How Dangerous Are Covid-19 Silent Spreaders?

A lot of people without symptoms are spreading the coronavirus, a new study suggests

Before boarding for a mid-March cruise from Argentina to Antarctica, passengers were screened for fever. In addition, any prospective passenger who had been to an area with Covid-19 outbreaks was not allowed to board.

“We thought we were pretty safe,” says Alvin Ing, MD, a professor of respiratory medicine at Macquirie University Hospital in Australia and one of several scientists on the cruise. “However, reality struck on the eighth day, when a passenger developed fever, and it sort of snowballed from there.”

Passengers were then confined to their cabins. The crew donned protective equipment and ended all but essential services to reduce crew-passenger contact (they still delivered meals).

Yet by the 20th day of the aborted cruise, before the 217 passengers and crew had any contact with people beyond the ship, 59% tested positive for coronavirus. Among them, 81% had no symptoms, Ing and his colleagues report in the journal Thorax.

Infections ‘much higher’ than expected

The finding illustrates not just how quickly Covid-19 can spread, but also the importance of testing and mandatory isolation of people who may have been exposed to the disease even if they don’t have symptoms, Ing says.

“As countries progress out of lockdown, a high proportion of infected, but asymptomatic, individuals may mean that a much higher percentage of the population than expected may have been infected with Covid,” says Alan Smyth, MD, joint editor-in-chief of the journal and a professor of at the University of Nottingham in the UK.

The study did not determine whether the single initially diagnosed case on the ship was responsible for all the others, or if other people might have been carriers when they came aboard but didn’t yet have a fever or other symptoms. Nor did it determine how many of the people who tested positive without symptoms developed symptoms later.

Either way, scientists have already established that people infected with Covid-19 can spread the disease before symptoms appear (presymptomatic), and those who never develop symptoms (asymptomatic) can also infect others. They’re called “silent spreaders” because they don’t feel sick and they’re not coughing or sneezing or otherwise making it obvious that they are in fact shedding a virus.

(The term “silent spreader” is distinct from “super spreader,” a person who may or may not know they are infected, and who by virtue of where they go and what they do might infect dozens or even hundreds of people.)

“The number of asymptomatic cases of COVID-19 is significant,” a group of UK doctors wrote April 16 in The Lancet journal. Among the data they cite: A small study in China found 78% of positive cases to be asymptomatic. On the Diamond Princess cruise ship in February, 57.7% of people who tested positive had no symptoms at the time of the test, and 17.9% never did develop symptoms.

The U.S. Centers for Disease Control and Prevention estimates that between 20% and 50% of Covid-19 infections may be asymptomatic. Younger people are more likely to be in this category, raising the risk for the those they come in close contact with, be it their peers or their parents.

Another small study, published May 27 in the journal JAMA Network Open, analyzed 78 people who were diagnosed with Covid-19 in a screening process of close contacts other people infected in the early days of the outbreak in Wuhan, China. Of those, 33 (42.3%) had no symptoms. The asymptomatic patients were younger (median age 37) compared to those with symptoms (median age 56).

The upshot: “We do know that asymptomatic transmission occurs,” says Amesh Adalja, MD, an infectious disease expert at the Center for Health Security at Johns Hopkins University. “But we still don’t understand fully everything that governs it.” Nor, Adalja says, is it clear just how prevalent the phenomenon is.

Indoors is worse than outdoors

In a phone interview today, Adalja said infections from people without symptoms are most likely to occur in crowded environments where close contact occurs, such as at home, in a nursing facility, or at an indoor space like a bar where people are touching or talking in close proximity for extended periods, or possibly sharing food or drink or touching the same surfaces.

That’s why facemasks are considered vital whenever strangers gather in close proximity (to protect others, more so than protecting the wearer) and health experts continue to advise people to observe 6-foot spacing and be vigilant about hand-washing.

Meanwhile, there is increasing evidence that the virus can become airborne beyond the standard 6-foot guideline, and that under the right circumstances — such as stagnant air or a long period of exposure — a person could be infected by airborne virus-containing droplets. Experts in building hygiene recommend upgrading air-conditioning filters to help reduce Covid-19 spread in offices and other workplaces.

Even while outdoors, “asymptomatic individuals who are speaking while exercising can release infectious aerosols that can be picked up by air streams,” Kimberly Prather, an atmospheric chemist at the Scripps Institution of Oceanography, and her colleagues write in a May 27 perspective in the journal Science. “Viral concentrations will be more rapidly diluted outdoors,” they write, “but few studies have been carried out on outdoor transmission of SARS-CoV-2.”

One way to think about how far the virus might travel is to imagine exhaled cigarette smoke, Prather and her colleagues say: “The distance from a smoker at which one smells cigarette smoke indicates the distance in those surroundings at which one could inhale infectious aerosols.”

Adalja contends, however, that you won’t catch Covid-19 just by walking past an infected but asymptomatic person on the street.

“The virus can’t teleport,” he says.