The Covid-19 virus spreads because of the large number of people who never develop symptoms from infection, according to two major new studies.
A study in a small town in northern Italy has found that 40% of those who became infected had no symptoms.
Almost everybody in the town of Vo, near Venice, was tested and the results have been analysed by Imperial College, London, UK, and the University of Padova, Italy.
Testing was undertaken at the start of lockdown and two weeks later – and at the beginning 2.6% of people, 73 in total, were positive for viral infection. Two weeks later this was down to 1.2%. No children under the age of ten became infected.
Researcher Professor Andrea Crisanti 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."
Fellow researcher Professor Enrico Lavezzo 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.
"The fact that the viral load is comparable between symptomatic and asymptomatic carriers means even asymptomatic infections have the potential to contribute to transmission, as some of the reconstructed chain of transmission obtained from the detailed contact tracing conducted in Vò confirmed."
* A study in Sweden suggests that many more people will have immunity to the virus than show up in antibody testing.
Researchers at the Karolinska Institutet found many people who had mild or asymptomatic infection had T-cell mediated immunity to the virus.
The findings come from a study of 200 people who had mild disease or were asymptomatic family members of those infected. They were compared with healthy blood donors.
Researcher Soo Aleman said: “One interesting observation was that it wasn’t just individuals with verified Covid-19 who showed T-cell immunity but also many of their exposed asymptomatic family members.
“Moreover, roughly 30% of the blood donors who’d given blood in May 2020 had Covid-19 specific T cells, a figure that’s much higher than previous antibody tests have shown.”
Robust T cell immunity in convalescent individuals with asymptomatic or mild COVID-19 bioRxiv 29 June 2020
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