Can anyone who tests positive for corona infect others? Or do some people carry so little virus that it becomes practically impossible? These are pressing questions that are being researched all over the world. In the Netherlands, Erasmus MC is now starting a study together with the Municipal Health Centres (GGDs) to gain more insight into this.
Internationally, there is discussion about the tests to determine whether someone has corona. For example, scientists at Oxford University warn that some of the people who test positive are no longer able to infect others. This is the so-called PCR test that determines whether genetic material from the virus is present in samples from, for example, the nose and throat.
Professor of virology Marion Koopmans and medical molecular microbiologist Richard Molenkamp of Erasmus MC are now investigating whether or not there is a link between the quantities of virus found in such tests and whether or not people are involved in a cluster of infections.
The gold standard
If some of the people tested positive turn out not to be contagious, this can have major consequences. The more positive results in the test street, the more source and contact tests have to start. And the more people have to stay at home.
“The GGD now monitors everyone who tests positive, but perhaps youd better focus the source and contact research on people who spread the virus most often,” says Koopmans. According to the virologist, its difficult to estimate how many percent of the people tested positive are now not contagious.
Koopmans calls the PCR test the gold standard because it is the most sensitive test for detecting the virus. But theres a downside to this sensitivity: people can still carry traces of genetic material weeks after an infection while they no longer have intact virus particles in their bodies with which they can infect others.
So the PCR test does not give a definite answer about infectivity. This is how it works: in the lab, the genetic material from the cotton swab is doubled about 40 times. This enables the test device to determine whether hereditary material of this specific coronavirus is present in the sample.
If the test is positive, most test devices report a so-called Cycle treshold value. This Ct-value indicates after how many times doubling it became clear that hereditary material of the coronavirus was found. The less virus there was in the sample, the more doublings are required and the higher the Ct-value becomes. A Ct value of 15 therefore indicates more virus than a Ct value of 37.
The question now is whether there is a Ct-value in which people carry so little virus material that they can no longer infect others. “There is a great need for a safe threshold value,” says Koopmans. “Suppose you say: from the Ct-value 35 you are not contagious, then the GGD doesnt have to follow those people”
No hard guarantees
“Up to now we have been busy scaling up and we are only now getting ready to examine those Ct values,” says Ann Vossen, chairman of the Dutch Association for Medical Microbiology. Doctor-microbiologists attached to the hospitals analyse the majority of the tests that the GGDs perform. Vossen also wants to know how many tests are actually positive. “I think its worth investigating what that means in terms of infectivity.”
In practice, it turns out to be difficult to determine a threshold value. “For example, we see people with low Ct values and few complaints. They carry a lot of virus, but may spread it less because they have fewer complaints,” says Koopmans.
There are more comments like that. “Suppose someone tests positive but carries little virus, then you dont know whether someone had a lot of virus two days earlier and could infect others,” says Vossen. “The amount of virus is not static.” According to Vossen, you also have to look at other factors, such as the first day on which someone had symptoms, the severity of the symptoms and the extent to which the virus circulates in the region.
Despite the reservations, Koopmans and Molenkamp try to set a threshold, partly because there are no national or international guidelines.
Such a threshold value wouldnt offer any hard guarantees, Molenkamp emphasizes. “You can only say: above this Ct value there is a good chance that someone isnt contagious. You can make policy on that and take the risk that you will miss a contagious person once in a while, but thats up to the policymakers”