The issue of asymptomatic spread of SARS CoV-2 has cast a cloud over the pandemic. We as humans are hardwired to fear that which is invisible more than that which is visible. The idea that thousands of individuals could be walking around spreading the virus without knowing it is truly terrifying. I sometimes find myself looking at everyone as as a threat, not just those with a cough. Unfortunately, as with everything else in this pandemic, the virus doesn’t seem to care about our anxieties and is intent on spreading any way it pleases. The day supporting asymptomatic spread has strongly influenced the recommendations for mask wearing that (almost) all of us have been following.

With this as the background there was, not unsurprisingly, great tumult created when the WHO recently published a report that the available evidence from contact tracing suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms.

There is real danger that the WHO report will lead to confusion in the public. Many news outlets strongly suggested in their reporting that the WHO is saying that there is NO asymptomatic spread. These conclusions are leading many to think that if you don’t have symptoms you are not contagious. This would be great news but don’t go burning your masks just yet.


Don’t go burning your masks just yet.


First, the report is very clearly not saying that asymptomatic transmission does not happen. It is merely suggesting that infected individuals without symptoms are less likely to transmit the virus than those who develop symptoms.

The WHO guidance admits that studies on transmission from asymptomatic individuals are difficult to conduct. Most studies included in the WHO guidelines have important limitations that are inherent in the study of the transmission of infectious disease:

  1. The studies rely on self reporting of symptoms. Human nature leads many to downplay or explain away their symptoms as being from something other than the infection (e.g. allergies). This would be especially pronounced in a situation where a diagnosis would lead to stigma which is the case with COVID-19. Therefore many people who are considered “asymptomatic” are really only mildly sypmtomatic.
  2. Many of the studies did not properly define which symptoms they were investigating. If they were looking for cough and fever but ignored diarrhea or vomiting they may label some patients as asymptomatic.
  3. Viable virus has been isolated from people before they develop symptoms and in those who have never developed symptoms. This would suggest that people who do not have symptoms may be able transmit the virus to others contrary to the conclusion for the report

If you read carefully, the report itself recommends that the general public continue to wear masks in public settings (e.g. grocery stores, at work, social gatherings, mass gatherings, closed settings, including schools, churches, mosques, etc) and situations where physical distancing is not possible (e.g. many jobs and public transportation). For people over 60 and those with other risk factors, they even recommend medical masks not just cloth face coverings in the above situations.


There are studies which show there is asymptomatic spread and people are notoriously unreliable reporters of whether they have symptoms.


Second, the conclusion may be wrong fundamentally. There are studies which show there is asymptomatic spread and people are notoriously unreliable reporters of whether they have symptoms. Further, coronavirus infections have continued spreading in situations where people with symptoms were isolating at home. This indicates taht asymptomatic and pre-symptomatic people were spreading the virus.  Spreading was also traced to schools around the world. Since kids rarely get symptoms, the spread in schools must be coming from asymptomatic carriers.

Further, there have been studies supporting a conclusion opposite that of the WHO. One in the Annals of Internal Medicine combined multiple studies and concluded that asymptomatic persons account for approximately 40% to 45% of infections. Another study found 23% of transmissions were from asymptomatic people

Another major problem is presymptomatic spread. This is not captured by the data incorporated into the WHO report. If you ask people at one point in time if they have symptoms and then never follow up you’ll never find out if they developed symptoms in time. One study estimated that 44% of people were infected by a person who was presymptomatic. Another found high transmissibility before symptoms developed. Imagine someone interprets the WHO report to mean that if they don’t have symptoms they are not contagious. This will lead them to conclude that they don’t need to wear a mask when recommended. If they get infected they would be contagious without symptoms for 2-4 days. During this time they will spread the virus.

It is my great hope that the WHO turns out to be correct. Public officials would then have more flexibility in reopening places such as schools and summer camps, mask-wearing might become less essential, and contact tracers would operate under different assumptions. I am not confident this will be borne out. Even if truly asymptomatic spread is very rare, pre-symptomatic transmission would still be important. We would still need to wear masks and distance to avoid spreading the virus during this period.

The bottom line is that the WHO report doesn’t change anything at this time. It is still recommended that you wear a mask in settings where physical distancing cannot be maintained. You should still isolate yourself if you are sick with COVID-19 and quarantine if you are exposed. Most importantly, you should not overreact to news reports (for more on that read this). The COVID-19 drama continues…

3 thoughts on “Are asymptomatic carriers a threat?

  1. Thanks for addressing this topic! I was hoping you would.
    Can you also speak on the idea of “super spreaders”? Is this relevant to the current pandemic? Are there really people who may be much more highly contagious than others? It is just because of their behaviors, or would a person’s unique biochemistry also factor in?

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    1. Contagiousness is probably influenced by a number of factors. It is likely related to the viral load of the infected person. It could also be related to how strong an individuals cough is. People who have a more pronounced cough reflex will spread more widely. There are also factors such as how many people the infected indvidual comes in contact with. There may also be immune factors. Some people’s immune system are better at neutralizing virus on mucosoal surfaces. It is all very complicated.

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