Italian whole-town study finds 40% of coronavirus cases had no symptoms

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LONDON (Reuters) - A study of coronavirus infections that covered almost everyone in the quarantined north Italian town of Vò found that 40% of cases showed no symptoms - suggesting that asymptomatic cases are important in the spread of the pandemic.

The study, led by a scientist at Italy’s Padua University and Imperial College London, also produced evidence that mass testing combined with case isolation and community lockdowns can stop local outbreaks swiftly.

“Despite ‘silent’ and widespread transmission, the disease can be controlled,” said Andrea Crisanti, a professor at Padua and Imperial who co-led the work. “Testing of all citizens, whether or not they have symptoms, provides a way to ... prevent outbreaks getting out of hand.”

Crisanti has become something of a celebrity in Italy for advocating widespread testing well before it became official World Health Organization  guidance.

Vò, which has a population of nearly 3,200, was immediately put into quarantine for 14 days after suffering Italy’s first COVID-19 death, on Feb. 21.

During that fortnight, researchers tested most of the population for SARS-CoV-2, the coronavirus that causes COVID-19.

A analysis of the results, published in the journal Nature on Monday, showed that at the start of quarantine, 2.6% of Vo’s population - or 73 people - were positive. After two weeks, only 29 people were positive.

At both times, around 40% of positive cases showed no symptoms. But because all of the coronavirus cases found - whether symptomatic or not - were quarantined, the researchers said, this helped slow the spread of the disease, effectively suppressing it in a few weeks.

Crisanti said the success of Vo’s mass testing also guided wider public health policy in the wider Veneto Region, where it had “a tremendous impact on the course of the epidemic” there compared to other regions.

Reporting by Kate Kelland; Editing by Kevin Liffey