How high is the risk that a Covid 19 patient will infect other people? Apart from spatial and social factors, how many virus particles are in the patient’s throat and expelled from there when breathing, speaking or coughing plays a role. A team led by Charité virologist Christian Drosten has now published data on the viral load of over 25,000 patients from Germany in the specialist journal “Science”. According to this, a third of the people who had a particularly high viral load and thus infectiousness had no symptoms at all. There were hardly any differences between the age groups. Small children had slightly lower viral loads, but their infectivity is not necessarily lower, according to the scientists. The study also confirmed that the so-called British variant B.1.1.7 is associated with particularly high viral loads.
On average, a person infected with Sars-CoV-2 infects three to five other people. At the population level, this so-called R-value is important because it can be influenced by protective measures such as distance requirements and wearing a mask. The fewer close contacts an infected person has, the lower the risk that they will transmit the virus to others. However, regardless of individual behavior, the infectiousness can differ from person to person. Because how many virus particles are in a person’s throat and can be passed on from there varies considerably. Can you predict who is particularly contagious? For example, are certain age groups more infectious than others, or does the severity of the symptoms suggest infectivity?
Data from over 25,000 infected people in Germany
In search of answers to these questions, a team led by Terry Jones, led by Christian Drosten from Charité Universitätsmedizin Berlin, has been collecting how high the viral load is in those who test positive for the coronavirus since the beginning of the pandemic. Between February 2020 and April 2021, they evaluated almost a million tests from over 415,000 people of all ages. 25,381 people had at least one positive PCR test. In order to estimate how many virus particles were in their throat, the researchers analyzed the so-called CT value (Cycle Threshold) of the PCR tests. This indicates how many amplification runs were necessary in the PCR before the virus material was detectable in the sample. The fewer runs are needed, the higher the viral load.
The researchers have now published their results in the journal “Science”. The evaluations show: Around nine percent of those infected had a particularly high viral load of one billion virus copies or more. More than a third of these potentially highly infectious people had no or only mild symptoms. “These data provide a virological basis for the observation that only a minority of infected people cause the majority of all transmissions,” explains Drosten. “The fact that there are so many people without relevant symptoms of illness makes it clear why measures such as distance rules and the mask requirement are so important for controlling the pandemic.”
Children hardly less infectious than adults
There were hardly any differences between the age groups. Infected between the ages of 20 and 65 had an average of around 2.5 million copies of the virus in their throats. Children between the ages of zero and five had the lowest viral load. In them, the researchers found an average of only around 800,000 copies of the Sars-CoV-2 genome. In older children and adolescents, the values matched those of adults with increasing age.
However, these differences are not clinically relevant, emphasizes Drosten. One reason for this is the different sampling methods used by children and adults: “In children, significantly smaller swabs are used, which use less than half as much sample material in the PCR test. In addition, instead of the painful deep nasopharynx swabs, simple throat swabs are often taken, in which there is even less virus. That is why we expect lower viral load readings in the PCR from the outset in children with the same virus replication. ”An assessment of the infectivity using laboratory samples from children and adults resulted in around 80 percent of the value for adults for small children. “And even these data-based estimates of infectiousness have to be corrected upwards again because of the different samples taken from children,” says Drosten. “My initial impression that all age groups were almost equally infectious has been confirmed, not only here, but also in other studies.”
Distance and masks as keys to preventing further outbreaks
While according to the evaluation it makes no difference in terms of infectivity how old a person is and whether they have symptoms or not, the virus variant played an important role: people who were infected with the so-called British variant B.1.1.7, had an average ten times higher viral load. The researchers estimated their infectivity in the laboratory to be 2.6 times as high. The researchers have not yet been able to answer what exactly makes the mutated virus so contagious. On the basis of the current data, Drosten sums up: “Even if laboratory tests have not yet been able to conclusively explain it: the B.1.1.7 virus is more infectious than other variants.”
As far as the data allowed, the researchers also reconstructed how the infectiousness developed over the course of the disease. To do this, they used the data from 4,344 patients, of whom several tests were available. The infectiousness is highest one to three days before the onset of the first symptoms – if the patient develops any at all. “Based on our estimates of the infectivity of those without symptoms and the higher viral load found in those infected with the B.1.1.7 variant, we can safely assume that non-pharmaceutical interventions such as social distancing and wearing of masks were key to preventing many more outbreaks, ”the researchers write. “Such measures should be used in all social settings and in all age groups, wherever the virus is present.”
Source: Terry Jones (Charité Universitätsmedizin Berlin) et al., Science, doi: 10.1126 / science.abi5273