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COVID-19: Asymptomatic Infection

Q: My mate says that he’s listened to loads of so-called experts who said that lots of people are asymptomatic which is obviously how it’s getting around so fast. I thought that you could only be asymptomatic during the incubation period and then you start developing symptoms, mild or otherwise - can you have COVID-19 and never develop symptoms?


This question is asking (a) can you have COVID-19 asymptomatically and (b) can someone who is asymptomatic infect others - we hope that this post will help you understand to what extent the answers to these questions are currently known:


Asymptomatic SARS-CoV-2 infections were actually observed relatively early on - back when the outbreak was still localised to China and classed as an epidemic (seems like a century ago, right!). One of these early studies in China examined close-contacts of people with confirmed COVID-19 who were asymptomatic at the time of testing. The researchers discovered 24 asymptomatic cases of infection. Of these 24 cases, 7 (29.2%) cases displayed no symptoms during the period of investigation and also showed normal results on CT scans of their lungs. It was also observed that these 7 asymptomatic cases were mostly in young people (median age 14 years old). This was a very small study, from which it is hard to draw definitive conclusions about transmission in the asymptomatic population, however it is suggestive that there is an unknown population of asymptomatic cases. This highlights the impact that contact tracing and case isolation can have.


As the outbreak progressed, researchers from the University of New York published a larger investigation looking into asymptomatic infection and transmission in China. They concluded that 86% of cases were asymptomatic or had only very mild symptoms (‘undocumented’ cases). People who fall into this category were shown to be 55% as contagious as people with symptoms, but were responsible for 79% of onward transmission. This may be because there are (a) more of them and (b) they are more likely to be carrying on with their normal lives and therefore have more social contact. However, this study was based on mathematical modelling - the data for this model was gathered between 10th and 23rd January 2020, and was extrapolated to infer the proportion of undocumented cases in 375 cities in China. The data used in this model was limited in its diversity and origin, but the two studies seem to indicate that it is possible to become infected with SARS-CoV-2 and not show any symptoms.


We saw further evidence for asymptomatic populations early on in outbreaks across the world - the first case in Iran arose from an unknown source as the country quickly became the new epicentre of the pandemic in late February, and this article from the beginning of March details the first cases in Stockholm that could not be linked to travel from high-risk areas or other confirmed cases. Of course, there have been many more examples of similar situations throughout the world. More recently, this information has been added to the World Health Organisation website; ‘some people become infected but don’t develop any symptoms and don’t feel unwell’.


Despite the fact that there is clearly a significant asymptomatic population, we know very little about disease transmission from within this population. Since the disease is primarily spread through infected respiratory droplets (expelled when we cough or sneeze, for example) the risk of catching COVID-19 from someone who is asymptomatic is thought to be much lower. What might appear to be a case of asymptomatic transmission may actually have resulted from the fact that the infected person has very mild symptoms and continues with their daily life.


Confusion surrounding asymptomatic transmission seems to stem from distinguishing pre-symptomatic (the time between becoming infected and the onset of symptoms: ‘incubation period’) cases from truly asymptomatic cases (test confirmed cases in which the patient does not develop symptoms at any point during their infection). Current data suggests that people are most contagious within the first 3 days from the onset of symptoms. The World Health Organisation report contained within the previous link notes the possibility of pre-symptomatic transmission - transmission during the incubation period. They also note that ‘it is important to recognize that pre-symptomatic transmission still requires the virus to be spread via infectious [respiratory] droplets or through touching contaminated surfaces’. Respiratory droplets are defined as being >5-10µM in diameter, and droplet transmission occurs when a person has close contact with someone with respiratory symptoms - therefore having their mouth, nose or eyes exposed to infected respiratory droplets. This exposure can also result from contact with objects or surfaces which are contaminated with infected respiratory droplets. This highlights the importance of following guidance on social distancing and personal hygiene.



With regards to truly asymptomatic cases, the report states that to date there have been no reported cases of asymptomatic transmission. However, they do not exclude the possibility that it may occur.



Since this post focuses mainly on asymptomatic populations and transmission, we wanted to highlight two excellent resources laying out the most up-to-date information on transmission more generally - both are already referenced within this article. The first is a World Health Organisation publication summarising the most current and reliable information on COVID-19 transmission. The second is a situation update report from the World Health Organisation published a few days later with some really useful additional information. Both of these sources are great because they reference many peer-reviewed publications and the World Health Organisation will continue to publish updates as new evidence emerges.

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