Notes about asymptomatic spread of SARS-CoV-2

You will need to wear a mask for a while

Logo courtesy of The Masks Now Coalition

In this current worldwide COVID-19 pandemic, the head of the US Nursing Home Association has rightly called the SARS-CoV-2 virus ‘almost the perfect killing machine‘ for elderly patients. Compared to SARS-CoV (aka ‘SARS Classic’ circa 2003), it is clear that SARS spread at the same time symptoms appeared, thus by screening for fever the spread of SARS could be quickly contained.

A few things that make the current coronavirus SARS-CoV-2 especially difficult to control and take a terrible toll on society:

  • Highly contagious, r0 values (average number one person infects) recently adjusted to 5.7 (was 2.2) from reanalysis of Wuhan data
  • Contagious-state timeframe from 14 to 21 days (mild cases), thus self-quarantine of infected and possible infected takes people out of the workforce or regular duties for a long time
  • Asymptomatic and pre-symptomatic spread (CDC internal estimates ~25% of cases)
  • Serious and critical cases require long hospitalizations; per this study a non-ICU bed for COVID-19 disease has an average stay of 11.5 days, and an ICU bed the average is 14.4 days
  • One or more underlying comorbidities accompanies fatal cases. “Overall, only 3 patients (0.8%) had no diseases, 89 (25.1%) had a single disease, 91 (25.6%) had 2 diseases, and 172 (48.5%) had 3 or more underlying diseases.” JAMA ref. and handy infographic (PDF)

Let’s take a look at accumulating evidence for asymptomatic spread, and some evidence that spending hours together (or longer) in large groups can be a source of mass infection.

Asymptomatic spread in Iceland

With SARS-CoV-2, there is clear evidence of asymptomatic spread. In Iceland, health authorities tested almost 5% of the entire population of the island country (17,900 tested out of 360,000). They used an rRT-PCR molecular test, looking for the viral RNA.

deCODE Genetics (now a part of Amgen) ran about 9,000 of these tests and told CNN about 50% of the positive tests were asymptomatic or pre-symptomatic. They have already started a randomized screening program, collecting blood to look for antibodies against the virus in their population.

Asymptomatic spread in a WA State nursing facility

The CDC, published in their weekly ‘Morbidity and Mortality Weekly Report’ on April 3, 2020 research data with the title “Asymptomatic and Presymptomatic SARS-CoV-2 Infections in Residents of a Long-Term Care Skilled Nursing Facility — King County, Washington, March 2020”.

Of the 23 positive individuals, a full 13/23 (56%) were pre-symptomatic (n=10) or asymptomatic (n=3). And the Ct values ranged from 37 down to 15, reflecting the wide range of Ct values noted in this post from yesterday. Since Ct values are semi-quantitative (and relative primer/probe efficiencies are specific to the sequences used), it is difficult to determine the exact numbers given only Ct values, but suffice it to say there are many orders of magnitude difference in levels, on the order of 105 (that’s 10,000-fold). See below for the figure from the publication.

Figure from Kimball et al MMWR Morb Mortal Weekly Rep 2020;69:377-381.

Asymptomatic spread in a choir practice

On March 6, 60 members of the Skagit Valley Chorale in Mount Vernon WA met for their usual 2.5 hour choir practice. They were aware of coronavirus infection in Seattle an hour’s drive to the south, did not shake hands, had no one coughing or having any kind of symptoms per several choir members.

Remarkably, per this reporting in the Los Angeles Times, 45 of the choir members ended up testing positive of the virus, and several dying of COVID-19. It is not known who was the ‘super-spreader’ individual at that event.

Examples of asymptomatic spread from Italy and Japan

The case of a small town in Italy, Vo, all 3,300 inhabitants were tested for the presence of virus, and yet again of the positive cases (about 100 individuals) again 50% showed no signs of symptoms (here’s a writeup from Newsweek).

In an analysis of all Japanese nationals evacuated from Wuhan China (n=565) with all of them tested, 11.2% (n=63) were symptomatic upon arrival, and a full 30.8% (n=174) tested positive for the virus but did not show symptoms.

You need to wear a mask

The main take-home message is you will need to wear a mask when outside for essential activities for a while. You will likely need to wear a mask in whatever restarting of work occurs, possibly in the month of May.

As I was convinced with some of this evidence a few weeks ago, I’ve been volunteering with The Masks Now Coalition, that connects sewists (those with sewing machines and the ability to use them) to hospitals (for use by non-COVID-19 patients and other staff as an emergency use due to shortages), nursing homes, grocery stores and other group settings. The coalition has requests for several hundred thousand masks, over four thousand volunteers nationwide (either making, distributing, soliciting or organizing), and lead volunteers on a state and local level.

On that note, if you want to volunteer in other ways, has put together this list with over 200 volunteer opportunities. You can use your time helping in a worldwide battle against a terrible scourge.

A different mask group, called Masks4All, has put together this YouTube video with the slogan, “I protect you, and you protect me.”

About Dale Yuzuki

A sales and marketing professional in the life sciences research-tools area, Dale currently is employed by Olink as their Americas Field Marketing Director. For additional biographical information, please see my LinkedIn profile here: and also find me on Twitter @DaleYuzuki.

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