A new modeling study published this Wednesday provides little reassuring information: people infected with Covid-19 could transmit the virus several days before the onset of the first symptoms. It comes at a time when several countries are stepping up measures to stem the pandemic.
The study seems to contradict certain measures already in place, such as “contact tracing”, that is, the search for people in contact with a positive case, because it only goes back to when the symptoms appeared. "More inclusive criteria must be taken into account 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," said the authors of the study published in the monthly journal Nature Medicine .
The highest viral load as soon as symptoms appear
Researchers compared clinical data on virus shedding in patients admitted to a hospital in Guangzhou (China) with other data on "peer-to-peer transmission", where it is assumed that a person in contaminated another, in order to draw conclusions about the contagion period.
The research team co-led by Eric Lau of the University of Honk Kong conducted throat swabs from 94 of these patients and measured the degree of contagion from the first day of symptoms and for 32 days. She found that the patients, none of whom was in a serious or critical condition, had the highest viral load as soon as symptoms appeared, before gradually decreasing.
Infectiousness 2.3 days before the onset of symptoms
The study used 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. The researchers assumed that the incubation period was just over five days. The authors concluded that infectivity started 2.3 days before the onset of symptoms and peaked at 0.7 days before the first signs of the disease.
According to the study, 44% of secondary cases in the chains of transmission were infected during the pre-symptomatic period. As for the degree of contagion, it would decrease rapidly in seven days.
Commenting on this study, Babak Javid of Tsinghua University School of Medicine in Beijing said the findings would have "important implications" for control measures, such as whether asymptomatic people should wear a mask.
Questioning strategies
"This is important because currently the control measures recommended for example by the WHO or the British government are based on the assumption that the maximum contagion occurs after the onset of symptoms and this is one of the reasons why the wearing a mask is not recommended for asymptomatic people, ”he added.
Read our file on the coronavirus
In early April, China announced it had identified more than 1,300 cases of infected but asymptomatic people. And according to the United States Centers for Disease Control and Prevention (CDC), a quarter of those infected were asymptomatic.
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