Viral load is comparable between symptomatic and asymptomatic carriers of SARS-CoV-2 suggesting asymptomatic or pre-symptomatic people are an important factor in the transmission of Covid-19.

The 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 in Nature, say 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 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 (73 people) were positive for SARS-CoV-2, while after a couple of weeks only 1.2% (29 people) were positive. At both times, around 40% 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.

Mass testing suppressed the disease in a few short weeks.

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."

The results of the mass testing programme in Vò 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," added Professor Crisanti.

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.

Early identification of infection clusters can suppress transmission

Viral load also appeared to decrease in people who had no symptoms to begin with but later developed symptoms, suggesting that 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 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.

"On the one hand, it is likely that a symptomatic infection transmits large quantities of virus, for example via coughing, but it is also reasonable to think that symptoms may induce a person with a symptomatic infection to stay at home, limiting the number of contacts and hence the transmission potential. On the other hand, 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."

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