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:
|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
In short, you can be asymptomatic and contagious for 14 days:
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.”
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:
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
Many of these studies we've cited above, but their analysis provides a clearer picture:
|Cohort||Tested||SARS-CoV-2 positive||%||Positive, but asymptomatic||%|
|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:
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.