Asymptomatic carriers transmit SARS-CoV-2 with normal breathing

The coronavirus disease (COVID-19) first emerged in a seafood market in Wuhan City, Hubei Province, China, in December 2019. Since then, it has spread to 187 countries, infecting more than 3.58 million people. The novel coronavirus spreads rapidly partly due to asymptomatic carriers. Now a new study shows that asymptomatic carriers may transmit the virus by merely breathing.

A team of researchers at the Swiss Centre for Occupational and Environmental Health and the University of Zurich wanted to estimate virus release from non-symptomatic carriers into different aerosol sizes by coughing and normal breathing. Further, they aimed to see what exposure can result from these activities in a room shared with other people.

Study: Estimation of SARS-CoV-2 emissions from non-symptomatic cases. Image Credit: JHDT Productions / Shutterstock
Study: Estimation of SARS-CoV-2 emissions from non-symptomatic cases. Image Credit: JHDT Productions / Shutterstock

Spreading the virus

Since the emergence of the novel coronavirus, now called the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), about five months ago, many scientists have studied its behavior and mechanism of infection. They found that the virus may cause severe disease in around 20 percent of those infected. Meanwhile, other patients may only experience mild to moderate symptoms, while some are asymptomatic. This means that they do not manifest any symptoms of the viral infection at all.

The worry of most scientists is, these asymptomatic carriers are still capable of spreading the virus, making it more difficult to contain and isolate active cases. Now, a new study, published on the preprint server medRxiv*, shows that even asymptomatic carriers can spread the virus by merely breathing in a room.

To arrive at their findings, the team modeled the release of viruses from individual patients by calculating the viral load per exhaled droplets formed during normal breathing and while coughing. The size distribution gave a base estimate of the concentration of the virus copes released by a regularly breathing or coughing patient. The team also simulated the situation when a carrier is in an enclosed room with different ventilation air exchange rates.

Virus spread through breathing and coughing

They found that when they computed the viral emissions from carriers who are breathing normally, they found that the highest virus load is present in the largest aerosol size with a cumulative total emission per breath of 0.34 copes/cm3 (air) for an average patient, while 11.5 copies/cm3 for high emitters. The total emissions in the PM10 fraction were approximately one-third of these values with 0.12 copies/cm3 (average) and 4.1 copes/cm3 (high) per breath.

This means that just by merely breathing, patients can still transmit the novel coronavirus. Further, the team also calculated the virus emissions from a coughing patient. They found that the total emission per cough was about 19,400 copies/cm3 for an average patient, and 651,315 copies/cm3 for high emitters. The cumulative emissions in the PM10 fraction were half of these values, with 10,900 copies/cm 3 (average) and a staggering 366,000 copies/cm3 (high) per cough.

To compare, the team also estimated the risk of exposure of bystanders who are spending time in the same room with a COVID-19 positive patient. This may help shed light on how the virus spreads in closed areas with asymptomatic patients who are not aware that they are carriers. The team revealed that for a typical hospital ventilation situation of ten air exchanges per hour, the concentration plateaus after about 30 minutes, while for a workplace with about three air exchanges per hour, concentrations continue to increase for almost three hours.

What does this mean?

The study sheds light on how the virus behaves and how it spreads across populations via local transmission. An increased number of viruses are expected to be released by COVID-19 positive patients in the form of airborne aerosols, particularly when they are coughing. However, infected patients, even asymptomatic ones, can still transmit the virus even through normal breathing.

Though a large proportion of the virus is in the form of large particles that can deposit rapidly, there are still viruses suspended in smaller size aerosols, which can remain airborne for an extended time. These smaller air particles are more potent since they can reach the deeper regions of the lungs.

The team also emphasized the risk of staying in a room with a COVID-19 patient. An average person breathes about one-half m3 per hour in resting state, and it can increase during exercise.

“Thus, a person spending time in a room with an average emitting patient breathing normally has a realistic chance of inhaling tens to hundreds of copies of the virus even when keeping distance from that person. The situation is worse in the presence of a high emitter and worst if the patient is a coughing high emitter,” the researchers wrote in the paper.

The situation is grim in the presence of a high emitter of the virus and even worse if the patient is coughing.

“The very high virus load in exhaled respiratory aerosols proposed by our assessment may be an explanation why COVID-19 was associated with much more transfections to hospital staff than what was expected from SARS,” the team added.

Also, while mandating everyone to wear a surgical mask as effective infection control and prevention of infection, the protective factors may still be inadequate if long periods are spent in the same room with a coronavirus positive patient.

The researchers suggest that workplaces and other buildings should improve ventilation in the room. Workplaces may need to revamp designs and the number of people staying in an office. The risk of infection is real when being near an asymptomatic carrier in a room for more than a few minutes, even if social distancing is observed.

Sharing workplace rooms with asymptomatic carriers is not advised. Hence, the team suggests that offices should not be shared if there is no mass testing available to differentiate healthy and non-symptomatic cases. Hospitals should also provide adequate personal protective equipment (PPE)s to health workers, especially when in the same room as a patient who is coughing. All patients should wear a well-fitting surgical face mask to reduce emissions, even if they are not coughing.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Angela Betsaida B. Laguipo

Written by

Angela Betsaida B. Laguipo

Angela is a nurse by profession and a writer by heart. She graduated with honors (Cum Laude) for her Bachelor of Nursing degree at the University of Baguio, Philippines. She is currently completing her Master's Degree where she specialized in Maternal and Child Nursing and worked as a clinical instructor and educator in the School of Nursing at the University of Baguio.


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