Asymptomatic Transmission? We Just Don't Know Yet for Certain

Natalie E. Dean, PhD


June 15, 2020

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Natalie E. Dean, PhD

There has been confusion about whether people infected with SARS-CoV-2 who do not have symptoms can transmit to others. In fact, we do have evidence that individuals without symptoms can spread the virus. This is the likely reason why SARS-CoV-2 has been harder to contain than its relative, SARS-CoV. Nonetheless, it is hard to detect this type of transmission, and it is even harder to measure how frequently it occurs.

To understand the science, we must first break down the concepts. It is useful to draw a distinction between people who are infected and never develop symptoms — "fully asymptomatic" individuals — and those who have not yet developed symptoms. We know that it is often also true that when people without symptoms test positive for the virus, they will go on to develop symptoms in a few days.

We have evidence that presymptomatic individuals can transmit to others. This is observed by tracing data. There are instances in which a person was infected but their only possible exposure was a presymptomatic contact. To be sure that this contact was the source, we must try to rule out all other potential sources of infection, including untraced contacts or fomites. 

While we can establish that presymptomatic transmission is possible, it is difficult to measure how frequently it happens and how much it contributes to the overall spread of the virus. To study this further, we can analyze high-quality contact tracing data.

Imagine that we have a pair of cases that we believe are linked, with one person infecting the other. We can look at when each person first developed symptoms. We know that there is a delay between when someone is infected and when they develop symptoms. For COVID-19, the incubation period is about 4-5 days on average. We can use this to approximate when we think each person was infected. If the second person was probably infected after the first had already developed symptoms, this suggests well-recognized symptomatic transmission. But if the second person was probably infected earlier, then presymptomatic transmission may be the cause. While we can't sort out exactly what happened with each pair, we can study lots of pairs to look for patterns in the data.

We can couple these types of analyses with other sources of data. Another source of evidence for presymptomatic transmission is that people have a high viral load around the time that they develop symptoms. This is a useful clue, but having a high viral load is not the same as knowing they can infect others. People without symptoms are not coughing or sneezing, for example, though they could spread the virus in other ways.

Some people never develop symptoms (fully asymptomatic). It is unclear how frequently this occurs, and the estimates range widely in the literature. The uncertainty is mostly because these people are so difficult to find. Most testing is reserved for people with symptoms, and it is only when we start testing many people without symptoms that we can identify asymptomatic infections. Besides being difficult to find, they are even more difficult to link in chains of transmission. So however much uncertainty we have about presymptomatic transmission, there is even more uncertainty about transmission from people who are fully asymptomatic. 

We need better contact tracing data and more testing and follow-up of people without symptoms to sort this out. It is important to remember that this is a new virus. There is still a lot of uncertainty, and it is hard to communicate this level of uncertainty to the general public. But we know that people infected who aren't exhibiting symptoms can inadvertently transmit the virus to others, and so our policies and actions must reflect this.

Natalie Dean, PhD, is an assistant professor of biostatistics at the University of Florida in Gainesville. She specializes in emerging infectious diseases and vaccine study design. Follow her on Twitter

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