More than 40 percent of people diagnosed with COVID-19 in one Italian town showed no signs of being ill, according to research published Tuesday indicating that asymptomatic carriers may be significant spreaders of the virus.
The authors said their research showed how important mass testing and isolating carriers was in containing clusters of the virus.
The town of Vo, population 3,200, registered Italy's first death from the disease in late February. It was immediately placed in a two-week lockdown, during which researchers were able to test more than 85 percent of the population for COVID-19.
They found that 2.3 percent of Vo was infected at the beginning of quarantine, compared with 1.2 percent at the end of lockdown, and that more than 40 percent of those who tested positive showed no symptoms.
The authors of the research, published in the journal Nature, said their findings showed how rapid case isolation and mass testing was able to effectively eliminate the virus from Vo.
"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," said Andrea Crisanti, of the Department of Molecular Medicine of the University of Padua and the Department of Life Sciences at Imperial College London.
"Despite 'silent' and widespread transmission, the disease can be controlled."
The team found that asymptomatic COVID-19 carriers had a similar viral load to those who got sick, suggesting that while not ill themselves they could still spread the virus.
"Even asymptomatic infections have the potential to contribute to transmission," said Enrico Lavezzo, from the University of Padua, who contributed to the study.
This was particularly noteworthy for policymakers seeking to limit COVID-19 clusters from spreading, he said.
"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," said Lavezzo.
The data from Vo also showed that none of the children under the age of 10 tested positive for COVID-19 despite living with several adults who did.
A Europe-wide study released last week showed that children are extremely unlikely to die from COVID-19, and age is known to be a key risk factor for the virus' mortality.
Co-author Ilaria Dorigatti, from the MRC Centre for Global Infectious Disease Analysis at Imperial, said the findings were relevant for governments as lockdowns are eased around the world.
"The Vo 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," she said.