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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.
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.
40% had no symptoms
A study of COVID-19 in the quarantined Italian town of Vò, where most of the population was tested, reveals the importance of asymptomatic cases.
The authors of the new research, from the University of Padova and at Imperial College London, published on June 30 in `Nature’, suggest asymptomatic or pre-symptomatic people are an important factor in the transmission of COVID-19. They also argue that widespread testing, isolating infected people, and a community lockdown effectively stopped the outbreak in its tracks.
The town of Vò, with a population of nearly 3,200 people, experienced Italy’s first COVID-19 death on 21 February 2020. The town was put into immediate quarantine for 14 days. During this time, researchers tested most of the population for infection of SARS-CoV-2, the virus that causes COVID-19, both at the start of the lockdown (86 percent tested) and after two weeks (72 percent tested).
The testing revealed that at the start of the lockdown, 2.6 percent of the population (73 people) were positive for SARS-CoV-2, while after a couple of weeks only 1.2 percent (29 people) were positive. At both times, around 40 percent of the positive cases showed no symptoms (asymptomatic). The results also show it took on average 9.3 days (range of 8-14 days) for the virus to be cleared from someone’s body.
None of the children under ten years old in the study tested positive for COVID-19, despite several living with infected family members. This is in contrast to adults living with infected people, who were very likely to test positive.
As a result of the mass testing any positive cases, symptomatic or not, were quarantined, slowing the spread of the disease and effectively suppressing it in only a few short weeks.
Co-lead researcher Professor Andrea Crisanti, from the Department of Molecular Medicine of the University of Padua and the Department of Life Sciences at Imperial, said: “Our research shows that testing of all citizens, whether or not they have symptoms, provides a way to manage the spread of disease and prevent outbreaks getting out of hand. Despite ‘silent’ and widespread transmission, the disease can be controlled.”
“There are still many open questions about the transmission of the SARS-CoV-2 virus, such as the role of children and the contribution of asymptomatic carriers to transmission. Finding answers to these questions is crucial to identifying targeted and sustainable control strategies to combat the spread of SARS-CoV-2 in Italy and around the world.”
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