• Wide-spread testing in the Italian town of Vò has shown that 40% of all its cases were asymptomatic.
  • These figures are important in understanding the 'silent' spread of COVID-19 throughout the world.
  • Vò was immediately put into quarantine for four days after suffering Italy’s first COVID-19 death, on February 21st.

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.

Life in Italy's original 'red zones': people gather in a square in front of town hall as Italian President Sergio Mattarella visits Codogno, the small northern town where cases and deaths immediately surged after the country's first coronavirus disease (COVID-19) patient was diagnosed there on February 21, in this picture taken by schoolteacher Marzio Toniolo in Codogno, Italy, June 2, 2020. Toniolo has been documenting what life has been like for the small cluster of northern Italian towns since they were put on lockdown weeks before the rest of the country. REUTERS/Marzio Toniolo REFILE - CORRECTING INFORMATION - RC211H9KA5RC

“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.