Is asymptomatic transmission the major driver of the COVID-19 pandemic?

The SARS-CoV-2 virus that causes COVID-19 can be spread readily during close contact. The extent to which asymptomatic infection (that is, an infection without symptoms) drives transmission is important but not clear. Evidence suggests that asymptomatic transmission of SARS-CoV-2 can occur, and that this is one of the reasons why SARS-CoV-2 has been capable of causing a pandemic.

Definitions

Before we get to the science, here’s a brief definition of some terms:

• Asymptomatic infection: an infection without symptoms.

• Symptomatic infection: an infection with symptoms.

• Pre-symptomatic infection: an infection identified before symptoms subsequently develop.

How many people who are infected remain asymptomatic?

There is evidence that around 20% of people who are infected with SARS-CoV-2 remain asymptomatic throughout the course of their infection. Also, around half of those who are first identified as being infected with SARS-CoV-2 whilst being asymptomatic will go on to develop COVID-19 symptoms.

Symptomatic vs. asymptomatic transmission

It is clear that people with symptomatic infection transmit SARS-CoV-2 considerably more frequently that those with asymptomatic infection. For example, in one review and meta-analysis, the secondary attack rate (the rate of transmission from a single index case) was two thirds lower for an asymptomatic person compared with a symptomatic person.

However, one of the defining factors of SARS-CoV-2 seems to be the ability to transmit at all when a person is either asymptomatic or pre-symptomatic. Even though the rate of transmission is lower, the opportunity for asymptomatic transmission is considerably higher on a population level, because most (hopefully all!) people with symptomatic infection will be isolated away from others.

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Whilst it’s difficult to put a number to the risk and rate of transmission from individuals with asymptomatic infection, mathematical modelling can help us to estimate the likely contribution of asymptomatic infection to overall transmission. One such modelling study concluded that at least 50% of all transmission originated from asymptomatic people.

Therefore, transmission from asymptomatic people seems to be a major driver of the COVID-19 pandemic.  

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