Can anyone who tests positive for corona infect others? Or does some of the people carry so little virus that it becomes practically impossible? Research is being conducted worldwide into these pressing questions. In the Netherlands, Erasmus MC is now starting an investigation together with the GGDs to gain more insight into this.
There is international discussion about the tests to determine whether someone has corona. For example, warn scientists from Oxford University that some of the people who test positive are no longer able to infect others. This concerns the so-called PCR test that determines whether genetic material of 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 there is a link between the amounts 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 positively tested people turn out not to be contagious, this can have major consequences. The more positive results in the test street, the more source and contact investigations the overloaded GGDs have to start. And the more people have to stay at home.
“The GGD is now monitoring everyone who tests positive, but it might be better to focus the source and contact research on people who spread the virus most often,” says Koopmans. Estimating the percentage of positively tested people is now not contagious, according to the virologist.
Koopmans calls the PCR test the gold standard because it is the most sensitive test to detect the virus. But this sensitivity has a downside: for weeks after an infection, people can still carry scraps of genetic material while they no longer have intact virus particles in their bodies with which to infect others.
Duplicate virus
The PCR test is therefore inconclusive about infectivity. It works like this: in the lab, the genetic material from the cotton swab is doubled about 40 times. This allows the test device to determine whether any genetic material of this specific coronavirus is present in the sample.
If the test is positive, most test devices report a so-called Cycle threshold value. This Ct value indicates after how many times it became clear that hereditary material of the corona virus has been found. The less virus there was in the sample, the more doubling is required and the higher the Ct value becomes. Thus, a Ct value of 15 indicates more virus than a Ct value of 37.
The question now is whether there is a Ct value in which people still 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 of 35 you are not contagious, then the GGD does not have to monitor those people.”
No hard guarantees
“Up to now we have been busy scaling up and we are only now getting to researching those Ct values”, says Ann Vossen, chairman of the Dutch association for medical microbiology. Medical microbiologists associated with the hospitals analyze most of the tests that the GGDs conduct. Vossen also wants to know how many tests are just positive. “It seems worthwhile to investigate what that means for infectivity.”
In practice it turns out to be difficult to set a threshold value. “For example, we see people with low Ct values and few complaints. They have a lot of virus with them, but may spread it less because they have fewer complaints,” says Koopmans.
There are more caveats, for example. “Suppose someone tests positive but has little virus with them, then you don’t know whether someone had a lot of virus two days earlier and could infect others,” says Vossen. “The amount of virus is not static.” In addition, according to Vossen, you should actually also look at other factors such as the first day on which someone got complaints, the severity of the complaints and the extent to which the virus circulates in the region.
Despite the caveats, Koopmans and Molenkamp are trying to set a threshold, partly because there are no national and international guidelines.
Such a threshold would not offer any firm guarantees, Molenkamp emphasizes. “You can then only say: above this Ct value there is a good chance that someone is not contagious. You can make a policy on that and take the risk that you occasionally miss an infectious person, but that is up to the policymakers. “
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