Asymptomatic cases

Many cases of COVID-19 are asymptomatic, Masks are effective at reducing transmission especially from asymptomatic carriers, who continue to expose others to the virus unknowingly.

Table of Contents

  1. Studies on asymptomatic cases
  2. The value of masks due to asymptomatic transmission
  3. Footnotes

Studies on asymptomatic cases

The scale of asymptomatic spread has been greatly underestimated.

That is, many people infected with the virus don't show any symptoms. They go about their day, silently spreading the disease to their family, friends, neighbors, coworkers, classmates, and anyone else they come into close contact with.

There's more and more data emerging on how many people are asymptomatic:

Report Conclusion Date
Covid-19: four fifths of cases are asymptomatic, China figures indicate A total of 130 of 166 new infections (78%) identified in the 24 hours to the afternoon of Wednesday 1 April were asymptomatic 02 April 2020
Suppression of COVID-19 outbreak in the municipality of Vo, Italy 43.2% (95% CI 32.2-54.7%) of the confirmed SARS-CoV-2 infections detected across the two surveys were asymptomatic 18 April 2020
Coronavirus infections could be much more widespread than believed, California study suggests Note Studies suggest that perhaps 30% to 50% of people who contract the new coronavirus have no symptoms 04 April 2020
Preliminary result and conclusions of the COVID-19 case cluster study (Gangelt Municipality) In Gangelt the segment of the population that has thus already formed an immunity to SARS-CoV2 is around 15% This is a lethality of 0.37%, 5 times lower than the expected 1.98%, implying many asymptomatic or mildly symptomatic cases. 09 April 2020
COVID-19 outbreak at a large homeless shelter in Boston: Implications for universal testing 147 (36%) out of 408 people tested positive for the #coronavirus at a large homeless shelter in Boston...only ~1/6 showed symptoms among those tested positive, i.e. 1:5 for symptomatic vs asymptomatic (via Twitter) 15 April 2020
Universal Screening for SARS-CoV-2 in Women Admitted for Delivery 29 of the 33 patients who were positive for SARS-CoV-2 at admission (87.9%) had no symptoms of Covid-19 at presentation 13 April 2020
Coronavirus clue? Most cases aboard U.S. aircraft carrier are symptom-free Roughly 60 percent of the over 600 sailors who tested positive so far have not shown symptoms of COVID-19 16 April 2020

Asymptomatic cases fall into two categories: pre-symptomatic, who eventually develop symptoms, and those who never do. The majority, estimated at about 60-80%, will develop symptoms, on average within about five days and almost all within two weeks (that's why you see quarantine periods of fourteen days). A minority may never develop notable symptoms but are nonetheless contagious for an unknown duration.

Italy is the first country to use large-scale serology (blood tests) to determine this. In the epicenter of the outbreak in Northern Italy, 70% of blood donors in a random sample (albeit a very small one) tested positive for COVID-19 without (yet) showing any symptoms

Half of those tested in Iceland and an estimated quarter of American subjects also displayed no symptoms despite being carriers.

In short, you can be asymptomatic and contagious for 14 days:

When symptoms are developed

Not new information

Asymptomatic transmission was well known in January. Any government agency telling you they're changing their views based on this new information is either negligent, not paying attention in a crisis, or is outright lying.

English language reports in January, not long after the first infection outside of China, warned of asymptomatic cases spreading the virus:

“A major concern is the range of severity of symptoms this virus is causing,” Jeremy Farrar, the director of the Wellcome Trust, said in a statement Tuesday. “It is clear some people are being affected and are infectious while experiencing only very mild symptoms or possibly without experiencing symptoms at all (asymptomatic). This may be masking the true numbers infected and the extent of person to person transmission. It is a matter of urgency to work this out.”

Reports from Germany on asymptomatic spreaders there came out at the end of January.

Professor Kwok-Yung Yuen from the University of Hong Kong-Shenzhen Hospital warned that:

“Because asymptomatic infection appears possible, controlling the epidemic will also rely on isolating patients, tracing and quarantining contacts as early as possible, educating the public on both food and personal hygiene, and ensuring healthcare workers comply with infection control."

The Lancet paper was published January 24th and warned that asymptomatic transmission was already demonstrated:

A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster
A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster (The Lancet)

The Wuhan government is considering costly testing of all residents to find the remaining asymptomatic carriers in an effort to prevent a second outbreak.

Update: detailed meta-research of SARS-CoV-2 testing studies

Scripps Research has recently put out a report (April 20, 2020, via Twitter) that presents a Summary of SARS-CoV-2 testing studies."

Many of these studies we've cited above, but their analysis provides a clearer picture:

Cohort Tested SARS-CoV-2 positive % Positive, but asymptomatic %
Iceland residents 13,080 100 0.8% 43 43.0%
Vo’, Italy residents 5,155 102 2.0% 43 42.2%
Diamond Princess cruise ship passengers and crew 3,711 712 19.2% 331 46.5%
Boston homeless shelter occupants 408 147 36.0% 129 87.8%
New York City obstetrical patients 214 33 15.4% 29 87.9%
USS Theodore Roosevelt aircraft carrier crew 4,954 856 17.3% ~500 58.4%
Japanese citizens evacuated from Wuhan, China 565 13 2.3% 4 30.8%
Greek citizens evacuated from UK, Spain, and Turkey Note 783 40 5.1% 35 87.5%
Charles de Gaulle aircraft carrier crew 1,760 1,046 59.4% ~500 47.8%
Los Angeles homeless shelter occupants 178 43 24.2% 27 62.8%
King County, Washington nursing facility residents 76 48 63.2% 3 6.3%
Arkansas, North Carolina, Ohio and Virginia inmates 4,693 3,277 69.8% 3,146 96.0%
New Jersey university and hospital employees 829 41 4.9% 27 65.9%

The value of masks due to asymptomatic transmission

We've already established that masks are extremely cost-effective in large part due to reducing asymptomatic spread:

The Case for Universal Cloth Mask Adoption & Policies to Increase the Supply of Medical Masks for Health Workers April 1 2020
Masks, including cloth masks and surgical masks, have measurable efficacy at preventing infected people from transmitting viruses to others.The Case for Universal Cloth Mask Adoption & Policies to Increase the Supply of Medical Masks for Health Workers April 1 2020

Every day, we find new evidence which helps estimate the number of asymptomatic case. Our goal with this article is to maintain an up to date collection of this evidence.

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Updated on


  1. This paper found that "The viral load that was detected in the asymptomatic patient was similar to that in the symptomatic patients, which suggests the transmission potential of asymptomatic or minimally symptomatic patients."
  2. COVID-19 Incubation Period: An Update concludes, "In the resulting models, estimated median incubation time (IT) of COVID-19 was 5.1 days; mean IT was 5.5 days. For 97.5% of infected persons, symptoms appear by 11.5 days. Fewer than 2.5% are symptomatic within 2.2 days. Estimated median IT to fever was 5.7 days. Among 108 patients diagnosed outside mainland China, median IT was 5.5 days; the 73 patients diagnosed inside China had a median IT of 4.8 days. Using exposures designated as high risk and a 7-day monitoring period, the estimate for missed cases was 21.2 per 10,000. After 14 days, the estimated number of missed high-risk cases was 1 per 10,000 patients."
  3. This occurred in Thailand and was first reported on January 13. The victim was a Thai national returning from Wuhan.
  4. Nb: this study is controversial and its findings and methodology disputed by many.
  5. Data clarified via personal communication with co-author