Estimating the Transmission Risk of Asymptomatic in a Community

It is generally known that symptomatic patient has a higher transmission risk.  According to this article by Yale Medicine,  a Delta symptomatic patient can spread to maybe 3.5 or 4 other people.  As for asymptomatic patients,  the common knowledge is that its transmission risk is relatively low. This article will try to examine from a different approach why some were saying that asymptomatic or people without symptoms are responsible for about 59% of the Covid transmission.

What happens at the site?

Ministry of Health (MOH) of Singapore has this pie chart showing that about 98.7% of the patients are asymptomatic or patients with mild symptoms.   This has been the case for many months.  It is quite clear that there are lots of asymptomatic/patients with mild symptoms in the community.  

How are they Responsible for the Transmission?

One can always argue that asymptomatic is not responsible for the spreading of Covid in the community.    It is always the symptomatic patients passing on the Covid to the asymptomatic.

According to this study which examined and traced the contact histories of about 7,000 people,  asymptomatic patients have a transmission rate of 0.06% whereas symptomatic patients have a rate of 1.12%

In other words,  let's say we have 10,000 people in a community,  9,800 people are asymptomatic and 200 people are symptomatic or pre-symptomatic as found out according to the MOH Singapore,  we might have this situation:   

Therefore,  there is a possibility that the asymptomatic can be 75%  responsible for the spreading of Covid in the community.

Other Studies

There are also other studies that have similar findings;  for example,  this study from Oxford Academy found that the secondary attack rates of symptomatic and asymptomatic were 4.1% (128 of 3136) and 1.1% (12 of 1078), respectively.  This Oxford study indicates that asymptomatic can have an even higher transmission risk,  causing or contributing more infections in the community. 

Conclusion

This simple example confirms this article saying that asymptomatic or people without symptoms are responsible for about 59% of the Covid transmission.  Therefore,  we mustn't underestimate the importance of asymptomatic patients who are usually hiding in the community.   These patients will always stay and spread Covid in the community until there is a mass-testing done to flush them out.  In the case of Singapore,   instead of using mass testings,  they introduced in September 2021 the mandatory Antigen Rapid Testings (ART) for the workers and residents.  This has helped to flush out about 4 to 5 times the number of these asymptomatic patients hiding in the community.