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Infectious Diseases

Updated 30 September 2020

Coronavirus: ‘Silent spreaders’ may be carrying the same amount of virus as those with symptoms

The latest research adds to growing evidence that asymptomatic people can spread the virus as effectively as those who display symptoms.

  • Two recent studies indicate that people 'silently' infected with SARS-CoV-2 may spread the virus as efficiently as those with symptoms
  • Both studies were published in journals this month
  • The researchers recommend extensive testing, as well as continued use of face masks

The true extent of asymptomatic spread of SARS-CoV-2, the virus that causes Covid-19 disease, is still unknown.

In June, a news update by the World Health Organization (WHO) stated that current evidence from contact tracing suggests that asymptomatic (displaying no symptoms) infected individuals are much less likely to transmit the virus than those who develop symptoms.

However, a more recent study, published this month in the journal Thorax, found that people with "silent" Covid-19 infection have as much of the virus in their noses and throats as those with symptoms.

A second study, also published this month in PLOS Medicine, found that while the majority of infected individuals do develop symptoms, they may test positive for the virus before those symptoms appear.

Thorax study: no significant difference between symptomatic and asymptomatic cases

In the Thorax study, researchers at the Asan Medical Centre in Seoul, South Korea, sought to determine how large the viral load of asymptomatic individuals from South Korea might be, as well as to what extent they might contribute to the spread of the virus.

To do this, they compared the viral load of 213 people, all of whom tested positive for SARS-CoV-2, but not all of whom had symptoms of Covid-19 infection, such as fever, dry cough, and sore throat.

Following a large cluster outbreak of Covid-19 in Daegu City, the close contacts of this cluster (a religious group) were traced, which turned out to be more than 3 000 cases. Infected individuals ranged in symptom severity from none to severe.

Those with mild or no symptoms were admitted to dedicated care facilities for isolation and monitoring. For this particular study, a total of 213 participants were admitted to one such facility.

If they didn’t display any of the following symptoms, they were classified as symptomless: fever; chills; muscle pain (myalgia); fatigue; runny nose (rhinorrhea); blocked nose; loss of taste or smell; sore throat; swallowing difficulties; cough; phlegm production; coughing up blood; headache; dizziness; loss of appetite; nausea; vomiting; abdominal pain; and diarrhoea.

More than half of those without symptoms (21, or 54%), and nearly two-thirds of those with mild symptoms (92, or 64%) tested positive for SARS-CoV-2. The researchers also noted that there was no significant difference in the viral load between the two groups.

Although the research had some caveats, including that most participants were in their 20s or 30s – meaning the findings might not apply to other age groups – previous research found that children under the age of five can carry just as much viral load as older children and adults.

PLOS study: pre-symptomatic cases found

In the PLOS study, researchers at the University of Bern in Switzerland pooled data from 79 studies of people infected with SARS-CoV-2 from March to June. More than 6 000 patients with follow-up data were included in the investigation.

The researchers distinguished between asymptomatic and pre-symptomatic cases (people who test positive for the virus when they still feel well but end up developing symptoms later).

According to the findings, the researchers estimated that 20% of patients who tested positive for SARS-CoV-2 never developed symptoms, while others turned out to be pre-symptomatic.

What do these findings mean?

Until it is clear how long, and to what extent, asymptomatic individuals might be infectious, testing should be extended to certain groups as a precautionary measure, the researchers recommended.

In a linked podcast, lead author Professor Sung-Han Kim explained: "Our data add further support to the general public use of face masks, regardless of the presence of symptoms, and suggest that the scope of SARS-CoV-2 testing should be expanded to include asymptomatic individuals in high-risk settings, such as nursing homes or healthcare facilities."