Asymptomatic spread of SARS-CoV-2 appears to be rare, says WHO

By Angela Betsaida B. Laguipo, BSNJun 10 2020

The coronavirus disease (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has taken a toll on many countries, infecting more than 7.35 million people. Previously, scientists claimed that the virus could be transmitted via asymptomatic carriers, or those not showing symptoms of the disease.

SARS-CoV-2 - Transmission electron micrograph of SARS-CoV-2 virus particles, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID

Now, in a controversial statement, a World Health Organization (WHO) official said that based on the data the health agency has, it seems to be rare that an asymptomatic carrier can transmit the virus to another person.

"From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual," Van Kerkhove, World Health Organization's technical lead for novel coronavirus response and head of the emerging diseases and zoonoses unit, said in a press briefing.

"We are constantly looking at this data, and we're trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward," she added.

Silent shedders

Asymptomatic individuals have been dubbed as "silent shedders" because they appeared to transmit the virus even without any symptoms, such as cough. Some people, particularly young and healthy individuals who are infected never develop symptoms or only develop mild symptoms.

Now, WHO says while asymptomatic people can still transmit the virus, it is very rare.

"We have a number of reports from countries who are doing very detailed contact tracing. They're following asymptomatic cases; they're following contacts, and they're not finding secondary transmission onward. It is very rare -- and much of that is not published in the literature," Kerkhove said.

The comment falls in line with what WHO health experts have been stressing for months. They underlined that SARS-CoV-2 spread happens through sustained close contact between people indoors, such as offices, homes, hospitals, churches, and other settings where people congregate for hours and where there is inadequate ventilation.

Novel Coronavirus SARS-CoV-2 This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like. Credit: NIAID-RML

Using the right term

Health experts question the latest comment from WHO. However, Dr. Isaac Bogoch and Dr. Allan Detsky of the University of Toronto had previously pointed out that the misunderstanding may be due to an issue of semantics.

They said that the term "asymptomatic" was used to describe patients who are "presymptomatic" or those who are infected but took several days before the symptoms appear.

In medical jargon, asymptomatic pertains to a person who has absolutely no symptoms at all. Even when interviewed by clinicians, these people will deny they ever experienced signs and symptoms, such as fever and muscle pain.

On the other hand, some people have no symptoms for the first few days before manifesting the classic symptoms of COVID-19. These people are classified as "presymptomatic" patients.

Meanwhile, some are "paucisymptomatic" or "subclinical" patients, wherein they have mild symptoms but do not seek medical treatment. Amid the coronavirus pandemic, most often, people who are presymptomatic and paucisymptomatic are mislabeled as asymptomatic.

Novel Coronavirus SARS-CoV-2: This scanning electron microscope image shows SARS-CoV-2 (round gold objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2, also known as 2019-nCoV, is the virus that causes COVID-19. The virus shown was isolated from a patient in the U.S. Credit: NIAID-RML

Focus on detecting and isolating symptomatic patients

Kerkhove said that government responses should focus on the detection and isolation of infected people with symptoms. Extensive contact tracing is also essential to determine all individuals who might have come into contact with them.

Kerkhove added that she acknowledges that some studies have shown the presence of asymptomatic and presymptomatic spread in household settings and nursing homes. Further studies are needed to determine whether the coronavirus can spread widely through asymptomatic carriers.

Presymptomatic patients drive virus spread

A report released by the U.S. Centers for Disease Control and Prevention (CDC) revealed the occurrence of presymptomatic transmission of SARS-CoV-2. The study shows the investigation of 243 cases of COVID-19 in Singapore between January 23 and March 16, revealing that seven clusters of cases point to presymptomatic transmission as the reason for secondary cases.

The report emphasizes the importance of containment measures, such as social distancing, including the closure and ban of crowded coming together.

"Public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of presymptomatic transmission. The potential for presymptomatic transmission underscores the importance of social distancing, including the avoidance of congregate settings, to reduce COVID-19 spread," the report said.

Contact tracing a potent weapon against SARS-CoV-2

In a media briefing, WHO director-general Tedros Ghebreyesus reiterates that contact tracing remains a vital element of the response to combat COVID-19.

He added that existing polio surveillance networks could be utilized in the COVID-19 response. In some countries, health works for polio are now being deployed for COVID-19.

Further, Dr. Tedros noted the importance of using digital tools for contact tracing. These tools can help health workers to determine all the contacts of an infected person and identify all possible cases.

"As part of a comprehensive approach, digital contact-tracing tools offer the opportunity to trace larger numbers of contacts in a shorter period, and to provide a real-time picture of the spread of the virus," Dr. Tedros said.

"We also emphasize that digital tools do not replace the human capacity needed to do contact tracing," he added.