As many as four in 10 people infected with coronavirus may display no symptoms, according to a study involving UK researchers that focused on the experiences of a town in Italy.
The study looked at Covid-19 in the quarantined Italian town of Vò (pictured above), where most of the population of nearly 3,200 people was tested.
“Notably, 42.5% of the confirmed SARS-CoV-2 infections detected across the two surveys were asymptomatic”
The findings, published in the journal Nature, reveal the importance of asymptomatic cases, said the researchers from the University of Padova and at Imperial College London.
Based on their results, the authors suggested asymptomatic or pre-symptomatic people were an important factor in the transmission of Covid-19.
They also argued that widespread testing, isolating infected people, and a community lockdown had effectively stopped the outbreak in its tracks.
Vò, located in the Veneto Region, experienced Italy’s first Covid-19 death on 21 February and the town was put into immediate quarantine for 14 days.
During this time, researchers tested most of the population for f SARS-CoV-2, the virus that causes Covid-19, both at the start of the lockdown (86% tested) and after two weeks (72% tested).
The testing revealed that, at the start of the lockdown, 2.6% of the population were positive for SARS-CoV-2, while after a couple of weeks only 1.2% were positive.
At both times, around 40% of the positive cases showed no symptoms. “Notably, 42.5% of the confirmed SARS-CoV-2 infections detected across the two surveys were asymptomatic,” said the study authors.
The study also found that, on average, it took 9.3 days – ranging from eight to 14 days – for the virus to be cleared from the body.
“We took a picture of the population and found that about half of the population testing positive had no symptoms”
Meanwhile, none of the children aged under 10 years old in the study tested positive for Covid-19, despite several living with infected family members.
This was in contrast to adults living with infected people, who were very likely to test positive, said the researchers.
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, noted the authors.
As well as identifying the proportion of asymptomatic cases, they also found asymptomatic people had a similar viral load – total amount of virus a person has inside them – as symptomatic patients.
Viral load also appeared to decrease in those who had no symptoms to begin with but later developed them, suggesting asymptomatic and pre-symptomatic transmission could contribute significantly to the spread of disease.
Professor Enrico Lavezzo, from the University of Padua, said: “The result concerning asymptomatic carriers is key.
“We took a picture of the Vò population and found that about half of the population testing positive had no symptoms at the time of testing and some of them developed symptoms in the following days.
“This tells us that if we find a certain number of symptomatic people testing positive, we expect the same number of asymptomatic carriers that are much more difficult to identify and isolate,” he said.
He added: “The fact that the viral load is comparable between symptomatic and asymptomatic carriers means even asymptomatic infections have the potential to contribute to transmission.
“Someone with an asymptomatic infection is entirely unconscious of carrying the virus and, according to their lifestyle and occupation, could meet a large number of people without modifying their behaviour,” he noted.
“There are still many open questions about the transmission of the SARS-CoV-2 virus”
Co-lead researcher Dr Ilaria Dorigatti, from Imperial College London, said: “The Vò study demonstrates that the early identification of infection clusters and the timely isolation of symptomatic as well as asymptomatic infections can suppress transmission and curb an epidemic in its early phase.
“This is particularly relevant today, given the current risk of new infection clusters and of a second wave of transmission,” said Dr Dorigatti.
She added: “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.”
The study was funded by the Veneto Region, Wellcome Trust, Royal Society, the European Union’s Horizon 2020 research and innovation programme, the Medical Research Council and the Department for International Development.