Study of Italian town reveals more than 40% of Covid-19 cases were asymptomatic

Nina Massey, PA Science Correspondent

Research reveals more than 40% of coronavirus cases in an Italian town were asymptomatic.

A study of Covid-19 in the quarantined town of Vo, where most of the population was tested, indicates the importance of asymptomatic cases.

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, researchers say.

They suggest this is particularly important when considering a potential second wave of transmission.

Researchers from the University of Padua and at Imperial College London indicate asymptomatic or pre-symptomatic people are an important factor in the transmission of the disease.

Pre-symptomatic people are those who are carrying the virus and later go on to develop symptoms, while asymptomatic refers to people who do not show any symptoms at all.

According to the scientists, widespread testing, isolating infected people, and a community lockdown effectively stopped the outbreak in its tracks.

Vo, which has a population of nearly 3,200 people, experienced Italy’s first Covid-19 death on February 21, and 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% tested) and after two weeks (72% tested).

They found that at the start of the lockdown, 2.6% of the population (73 people) were positive for Sars-CoV-2, while after a couple of weeks only 1.2% (29 people) were positive.

Both times 42.5% of the positive cases were asymptomatic, according to the research published in Nature.

The results also suggest that it took on average 9.3 days for the virus to be cleared from someone’s body.

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 a number of 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.”

The results of the mass testing programme in Vo informed policy in the wider Veneto Region, where all contacts of positive cases were offered testing.

“This testing and tracing approach has had a tremendous impact on the course of the epidemic in Veneto compared to other Italian regions, and serves as a model for suppressing transmission and limiting the virus’ substantial public health, economic and societal burden,” Prof Crisanti added.

As well as identifying the proportion of asymptomatic cases, the team also found that asymptomatic people had a similar “viral load”, the total amount of virus a person has inside them, as symptomatic patients.

The researchers found that viral load, how much virus a person had in their system, also appeared to decrease in people who had no symptoms to begin with but later developed symptoms.

This suggests asymptomatic and pre-symptomatic transmission could contribute significantly to the spread of disease, making testing and isolating even more important in controlling outbreaks.

Co-lead researcher Dr Ilaria Dorigatti, from the MRC Centre for Global Infectious Disease Analysis, Jameel Institute (J-Idea), at Imperial College London, said: “The Vo 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.”