One reason COVID-19 has spread so rapidly is that people infected with the SARS-CoV-2 coronavirus can show no symptoms of disease and yet are still contagious — they don’t look or feel sick but still transmit the virus without realizing.
Spreading disease without illness is called ‘asymptomatic’ transmission. An infected person with signs of disease is a ‘symptomatic’ case, whereas someone who has no noticeable symptoms is asymptomatic. So far the terminology is clear. But you may have heard another term that causes confusion: ‘presymptomatic’.
At first it sounds straightforward, a presymptomatic case is simply someone who hasn’t developed any symptoms… yet. It’s more complicated than that, however, as presymptomatic can also mean asymptomatic, an important point that gets more confusing the more you think about it.
On 8 June, Maria Van Kerkhove, technical lead of the World Health Organization (WHO) health emergencies programme, discussed the role of asymptomatic cases in propagating the pandemic. She made statements that sparked further confusion by using a definition of “asymptomatic cases” that excluded individuals who might have actually been presymptomatic.
Here’s the issue. If someone tests positive for Coronavirus (a confirmed case) and has no symptoms then they’re classified as an ‘asymptomatic’ case. But if that infected individual later becomes a symptomatic case, you reclassify their initial phase of infection as ‘presymptomatic’.
You can read dozens of paragraphs on the distinction between ‘asymptomatic’ and ‘presymptomatic’ and still be left none the wiser. Sometimes a picture is worth a thousand words, so to clarify the confusion, let’s visualize the difference.
In the Venn diagram above, the circles are the two types of cases while the sections (including the overlapping part) represent different phases of infection.
The WHO defines asymptomatic in a narrow way. It only includes infected people who never develop symptoms (blue part of the diagram), which ignores those who go on to become symptomatic cases after going through a presymptomatic phase where they are involved in asymptomatic transmission of the virus. By a broader definition (blue plus green sections), people who have no symptoms when they spread COVID-19 can also be considered asymptomatic cases.
According to a study led by epidemiologists at Columbia University, which put real data into a computer model to estimate the proportion of undocumented cases that escaped China before lockdown, around 86% of Coronavirus cases weren’t detected.
Though the message behind what ‘asymptomatic’ means is confused, it’s clearly crucial to understanding how COVID-19 continues to spread.
I’m a science communicator specialising in public engagement and outreach through entertainment, focusing on popular culture. I have a PhD in evolutionary biology and spent several years at BBC Science Focus magazine, running the features section and writing about everything from gay genes and internet memes to the science of death and origin of life. I’ve also contributed to Scientific American and Men’s Health. My latest book is ’50 Biology Ideas You Really Need to Know’.