How common are asymptomatic COVID19 infections?

How common are asymptomatic COVID19 infections and why is this so important?

By Candela Iglesias and Joanne Fielding

Since the start of the pandemic there have been ongoing discussions on whether COVID19 could cause asymptomatic infections and how often, and what role – if any – did people with asymptomatic infections play on virus transmission. Today the science is consolidating and clearer answers are emerging. Let’s dive into it.

First, what does asymptomatic mean?

Asymptomatic means without symptoms. Anyone who has a positive test for COVID19 (in this case) and doesn’t report symptoms would fall into this category. Seems simple enough, but there is a lot of variation in what people will call a symptom. What I might call a headache you would dismiss as nothing at all.

Also, making things more complicated, someone might not have symptoms at the time of testing but might develop symptoms later. Some scientists prefer the term pre-symptomatic to refer to these cases, to help distinguish between true asymptomatic cases and those that will eventually develop symptoms.

How many asymptomatic COVID19 cases are there?

There have been multiple papers written on the topic with huge variations in the numbers. In this compilation of reports we can see that studies have found that anywhere between 8-80% of people testing positive are asymptomatic.

Studies on asymptomatic cases have two challenges. First, not all of these studies correct for the pre-symptomatic individuals that will develop symptoms later, and would strictly speaking, not count as asymptomatic.

Second, many studies don’t have a sample that is representative of the whole population. Think about it, if you test in a school your sample is only young people, so it will probably have more asymptomatic cases than the general population. If on the other hand you test in a hospital, you are enriching for people that might be more fragile and potentially more likely to have symptoms if they get infected with COVID19.

Some of the best studies have tested a large number of people in the general population. For example, a New England Journal of Medicine article reported results from tests in over 10 000 individuals living in Iceland who accepted an open invitation for testing. Forty-three percent of those who tested positive reported having no symptoms at the time of testing.

Why is the percentage of asymptomatic people so important?

If asymptomatic people are able to transmit the infection, the efforts needed to control the pandemic become more complicated. People who have no symptoms will not know to self-isolate, to alert the authorities or to request a test, and thus risk unwittingly becoming sources of transmission.

Data indicate that asymptomatic individuals are indeed able to transmit the infection. According to a study performed in Singapore and Japan shedding of the virus can occur in the absence of symptoms. They identified seven clusters of cases in which transmission occurred from an asymptomatic/pre-symptomatic person. Similar results were found in a nursing home in the United States and other studies. Asymptomatic individuals may transmit less easily than symptomatic ones, and this is still under research.

This means that symptom-based screening is not enough and wider testing in some settings, such as contact-tracing efforts and among vulnerable populations (e.g. in nursing homes), are needed to try and curb the spread of COVID19, particularly as societies reopen.

What does this mean for me?

It means that if we unknowingly become infected and don’t develop symptoms, we may transmit the virus to other people, included our loved ones. This calls for caution, adequate physical distancing and hygiene measures with people in our circle, particularly those who are elderly or have other conditions that increase their risk. It also means following local guidance on wearing a mask and other measures when we are in public or common spaces. Finally it is also important to be attentive to our surroundings as we start to go out more. If someone in our workplace or other places where we have been has tested positive, it is important to try and get tested and maintain adequate distancing measures with others.

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