Those who carry the Sars-CoV-2 virus are contagious two days before the first signs of the disease. Most likely just before the onset of symptoms.

It became clear early on during the coronavirus pandemic that asymptomatic infections play an important role in the spread of the pathogen. An analysis by a team led by Xi He from Guangzhou Medical University and Eric Lau from the University of Hong Kong shows how big this problem is.

In their study in “Nature Medicine”, the working group analyzed the viral load of 94 patients and the transmission dynamics in 77 infections. Using a computer model, were used the results to calculate that the infected people were probably contagious on average 2.3 days before the first symptoms. In the case of the contagions examined, the peak of infection ability was 0.7 days before the onset of symptoms, although the viral load was at its highest when the signs of the disease began to show up.

The contagion potential of mild and asymptomatic cases is the reason for the dramatically different course of the current pandemic compared to the Sars outbreak in 2003.

The fact that people without any disease signs can already transmit the virus makes it very difficult to treat the disease with measures such as contact tracking and Narrow quarantine.

As the team led by He and Lau reports, about half of the transmissions examined took place before the first symptoms – this fits with epidemiological analyzes, according to which infected people without symptoms are responsible for a significant part of the infections. Experts had also suspected that the virus was excreted about two days before the onset of the disease. However, the time of the first symptoms is based on subsequent reminders, so that the period and number of asymptomatic transmissions are too high, writes the working group.