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Infectious Diseases

Updated 23 September 2020

Latest science on Covid transmission puts focus on those infected, but not yet feeling ill

Roughly 40% of SARS-Cov-2 transmissions occur extremely early, i.e. in the few days before or after the person passing on the virus shows any symptoms, new research suggests.

  • Many guidelines call for self-isolation as soon as someone experiences symptoms of Covid-19
  • However, researchers found that a large portion of transmissions occur before the onset of symptoms
  • This could influence measures like contact tracing and other non-pharmaceutical interventions

The case for wearing masks and doing everything we can to protect ourselves and others may be less effective than believed, as we might carry – and unknowingly spread – the novel coronavirus before we are even aware of being ill.

According to a new study from the University of Oxford, a large portion of transmissions are from people who carry the virus before symptoms appear, or very shortly after they start experiencing symptoms.

The full study is published on the database MedRvix and is awaiting peer review.

Timing from transmission, symptoms and infection

Luca Ferretti and his colleagues, all from the University of Oxford,  investigated 191 cases of SARS-CoV-2 transmission from an infected person to an uninfected person to determine the timing of the initial infection and onset of symptoms, and exactly when infection of the other person occurred.

In their study, they found that as many as 40% of all transmissions occurred before the onset of any symptoms, and 35% occurred on the first or second day of symptom onset.

People with symptoms are easier to identify and can immediately take measures to avoid the spread. However, milder symptoms of Covid-19 can be similar to other respiratory diseases, which makes it difficult for an individual to determine whether they should self-isolate or not.

In the case of asymptomatic spreaders, however, the researchers suggest further interventions, as such a high proportion of transmission occurs without symptoms. They suggest actively finding such individuals through contact tracing, and curbing the spread through face masks, hand hygiene and various degrees of distancing.

Understanding the trajectory of the virus

According to the team, an investigation into this timing is crucial to understand the epidemic better. This may also help shape non-pharmaceutical measures such as isolation, contact tracing and other interventions to help curb the spread of the virus in the absence of a vaccine.

Current algorithms from tracing apps may only account for symptomatic spreaders, which means that many potential transmissions are missed. They state that the large fraction of transmissions that occur before symptoms appear is impeding efforts to curb the spread of Covid-19.

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READ | SA scientists to explore asymptomatic spread of Covid-19 in the country 

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