How can people spread the new coronavirus if they don’t have symptoms?

  • Asymptomatic coronavirus transmission appears to be low, but it does happen
  • In these individuals, the virus spreads pretty much the same way it would in a symptomatic person
  • This reinforces the importance of physical distancing and wearing face masks


While governments around the world have imposed strict measures to minimise the chances of people contracting the SARS-CoV-2 virus, asymptomatic (displaying no symptoms of disease) spread throughout populations continues to be a threat.

The US Centers for Disease Control and Prevention (CDC) recently estimated that 35% of cases are asymptomatic, and that 40% of transmissions occur before people develop symptoms.

Although the rate of virus transmission by asymptomatic individuals appears to be lower compared to those with symptoms, and numbers may change as more is learned about Covid-19, personal protective measures, such as physical distancing and wearing face masks are strongly encouraged. But how can an asymptomatic individual spread the virus?

Professor Shabir Madhi, professor of vaccinology at the University of the Witwatersrand (Wits) in Johannesburg, who is leading the South African Ox1Cov-19 Vaccine VIDA-Trial, explained during a MyHealthLive webinar last week. 

How this happens

The new coronavirus mainly spreads from one person to another via human respiratory droplets and direct physical contact. The infected droplets of saliva or mucus can be carried through the air when one sneezes, for example.

When this happens, the viral particles emitted can be breathed in by the next person, or it can end up on surfaces that people touch, such as door handles. The same concept applies to an infected person who is asymptomatic.

“As you speak, a whole lot of droplets leave your mouth. These droplets are so small that you simply can’t see them, and this is what happens on a daily basis. So in an asymptomatic person, they’ve got a lot of [the SARS-CoV-2] virus at the back of their throat, and those viruses are intermingling with their saliva.

“And as they speak, the contaminated microdroplets – as well as the droplets you might be able to see with the naked eye – are then dispersed in the air,” explained Madhi.

Madhi added that the larger droplets fortunately settle down on surfaces immediately due to gravitational pull, but this also means that it might contaminate surfaces.

According to a study published in the New England Journal of Medicine, the virus can remain on plastic for 72 hours and on cardboard for 24 hours.

Asymptomatic patients may contaminate many surfaces 

Additional reporting by HealthDay notes that asymptomatic Covid-19 patients are as likely as those with symptoms to contaminate several surfaces in their rooms. This reporting is based on a new study of sampling the surfaces and air of six negative pressure, non-intensive care unit (ICU) rooms against 13 laboratory-confirmed Covid-19 patients – two of whom displayed no symptoms.

The researchers collected samples from surfaces such as door handles, bedrails, light switches and pillows, among others, and found that patient surroundings in the non-ICU negative pressure isolation ward for asymptomatic Covid-19 patients, or with mild disease, were heavily contaminated by the virus. The team stressed the need for thorough cleaning of areas occupied by patients.

Microdroplets disperse faster than large droplets

In the case of microdroplets, Madhi explained that they can continue being airborne for up to 10 minutes, depending on the space in which it occurs.

“It will disperse much faster in an open space but if you’re in a closed room with all of the windows shut, those droplets can circulate in the air for as many as five to 10 minutes,” he said, further commenting:

“So if someone else is in that space and breathes in that air, they could become infected. But more common than that is when you’re in close proximity to the person you’re speaking to, the contaminated microdroplets that are released can travel at a distance of up to about 1.5 metres.”

A previous HealthDay article reported on studies that found that people infected with the virus could easily spread aerosolised viral particles as they cough, breathe or talk.

Two interventions therefore critical

This is exactly why wearing face masks is critical in helping to prevent the spread of the virus, Madhi stressed. 

“These masks are not so much for your own protection, but more for the protection of others that you’re engaging with and who might be exposed to the virus that you are unknowingly infected with (because you’re asymptomatic),” Madhi said, adding that we need to be mindful that the droplets don’t only spread when one sneezes, coughs, or speaks loudly, for instance: 

“Just normal breathing, in fact, releases droplets into the atmosphere.” Supporting this point is a study that was published in May that explained the link between breathing and talking, and the spreading of the virus.

While studies on the relatively new virus continue to emerge, a recent one found that exhaled aerosols can spread the virus nearly four metres, reminding us of the importance of physical distancing. 

READ | 6 things you need to know about the first SA Covid-19 vaccine trial

READ | Why vaccines need to be tried and tested in different populations across the world

READ | Coronavirus in SA: All the confirmed cases

Image: Helena Lopes on Unsplash

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