A preliminary study suggests that more than 80% of people aged 20 and under may show no symptoms after contracting SARS-CoV-2. This may have important implications for virus transmission.

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Italian researchers show that over 80% of young people with a SARS-CoV-2 infection might be asymptomatic.

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Experts from the Bruno Kessler Foundation in Trento, Italy, in collaboration with colleagues affiliated to the ATS Lombardy COVID-19 Task Force and various research institutions, conducted a study assessing what percentage of people who have contracted SARS-CoV-2 are likely to experience any symptoms.

This preliminary study does not yet appear in a peer-reviewed journal, but its authors have made their findings available online, on the preprint platform arXiv.

The rate of likely asymptomatic carriers of the coronavirus could have important implications for viral transmission, the study authors point out.

“This work allows us to clearly show the difficulties in identifying infections with surveillance since the majority of these are not associated with respiratory symptoms or fever,” says study co-author Stefano Merler, who specializes in mathematical modeling of infectious disease transmission.

The researchers analyzed data on 5,484 individuals from the Lombardy region of Italy who had been in contact with people with a new coronavirus infection. Of these, 2,824 individuals had laboratory-confirmed SARS-CoV-2 infections.

Looking at the data, the investigators found that, of the 2,824 individuals with known coronavirus infections, only 876 (31%) had presented any symptoms.

Looking at data on age, the researchers calculated that, among people aged 20 years and under, a probable 81.9% would present no symptoms following infection with the coronavirus.

Among those 80 years of age or over, the investigators estimate that only 35.4% are likely to show no symptoms following infection with the new coronavirus.

Among individuals with a confirmed SARS-CoV-2 infection who were 60 years old and under, 73.9% were likely not to experience fever — body temperature of at least 37.5oC — or respiratory symptoms, the study authors noted.

In terms of general trends, the researchers concluded that the likelihood of experiencing SARS-CoV-2 infection symptoms increased with age.

They also noted that 6.6% of people aged 60 years and older had a severe form of COVID-19 as a result of contracting the new coronavirus, and males had a “significantly higher risk” than females of experiencing a critical state.

Part of what makes the current study important, according to Merler, is that it “also represents a useful piece of information to better understand the role of children in the epidemiology of COVID-19, which we still know very little about.”

“Everyone knows that few positive children were identified during the pandemic,” he says, “but this study allows us to determine the contribution of a possible lower susceptibility to infection in children, which we had identified in a previous study conducted in China, compared to the probability of developing clinical symptoms once infected.”

However, the study also gave rise to more questions, which, as yet, remain unanswered.

“Finally, there are other interesting research questions. We estimated the probability of developing critical symptoms following the infection, finding that it is particularly high in the older age groups […], as expected. But we have also seen that women have a lower risk and, even while we are waiting for other studies to confirm these results, the reason why is still unclear.”

– Stefano Merle

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