A study published by researchers at the Imperial College of London and University of Padova says that 40% of the COVID-19 cases in the Italian town of Vò were in people who had no symptoms — but who were able to spread the virus as effectively as those with symptoms.

The town of Vò, with a population of nearly 3,200 people, experienced Italy’s first COVID-19 death on Feb. 21, 2020. The town was put into immediate quarantine for 14 days. During this time, researchers tested most of the population for the novel cornavirus SARS-CoV-2 which causes COVID-19, both at the start of the lockdown (86 percent tested) and after two weeks (72 percent tested).

The testing revealed that at the start of the lockdown, 2.6 percent of the population were positive for SARS-CoV-2, while after a couple of weeks only 1.2 percent 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.

The team also found that asymptomatic people had a similar ‘viral load’ – the total amount of virus a person has inside them – as did patients with COVID-19 symptoms.

The researchers said their work suggested that people with no symptoms could spread the disease, and said it is crucial to test all contacts of those with COVID-19 — with or without symptoms.  They noted that people with symptoms are likely to isolate, to stay home, whereas people with no symptoms are not likely to do that–so can spread the virus unknowingly.

Researchers also found that few children in homes with COVID-19 patients tested positive for the virus, whereas adults in the homes often did.

The study was published in Nature.