Asymptomatic, pre-symptomatic, symptomatic: what is the difference?

Before a vaccine is available, our best hope of combating coronavirus lies in preventing its spread. But how to do this when people may be unaware they have it? And what’s the difference between asymptomatic, pre-symptomatic and symptomatic cases? Here’s a brief explanation.

Asymptomatic cases

People are 'asymptomatic' when they test positive for COVID-19 without having shown any symptoms.

The WHO says it’s unclear how many transmissions happen from these asymptomatic cases. Further studies to understand the transmission of the virus from asymptomatic carriers are hard to conduct, as they would require widespread testing and more data than is currently available.

Additionally, rather than being asymptomatic, it is thought that in many COVID-19 cases, the infected person has a very mild form of the disease. Common symptoms – such as a cough, fever or tiredness – may not develop to any noticeable extent, or symptoms that do exist may be atypical and not look like the most common forms of the disease.

Pre-symptomatic cases

Given the incubation period for COVID-19 – the time between exposure to the virus and symptom onset – can be up to 14 days, there are also circumstances where people could have caught the disease but are not yet aware of it so are continuing as normal.

Some of the cases of coronavirus that have been picked up through tracing efforts fall into this camp, and people later go on to develop symptoms. This is backed up by data that suggests some people can test positive for the virus 1-3 days before they develop symptoms.

In this pre-symptomatic stage, some people are still contagious, so onwards transmission of the virus is still possible, despite no outward display of illness. This is one of the reasons why governments are asking whole households to isolate when one member gets sick.

Asymptomatic and pre-symptomatic cases have been dubbed “silent spreaders”, and underline the importance of continued social distancing, regular hand washing and good hygiene when out and about.

Transmission of the disease from a symptomatic case to another person is the easiest form to trace. The virus is primarily spread to other people through close contact with infected respiratory droplets, direct contact with infected people, or by contact with contaminated surfaces.

Although the disease can vary in severity from person to person, the majority of people who do catch it will have a mild form and will display symptoms. Common symptoms include fever, dry cough and tiredness. Less commonly, they can include aches and pains, nasal congestion, headaches, conjunctivitis, sore throat and rashes, among others.

Evidence to date suggests people shed the virus most during the first three days from the onset of symptoms. This suggests people are most contagious during the early part of the disease, when symptoms have just started.

The cases we don’t know about

Alongside any risk posed by silent spreaders, there are also cases that arise as a result of non-documented infections – people who have symptoms but don’t get tested for one reason or another. By definition, the volume of undocumented cases is much harder to get a grip on than documented cases, where people are identified as having the disease because they sought healthcare, or were tested through contact tracing.

A study published in March suggests that, based on statistical models, 86% of all infections in China were undocumented before late January when the authorities quarantined Wuhan. This would help explain the rapid transmission of illness around the world, the authors say.

Effective tracking of infections, and contacting people who may have picked up the disease from confirmed cases, is now a focus for many countries. As lockdowns start to ease across Europe and parts of the US, such tracing schemes are likely to prove crucial in efforts to prevent a rebound in infection levels.