The CoviD-19 saga: Symptomatic vs Presymptomatic vs Asymptomatic Transmission

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Back in May 2020, our politicians and health officials debated on the importance of mass testing out of fear of asymptomatic transmission. As much as I personally agree, backed with evidence, that asymptomatic transmission must not be downplayed – the capacity of healthcare and the ability to test remains hierarchical, and asymptomatic patients come only second to presymptomatic patients, and presymptomatic patients come second to symptomatic patients. 

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So our need to test asymptomatic patients is limited to how much the capacity of our healthcare can handle the more serious ones. If the healthcare capacities are stagnant, there’s basically less ways to conduct surveillance to detect asymptomatic transmissions in a community.

But let’s go back to basics, it’s important to know the differences of symptomatic, presymptomatic and asymptomatic transmission. 

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This is based on WHO’s situation report no. 73, which differentiates the three types.

Symptomatic Transmission

By way of definition, a symptomatic CoviD-19 case is a case that has developed signs and symptoms compatible with CoviD-19 virus infection (link to CoviD19 saga Symptomatology and Virology). Symptomatic transmission refers to transmission from a person while they are experiencing symptoms. 

Data from published epidemiology and virologic studies provide evidence that CoViD-19 is primarily transmitted from symptomatic people to others who are in close contact through respiratory droplets, by direct contact with infected persons, or by contact with contaminated objects and surfaces. 

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Data from clinical and virologic studies that have collected repeated biological samples from confirmed patients provide evidence that shedding of the CoviD-19 virus is highest in the upper respiratory tract (nose and throat) early in the course of the disease. 

That is, within the first three days from onset of symptoms. Preliminary data suggests that people may be more contagious around the time of symptom onset or the time symptoms just started to appear as compared to later on in the disease.

Presymptomatic Transmission

The incubation period for COVID-19, which is the time between exposure to the virus (becoming infected) and appearance of symptoms is on average 5-6 days, however can be up to 14 days. And in uncommon isolated cases, reach 19, 24 and 27 days. 

During this period, also known as the “presymptomatic” period, some infected persons can be contagious. Therefore, transmission from a pre-symptomatic case can occur before symptom appear. In a small number of case reports and studies, pre-symptomatic transmission has been documented through contact tracing efforts and enhanced investigation of clusters of confirmed cases. This is supported by data suggesting that some people can test positive for COVID-19 from 1-3 days before they develop symptoms. Thus, it is possible that people infected with CoviD-19 could transmit the virus before significant symptoms develop. It is important to recognize that pre-symptomatic transmission still requires the virus to be spread via infectious droplets or through touching contaminated surfaces.

Asymptomatic Transmission

An asymptomatic laboratory-confirmed case is a person infected with COVID-19 who does not develop symptoms. Asymptomatic transmission refers to transmission of the virus from a person, who does not develop symptoms. There are few reports of laboratory-confirmed cases who are truly asymptomatic. This does not exclude the possibility that it may occur. Asymptomatic cases have been reported as part of contact tracing efforts in some countries. WHO regularly monitors all emerging evidence about this critical topic and will provide an update as more information becomes available.

Asymptomatic Transmission is still an open question. Research on asymptomatic transmission currently lacks data. WHO’s Maria Van Kerkhove, released a statement on a press briefing that asymptomatic transmission is “very rare” sparking confusion.  She later re-clarified that she was referring to patients with NO symptoms in the duration of the disease, and not the fact that pre-symptomatic patients can actually infect people when they still have not developed symptoms and are thus, at that moment, asymptomatic. 

There’s currently a disparity in the data of asymptomatic transmission, some studies estimate fewer than 20% of cases are asymptomatic – the WHO’s recent estimate is at 16%. But according to a review study on the Prevalence of Asymptomatic SARS-CoV-2 Infection, published in the Anals of Medicine, it could be as high as 45%.

The importance of acknowledging asymptomatic transmission is not of fear mongering and panic, but to convince people that even if they feel “fine” they could possibly be carrying the disease, that is why one of the goals of our health authorities is to persuade the estimated 7 billion people across the globe to wear a masks to protect themselves and each other.

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