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Can asymptomatic individuals spread Covid-19?

Updated: Aug 11, 2021

Controversy just keeps hitting Covid-19.

Before this week, the standard concern for Covid-19 spread was that asymptomatic individuals were a great threat to spread the disease. This opinion was supported by the CDC, Harvard Global Health, and others. Since these individuals were asymptomatic and had undiagnosed disease, they were integrated into society with susceptible individuals and could theoretically create unchecked spread of disease.

In a filmed interview early this week, Dr. Maria von Kerkhove, Covid-19 technical lead for the World Health Organization (WHO), evoked a major reaction by effectively contradicting this notion. When referring to the spread of the disease, Dr. Kerkhove stated, “It appears to be rare that an asymptomatic person actually transmits onward to a secondary individual.” In the same interview, she also stated that many of the previously thought to be asymptomatic individuals who transmitted the disease, actually had mild symptoms and were not really asymptomatic.

On June 9, the day after this previous interview, Dr. Kerkhove backpedaled from her previous remarks by stating “there are so many unknowns” regarding the question of whether asymptomatic individuals can spread the infection.

So do asymptomatic individuals spread Covid-19 or not?

The main difficulty in answering this question is the difficulty in designing an accurate study to answer this question. Part of the issue is the definition around asymptomatic, presymptomatic, and symptomatic with “mild symptoms.” In various studies attempting to analyze this area, these three distinct characteristics are rarely defined clearly. The individual may have been asymptomatic (or presymptomatic) when the infection occurred but then later developed symptoms. The existence of this possibility is the scariest from a public health standpoint in these types of transmission are the hardest to prevent. In the studies of contact tracing, this type of individual is not properly represented as asymptomatic as they have developed symptoms in retrospect. Therefore, the risk of transmission from this person would not qualify as an asymptomatic transmission in WHO retrospective studies. However, the biggest issue may be that investigators never know when exactly the infection occurred and who definitely was the transmitting party.

Dr. Kerkhove clearly believes that the vast majority of infections are spread by individuals who have at least mild symptoms. She bases this on the few existing studies that try to tackle this question. These studies have been retrospective contact tracing studies that analyze infected individuals and try to determine how they became infected.

However, these studies contradict previous Singapore data cited by the CDC that found clusters of presymptomatic patients were capable of transmitting disease. An article last month in the NEJM illustrated this very point as presymptomatic individuals in a nursing home may have responsible for subsequent infections.

Additional problems include uncertainty in determining exactly who was the infecting individual to another person. The virus does not incorporate the DNA of the person who infected the next person, so the infecting individual is often subject to speculation.

So, knowing this is a difficult question to answer, how do we limit infections going forward?

A key point Dr. Kerkhove mentioned was that many previously thought to be asymptomatic actually had mild symptoms. Since there is no direct way to screen for asymptomatic individuals without everyday testing, the next best thing is to screen for those with mild symptoms and have a major concern that these individuals could be positive for Covid-19 and able to transmit the infection to others. In the workplace, an exhaustive symptom monitor would then become even more important and serve to screen those individuals who need expedient quarantine.

In the meantime, until the WHO can really explain their convoluted opinions on the subject with definitive studies, we must assume that asymptomatic, presymptomatic, or those with any Covid-19 symptoms have a chance of spreading the infection to others.

The stakes are too high.

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