More than 40% of the inhabitants of an Italian town called Vó, who tested positive for COVID-19, had no symptoms of the disease, according to a new study. This suggests the importance of asymptomatic cases in the spread of the pandemic and the need for widespread testing, the researchers argued.

The town, with a population of nearly 3,200 people, experienced Italy’s first COVID-19 death on 21 February. The town was put into immediate quarantine for 14 days and during this time the researchers tested most of the population for infection, both at the start of the lockdown and after two weeks.

The testing showed that 2.6% of the population tested positive, but after a few weeks the figure dropped to 1.2%. At both times, around 40% of the positive cases had no symptoms. The results show that it took an average of 9.3 days for the virus to be cleared from someone’s body.

Meanwhile, the results also indicated that 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.

Co-lead researcher Professor Andrea Crisanti, said in a statement: “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 program in Vò were useful for the wider Veneto region, where all the contacts of positive cases were offered testing. This had a big impact on the course of the epidemic in Veneto compared to other regions of Italy, said Crisanti, who described it as a model to follow to limit the spread of the virus.

Crisanti became a celebrity in Italy for advocating widespread testing well before it became official World Health Organization (WHO) guidance. He called for broad testing even before the first case came to light in Italy in February. However, his request was rejected by officials in his northern Veneto region, who relied initially on guidance from the WHO.

As well as identifying the proportion of asymptomatic cases, the researchers also found that asymptomatic people had a similar “viral load,” the total amount of virus a person has inside them, as symptomatic patients. The viral load appeared to decrease in people who had no symptoms at the onset of infection, but who later developed symptoms.

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

“The 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,” said co-lead researcher Ilaria Dorigatti. “This is particularly relevant today, given the current risk of new infection clusters and of a second wave of transmission.”

The study was published in the journal Nature.