A new study shows that people infected with Covid-19 can transmit the virus several days before the first symptoms appear. They would be contagious more than two days before the appearance of the first effects of the infection.
Every day we learn a little more about the coronavirus. A study, published Wednesday in the journal NatureMedicine and conducted by researchers from the University of Hong Kong, reveals that Covid-19 infection could take place before the onset of symptoms, which then only occur later. This therefore means that one can transmit the virus and be contagious even before having felt the effects of the infection oneself.
Infection occurs more than 2 days before symptoms
Researchers studied clinical data on the virus in patients admitted to hospital in Canton, China. For this, they took samples from the throat of 94 patients and measured the degree of contagion from the first day from the onset of symptoms until 32 days later. In these patients, none of whom was in serious condition, the viral load was highest at the start of the onset of symptoms before gradually decreasing.
In the study, these figures were compared with public data from 77 peer-to-peer transmissions, in China and around the world, to assess the time between the onset of symptoms in each patient. From this comparison, the researchers concluded that infectiousness begins on average 2.3 days before symptoms appear, with the peak of contagion 0.7 days before the onset of symptoms and declines rapidly during the first seven days. Among secondary cases in the chain of transmission, 44% were infected during the period preceding the onset of symptoms.
Questioning of control measures
This new discovery calls into question the control measures on the population. In the conclusions of the study, Professor of Medicine at Tsinghua University in Beijing, Babak Javid, explains that the measures currently recommended “are based on the assumption that maximum contagion occurs after the onset of symptoms, and this is one of the reasons why wearing a mask is not recommended for asymptomatic people.”
On the other hand, this calls into question the idea of tracing people in contact with a positive case since this is done when the symptoms have appeared. “More inclusive criteria must be taken into consideration in contact tracing in order to identify potential modes of transmission two or three days before the onset of symptoms and thus be able to control the epidemic more effectively”, the study authors said. The US Centers for Disease Control and Prevention (CDC) estimate that a quarter of infected people are asymptomatic, making this tracing complicated.
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