temporal dynamics in viral shedding and transmissibility of COVID-19

Abstract
We report temporal patterns of viral shedding in 94 patients with laboratory-confirmed COVID-19 and modeled COVID-19 infectiousness profiles from a separate sample of 77 infector–infectee transmission pairs.

We observed the highest viral load in throat swabs at the time of symptom onset, and inferred that infectiousness peaked on or before symptom onset.

We estimated that 44% (95% confidence interval, 25–69%) of secondary cases were infected during the index cases’ presymptomatic stage, in settings with substantial household clustering, active case finding and quarantine outside the home.

Disease control measures should be adjusted to account for probable substantial presymptomatic transmission.

I notice that the estimates of serial interval (almost?) all come from places that had pretty aggressive & successful containment measures in place, such as identifying & isolating potential carriers (including people who show symptoms, traced contacts, and high-risk travelers). That would tend to shorten the serial interval, since people who are identified early in their infection lose the opportunity to transmit during the later portion of their illness.

Are there estimates of what R was for these populations? If it's a lot less than the 2-3 that other studies have found that would be some evidence that a lot of later-stage transmissions were prevented.

Note: I've added this post to the Coronavirus Links Database.

Asymptomatic carriers seem to be rare, though not completely nonexistent.

I think one should expect asymptomatic people to be less infectious than pre-symptomatic people, so it may not be important from a "changing current responses" perspective to understand the prevalence of asymptomatic-ness. That being said, here is a paper that analyzes the Diamond Princess data and claims the asymptomatic rate to be >15%. https://eurosurveillance.org/content/10.2807/1560-7917.ES.2020.25.10.2000180

I haven't understood their methodology well enough to have a strong opinion about its validity. I think they're using time series data along with some priors about the shape of the distribution of the random variable "number of days between testing positive and showing symptoms, given that the patient will at some point show symptoms" to conclude that a decent fraction of the Diamond Princess people won't show symptoms.

The claim is made in paragraph 5 of the discussion section.

A question I always have about these studies is at what level symptoms are defined and self-reported. E.g. presumably "you have an itchy throat or a mild headache in the morning/mildly increased fever over your baseline" is pre-symptomatic. Self-isolating with mild symptoms is probably hard to measure but can be at least socially enforced.

If the thought here is true does that suggest the actual transmission mechanism is perhaps not yet understood as well?

The claim has been it is not an airborne transmission (which perhaps still makes sense on a technical level) but if infection is occurring without things like people coughing or sneezing (dispelling a larger amount of internal matter than simple breathing) how is it spread?

Is the view that it must be more about things like an infected person touches their mouth in some manner, then touches something else or shakes someone's hand...

What are the confidence internal for the estimates for the serial intervals? That doesn't seem like something that would have been well measured during the outbreaks so far but perhaps I'm not crediting the medical community enough here.

Thanks for this!

I thought I read that in contact-tracing they found that 25% of transmissions were pre-symptomatic, but now I can't find the reference :(