When Adriano Trevisan died of COVID-19 in early 2020, he became the disease’s first Italian fatality. His death, at age 77, spurred a chain of events that have shed light on a critical question: How often is the disease spread by people without symptoms?
Trevisan lived in a hillside village near Venice. Upon his death, the local governor not only quarantined the entire village but also ordered all 3,341 residents to be tested for SARS-CoV-2 twice, before the lockdown and two weeks later.
What the testers found: Among the 88 villagers infected, at least 50% were “completely asymptomatic,” reported Sergio Romagnani, M.D., a professor of clinical immunology at the University of Florence, who was involved in the testing.
Romagnani called asymptomatic transmission “a formidable source of contagion.”
That remains to be seen, but the Italian results, though unpublished and on the high side, generally align with COVID-19 studies of cruise-ship passengers, Icelandic volunteers, and hospitalized patients in Tibet.
The U.S. Centers for Disease Control and Prevention’s director, Robert Redfield, M.D., estimates the number of “covert cases” at 25%.
“This helps explain how rapidly this virus continues to spread,” Dr Redfield said.
The World Health Organization (WHO) maintains that most patients classified as asymptomatic are actually pre-symptomatic — people who felt fine when tested but later developed the telltale fever, cough, and aches. WHO has called the percentage of truly asymptomatic cases “relatively rare.”
Nonetheless, WHO agrees with the scientific consensus that asymptomatic transmission is not rare.
Whether asymptomatic people never fall ill or whether they go on to develop symptoms, the fact remains: infected people transmit SARS-CoV-2, the virus causing COVID-19, while feeling perfectly healthy.
“The bottom line is that there are people out there shedding the virus who don’t know that they’re infected,” said Jeffrey Shaman, Ph.D., a public health expert at Columbia University in New York City.
Silent spreaders, covert cases, submerged infections, stealth transmission — whatever term you use, the phenomenon poses no small challenge for controlling the spread of COVID-19, especially at healthcare facilities.
In the novel coronavirus, hospitals face a daunting combination of circumstances: a deadly disease that is likely transmitted through the air and spread by people who may never even get sick.
As evidence mounts that SARS-CoV-2 can be spread via aerosol, hospitals must respond by taking extra measures to prevent stealth transmission among healthcare workers.
How Common Are Covert Cases of COVID-19?
Submerged infection is nothing new. Norovirus, influenza, human respiratory virus, mild coronavirus strains — every day, millions walk around oblivious to the fact that they’re infected with one virus or another.
A few years back, before SARS-CoV-2 was unleashed upon humans, researchers stood at a New York City tourist attraction and swabbed 2,600 visitors who volunteered to be tested. Among the tourists, 6.2% tested positive for a virus; 65% to 97% of the infections were classified as asymptomatic.
As scientists work to determine where COVID-19 fits in, the most useful data comes from closed-off populations who’ve undergone comprehensive testing.
Adriano Trevisan’s Italian village was one such population; the Diamond Princess cruise ship was another. Quarantined for two weeks off the port of Yokohama, Japan, the Diamond Princess had over 3,700 passengers and crew aboard.
Two days after the ship docked, 21% of the passengers tested positive for COVID-19; among those infected, 17.9% never developed symptoms.
“The substantial asymptomatic proportion for COVID-19 is quite alarming,” said American epidemiologist Gerardo Chowell, Ph.D., part of the research team.
Chowell notes that most of the ship’s passengers were over 60 and therefore at elevated risk for developing symptoms; among the general population, he estimates, the proportion of covert cases may be as high as 40%.
In Iceland, about 50% of volunteers who tested positive for COVID-19 reported feeling no symptoms, though the sample of 9,000 was entirely self-selected, and it’s likely some of them were pre-symptomatic.
More compelling was an analysis of COVID-19 patients quarantined in a hospital in Tibet.
In this study, there were no doubts: All local residents who tested positive — 83 patients — were hospitalized for two weeks and thoroughly examined. Among them, 21.7% neither demonstrated nor reported symptoms during the surveillance period.
These patients weren’t feverish or coughing or achy, yet most had “viral pneumonia-like changes” in their medical workups. Some 83% had abnormal blood chemistry, and 39% had abnormal chest CT images.
How Contagious Are People Without Symptoms?
Clearly, plenty of people infected with the novel coronavirus are walking around without fevers or coughs. But are they contagious?
Certainly, covert carriers of other viral diseases can transmit infection.
“For measles and norovirus infections, it is well established that asymptomatic individuals are frequently able to transmit the virus to others,” wrote the Diamond Princess research team.
Typically, it takes an onslaught of viral particles to overwhelm the immune system, invade a cell, and start replicating. It’s generally thought that the higher a patient’s viral load — the more virus their cells are emitting — the more infectious the patient.
With COVID-19, at least some asymptomatic patients seem to be just as virally loaded as those with fevers, coughs, and breathing difficulties.
One Chinese study describes an infected 26-year-old who never developed symptoms but whose viral load, based on nose and throat swabs, “was similar to that in the symptomatic patients.”
Another Chinese study traced a family cluster of nine members. Among the eight who became infected, two never developed symptoms, and two were infected by members who had not yet fallen ill.
“Asymptomatic patients can still infect others,” the authors wrote, “These ‘silent patients’ may remain undiagnosed and be able to spread the disease to a large number of people.”
In cases of pre-symptomatic transmission, those infected appear to be most contagious in the two days or so before symptom onset; after that, viral load begins to decline.
One Chinese team, using a variety of models, reported that “infectiousness peaked on or before symptom onset” and estimated that between 44% and 52% of COVID-19 transmission occurs before illness.
It remains unclear whether asymptomatic or mild cases are as contagious as severe cases. However, to protect patients and healthcare workers alike, hospitals must take stealth transmission every bit as seriously as transmission from severely ill patients.
“Just because you get the disease from someone with mild symptoms does not mean yours are going to be mild,” cautions Columbia University’s Jeffrey Shaman. “You could still end up in the I.C.U.”
How Asymptomatic Transmission Occurs
No doubt, asymptomatic transmission is contributing to the spread of COVID-19. Infected people who feel just fine are passing on the virus simply by talking or breathing — no sneezing or coughing necessary.
It may be enough for an infected healthcare worker, one who may never become ill or may develop a fever tomorrow, to chat with a colleague. It may be enough for her to sit on a bench and sigh, releasing microscopic viral particles that waft about the hospital, only to be inhaled by someone else or to land on a computer keyboard that someone else touches.
“We can’t assume that any of us are not potential vectors at any time,” says Carl Bergstrom, Ph.D., an American expert in emerging infectious diseases.
Though droplet and contact transmission may be driving the pandemic, evidence for aerosol transmission of SARS-CoV-2 is accumulating.
In multiple hospital studies, air samples have tested positive for SARS-CoV-2 — in hallways and in rooms where healthcare workers removed protective clothing after treating COVID-19 patients.
And now a study conducted at two Wuhan hospitals has found the virus was “widely distributed” in the air and on surfaces in both the ICU and general COVID-19 ward, “implying a potentially high infection risk for medical staff and other close contacts.”
What’s more, the authors found the transmission distance of SARS-CoV-2 particles may actually be 4 metres, more than twice the distance that heavier droplets can travel.
The authors don’t know whether the airborne load of SARS-CoV-2 was potent enough to transmit infection. But they do not advise taking any chances and recommend their findings be “used to improve safety practices.”
Throughout the pandemic, hospitals have been doing all they can to maintain hand hygiene and surface cleaning and to provide healthcare workers with sufficient protective gear. But gear shortages have been dire, hand hygiene is notoriously inadequate, and cleaning crews simply cannot keep up.
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