The figure shows how quickly coronavirus decays in aerosols (the leftmost panels) and on different surfaces. For clarity, the top row in red is the data for this pandemic coronavirus, currently circulating in the world. The bottom row in purple is the data for the SARS virus, which circulated in Asia in 2003-2004--and which clearly behaves similarly.

I’m not so focused on how different cardboard is from steel or plastic. The virus is there on most surfaces, and it sits there ready to infect passersby for a pretty long time.

For me, the data raise two key questions:

If the Covid19 coronavirus is transmitted the same way as the 2003 SARS virus, why is Covid19 so much harder to control and whiy is it causing such a devastating global pandemic? Knowing this information, how do I protect myself?

Let’s take those in turn.

Why is Covid19 a pandemic, but SARS in 2003 was not?

We actually don’t know with any certainty--yet--but the answer appears to be in the timing of when Covid19 coronavirus is “shed”, and where it is coming from--the throat or the lungs. These factors determine the timeline of when a person infected with this coronavirus is most contagious, and how contagious they are. Again, it’s all about R0.

Three important studies appeared last week on this question of when is an infected person contagious? The key, worrisome finding: in Covid19, people may be highly contagious even before developing symptoms. They also can remain contagious for many days after symptoms start--peaking at 5 days, but extending outward of two weeks. Understanding the exact timing of contagiousness is essential for optimizing these social distancing control measures we are all undergoing--again, in order to drive R0<1. The findings from these studies has--I hope-- accelerated the past week’s global social distancing efforts--if you can’t rely on someone knowing they are sick and isolating themselves to avoid being exposed, let’s all just keep away from each other. Now.

To be clear: two of these studies fall under the category of what I would call “best effort mathematical speculation.” Meaning, they looked at data from hundreds of Covid19 cases in well-documented clusters in China and Singapore, and tried to calculate when, exactly, did one patient in the cluster pass coronavirus to the next one in the cluster? Both studies found that most infected patients develop symptoms from 5-14 days following their exposure, peaking at ~6 days.

One of the studies, from a team of scientists from Canada, the Netherlands, and Singapore, used a complex mathematical model of coronavirus transmission to calculate that people passed coronavirus one to the next for up to 2 days before they were really symptomatic. That raised a lot of expert eyebrows last week. Again--it’s “mathematical speculation,” and not clear scientific evidence. But it at least raises serious concerns about how tough this will be to control if we don’t adhere to strict social distancing protocols. In our understanding of prior pandemics, people were not highly contagious for this long before they developed symptoms.

I emphasize that this is only one study. Other similar studies--and our prior understanding of the same question in influenza--suggest that Covid19 contagiousness may begin several hours before symptoms appear, but not days. Still, in designing the best approach to social distancing, it sure sounds like we ought to isolate from any exposed/at risk person even before they show symptoms. Which is, in fact, what China did. And which is also the current global recommendation, though it remains disturbingly slow to be put into practice, especially in the US and UK.

“Shedding”

The other key study out this week was not mathematical at all, just straight biology. German researchers took hundreds of throat swabs and sputum samples from 9 patients followed closely from the moment they developed symptoms. (I know--many scientists are on the front lines of this pandemic, too, along with health workers). They then measured how much virus was present in each swab taken from each infected person at every time point in their illness.



The data confirmed the mathematical models:

There is a lot of virus in throat swabs on the first five days of Covid19 illness There is also a lot of virus in the lungs of Covid19 patients, even in mild illness Virus infectivity tails off quickly after the first 7 days of illness, but virus can continue to be shed upwards of 14 days

So, what’s different about the Covid19 coronavirus from SARS in 2003-04 is, likely, this: both SARS and the Covid19 enter through the mouth and throat (the “upper airways”), and get into the lungs and “lower airways.” Here’s where things are different: the 2003 SARS virus mostly settled in the lungs. Not a lot was being shed routinely onto surfaces, and the highest levels of contagiousness came late in the course of SARS pneumonia, around Day 7. So, once containment policies started in 2003, by the time most SARS patients were highly contagious, they were well isolated and the rest of us were out of harm’s way.

By contrast, Covid19 is a disease of the lower airways--pneumonia--and the upper airways--the nose and throat. Unlike SARS patients, people with Covid19 are shedding huge amounts of virus on to surfaces really early--even before they have symptoms--and they shed more of it, and they shed it for longer, even when they are only mildly ill.

These are subtle but significant differences from SARS--and they are enough to take us from a bad-but-controlled-outbreak-in-Asia in 2003-2004, to this global pandemic in 2020.

(As an aside, the figures in this publication are pretty technical; if you are visual and want to see the data, the money figure is Figure 1 here).

Household Transmission

Today’s piece has been long and with a lot of bold highlights. My apologies. I’ve been feeling a sense of urgency these days. There is just one more piece of data on this topic that caught my attention and is worth knowing about. It comes from a team at the Shenzhen CDC in China and Johns Hopkins.

From mid-January to mid-February, the Shenzhen CDC identified 391 cases of coronavirus infection in travelers coming in from the Wuhan region. They then tracked down 1,286 close contact--people who lived with, ate a meal with, traveled with, or interacted closely on a social basis, any time in those key 2 days before symptoms began, and up until the time cases were identified and isolated, on average an additional 3 days.

Once again, I don’t think the figures are all that great, but here goes: