50% of COVID-19 Infections Come From People Without Symptoms, says Yale Study

Half of the Covid-19 infections would be caused by people without symptoms: some in the pre-symptomatic stage and others completely asymptomatic. Therefore, isolating only people with symptoms would not be enough to stop a new outbreak of Covid-19.

This is pointed out by a new study from Yale University (United States) and published on July 6 in the scientific journal of the United States National Academy of Sciences.

According to the investigation, the transmission of Covid-19 is “silent.” “Our results indicate that the ‘silent transmission’ of the disease during the pre-symptomatic and asymptomatic stages is responsible for more than 50% of the overall attack rate in Covid-19 outbreaks. Furthermore, such silent transmission alone can lead to outbreaks, even if all symptomatic cases are immediately isolated,” the research notes.

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Scientific analysis

To perform the analysis, the researchers created a mathematical model with different categories: susceptible, infected and incubating, pre-symptomatic, asymptomatic, symptomatic with mild or severe disease, recovered or dead.

When running the model, the scientists found that 48% of the transmissions were caused by people in the pre-symptomatic stage of the disease. Asymptomatic patients, on the other hand, infected in a much lower proportion, with 3.4% of the transmissions.

The research found that people without symptoms not only help spread the virus, but are also capable of generating new ones. For this reason, it suggested that epidemiological follow-up should take them into account.

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Sympton-based surveillance

“In addition, symptom-based surveillance must be complemented by rapid surveillance based on epidemiological contact scanning, which can identify exposed people before their infectious period,” the research notes.

This is somewhat similar to what the Costa Rican Ministry of Health does when detecting a case, sending suspected cases to comply with a 14-day quarantine even if they do not show symptoms.

However, this monitoring has become difficult to carry out with the recent increase in cases of Covid-19, which is why the government declared community transmission in the Greater Metropolitan Area (GAM).