How Many Coronavirus Cases Are Symptomless?

A study carried out in the Italian town of Vo has revealed new insight into the dangers of asymptomatic COVID-19 cases. During the early stages of the pandemic round 3200 people were tested. Of those who tested positive, more than 40% were not showing any symptoms. In February, Vo was the first Italian community to report a coronavirus death. The town was immediately plunged into lockdown for two weeks, with the rest of the Veneto Region soon following.

Study warns 40% of COVID-19 cases asymptomatic

The study, which was carried out by researchers at the University of Padova and Imperial College London, saw almost all the population tested for the SARS-CoV-2 virus over the course of 14 days. Testing conducted at the start of the lockdown confirmed 73 cases, around 2.6% of the Vo population. After two weeks 1.2% of the population tested positive. At both the start and end of the quarantine period, roughly 40% of positive cases were asymptomatic.

The findings were published in the journal Nature, with the authors warning that asymptomatic and pre-symptomatic people play an important role in spreading COVID-19. The researchers also stress the importance of communitywide lockdowns, self-isolation and widespread testing, asserting these strategies played a critical role of stopping the spread in Vo.

Testing and isolation critical to disease control

"Our research shows that 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,” says Professor Andrea Crisanti, co-lead researcher of the study. “Despite 'silent' and widespread transmission, the disease can be controlled,” adds Crisanti, who works in the Department of Molecular Medicine at the University of Padua and the Department of Life Sciences at Imperial College London.

“This testing and tracing approach has had a tremendous impact on the course of the epidemic in Veneto compared to other Italian regions, and serves as a model for suppressing transmission and limiting the virus' substantial public health, economic and societal burden," adds Crisanti.

Asymptomatic patients a genuine threat

As well as exploring the likelihood of asymptomatic cases, the team also found that COVD-19 patients without symptoms had similar ‘viral loads’ to their symptomatic counterparts. They also found that viral loading significantly decreases in patients who are initially asymptomatic, then develop symptoms later in the infection. This suggests person-to-person transmission is more likely to occur via pre-symptomatic and asymptomatic patients, once again highlighting the importance of widespread testing.

“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,” says co-lead researcher Dr Ilaria Dorigatti, who represents the MRC Centre for Global Infectious Disease Analysis and the Jameel Institute (J-IDEA) at Imperial College London. “This is particularly relevant today, given the current risk of new infection clusters and of a second wave of transmission.”

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