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Infectious Diseases

Updated 14 July 2020

What we learned from Italy's outbreak: About 40% of coronavirus cases in one town were asymptomatic

In a study that focused on the Italian town of Vo, researchers found that community lockdown measures significantly reduced the infection rate.

  • A comprehensive study on an Italian town during the pandemic analysed the positive impact of lockdown measures
  • The researchers also found that about 40% of cases were asymptomatic
  • Without counter measures, about 86.2% of Vo's residents would have been infected

The first case of coronavirus in Italy was detected in the town of Vo. The Paduan town went into lockdown immediately for 14 days.

The findings of an in-depth study on Vo was recently published in Nature, where researchers wanted to asses transmission rates in the community, the risk factors of other underlying conditions and their treatment and whether a lockdown had any impact on the spread of the virus. 

They took 2 812 swabs from subjects at the first interval, and less that two weeks later swabs from 2 343 participants. 

READ: Top WHO official backtracks on comments that asymptomatic spread of Covid-19 virus is 'rare'

Reduced infection rate

At the beginning of lockdown they had an infection prevalence of 2.6% with 73 people testing positive, and at the end of lockdown it went down to 1.2% with only 29 positive cases. Only 4.9% of the total population became infected.

"Our analyses show that viral transmission could be effectively and rapidly suppressed by combining the early isolation of infected people with community lockdown," writes the researchers. 

Fever and cough were the most prevalent symptoms, while all the children under 10 that were tested had no infection. The chances of infection for those over 50 years of age were three times higher than younger people.

16% of total cases required hospitalisation.

Without the lockdown, the researchers estimate that 86.2% of Vo's total population would have been infected with the virus.

ALSO READ: How can people spread the new coronavirus if they don’t have symptoms? 

Asymptomatic cases high

The most interesting find however, was that 42.5% of the confirmed cases showed no symptoms and there was no difference in the viral load between asymptomatic and symptomatic cases.

Between the first and second survey, 61.4% of symptomatic and 55.2% of asymptomatic infections cleared the virus during the study period. 

These findings are similar to another Italian study in Lombardy, which found that only a third of Covid-19 cases had respiratory issues and/or a fever of 37.5 degrees Celsius or above - the two most common symptoms of the virus.

Scientists are scrambling to understand the effect of asymptomatic cases on the spread of the coronavirus, especially their infectiousness and how many there are.

It's difficult to track these cases outside of contact tracing because many don't get tested without symptoms.

READ: What researchers learned from Italy's Covid-19 outbreak: Many cases didn't show the usual symptoms