COVID19: Asymptomatic vs Presymptomatic Spread

by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

This is a bit of a rushed “emergency” post, brought about because of a poorly worded statement by the World Health Organization that was then amplified by the media. To wit, “Asymptomatic spread of coronavirus is ‘very rare.’

Ay ay ay. This is some Pig Latin Google Translate level of muddled miscommunication. And I’ve already spent my day fielding multiple requests from the media and general public who want clarity on this issue.

There is so much to unpack here. The reason it’s important to get ahead of this industrial mountain of linguistic fertilizer is that it is already  being used by COVID-deniers to promote dangerous conclusions, as in:

COVID misinformation


Asymptomatic vs Presymptomatic

The long and short of it is that the WHO was specifically talking about asymptomatic people and their role in spreading COVID-19. This is different from presymptomatic and subclinical people –a distinction that they really should have made.

I myself have been guilty of this on occasion. In media interviews, I have referred to COVID-19 as an “asymptomatic pandemic”, meaning that much of it is spread without the infected cases even knowing they were infected in the first place. It’s part of the rationale for why we encourage mask-wearing: if most people wear masks, thus limiting the distance traveled by their respiratory droplets, then even the people who don’t know they’re infected would be inhibited from spreading the virus.

Dr Eric Topol offers the following convenient definition:

Asymptomatic people will never show symptoms and so, absent a test for some reason, will never know that they were ever infected. And yet they might be spreading the disease to others. Presymptomatic people will eventually develop symptoms, but are suspected to be spreading the infection before those symptoms arise.

In a press conference, the WHO’s Dr van Kerkhove stated, “From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual.” Her position is likely based on studies like this one, which conclude that “the infectivity of some asymptomatic SARS-CoV-2 carriers might be weak.”

Whether or not this is true, Dr van Kerkhove’s statement was received by many to mean that only symptomatic people can spread the disease. Therefore there is no need for masks, no need for random screening, and no need for non-symptomatic people to be physically distancing.

But van Kerkhove’s statement does not apply to presymptomatic people. There is plenty of evidence that presymptomatic people spread the disease. This Nature article estimates that 44% of new cases were infected during the presymptomatic stage of the people who gave it to them. Meanwhile, this study estimates that 89% of supposedly asymptomatic cases are actually presymptomatic, in that they will eventually develop symptoms.

This earlier study from China estimated that 23% of infections in Shenzhen might have resulted from presymptomatic cases, and concluded that this type of spread is essential for containment of the disease.


This distinction is largely lost on the general public, who only hear that if you aren’t coughing, you’re probably safe. This is clearly not true.

Though media ran with the statement that “asymptomatic transmission is rare”, today Dr van Kerkhove walked back that headline with the statement, “I was responding to a question at the press conference. I wasn’t stating a policy of WHO or anything like that.

As someone who has been misquoted by media in the past, I’m sympathetic. But this slip-up really underlines the challenges that public health in general, and the WHO in particular, are having with COVID-19 communication. The conspiracy theorists look for any crack in the messaging to prise apart the science and sow discontentment amongst the population.

So let me reiterate: the best evidence thus far indicates that a great many cases are produced by people who do not (yet) show symptoms. So it is still very advisable for large numbers of people to wear masks and to physically distance

Dr Ashish Jha summarized the issue this way: “Are infected people without symptoms an important cause of spread? My best guess: yes.”


So What Is This Subclinical Thing?

A subclinical case is one that does not manifest sufficient symptoms to trigger a detection. Now we’re getting a little pedantic here. Truly asymptomatic cases will show no symptoms. But in actuality, most of them are likely subclinical, in that the symptoms are so mild that no one ever thinks to qualify them as actual COVID cases.

Yeah, this stuff is confusing, which is why it’s so important to be clear what we’re talking about when we talk to the public.


Distribution is not Risk

There is an important epidemiological aspect to this confusion, beyond the distinction between asymptomatic and presymptomatic carriers.

The WHO directive is actually better described in this document, which states, “One recent systematic review found that the proportion of asymptomatic cases ranged from 6% to 41%, with a pooled estimate of 16%.”

Some people are taking this to mean that asymptomatic cases only infect others 16% of the time. This is a classic misreading of a distribution as a risk.  The WHO claim that the “pooled estimate” of asymptomatic cases is 16% is a distribution. The probability of a an asymptomatic case infecting another person is a risk. The two are definitely not the same.

Let me give you an example.

In Canada, 71% of Parliament is male. That is a distribution. This does not mean that if you are a Canadian male, you have a 71% chance of being a Member of Parliament. To compute the latter, you would divide the number of men in Parliament (240) by the number of men in Canada (19 million) to get a risk of 0.001%.


So What Does All This Mean?

Parsing all this noise, this is what you should take away: the best evidence still shows that a lot of people are getting COVID from people who are not showing symptoms when they are infectious.

It’s as simple as that.

It means that we can’t let our guard down. Masks and physical distancing and avoiding large gatherings of people are still the best way to protect the population from community transmission.

I will leave you with my new virtual background…. the bridge of the Enterprise! Yasss!