Can Covid-19 affect healed people again?

A patient is discharged as cured after a negative virus test. Days later, a new test shows: the virus is there. How can that be? The explanation is probably simple - and there is no cause for concern

Several times there were reports from Asia about individual corona patients who were released from the hospital as recovered and tested positive for the virus a few days later. Most recently there was a message from the South Korean disease protection authority KCDC on 91 such cases. Weren't those affected immune and got infected again straight away? Or did the pathogen, like herpes viruses do, for example, come back after a period of rest?

Phenomenon in the decay phase of the disease

Even if the ultimate certainty is still lacking due to the lack of large studies: It is very likely that another, completely harmless phenomenon is the main cause of the alleged flare-up. "As the disease subsides, the remaining virus quantities are sometimes above and sometimes below the detection limit of the PCR test," explain Melanie Brinkmann from the Technical University of Braunschweig and Friedemann Weber from the University of Gießen in a joint statement. "In this phase the test works more at random."

Virus excretion in throat swabs, lung secretions, stool samples from nine patients: When healing, the excretion often falls below the detection limit (dashed line).

© Nature com

The Berlin virologist Christian Drosten from the Charité also emphasizes that the pathogen can be detectable at times, especially at the end of the illness, and not on a few days in between. "The virus has been there all the time, but the test cannot always capture that," he said on the NDR podcast.

The parable of the goldfish in the paddling pool

Drosten uses a three-dimensional image to illustrate this: goldfish in a paddling pool. If someone blindfolded takes a sample from it with a bucket, be a goldfish - as an image for a virus particle - in it and sometimes not. At the end of the disease, the virus load is only low, so in comparison there are fewer and fewer goldfish in the tank.

This increases the likelihood that a test will turn out negative - even though there is still viral material in the patient. Especially with swab samples from the throat, this is not surprising, explains Drosten in the NDR podcast. The virus was clearly detectable in lung secretions and stool for much longer - and at least in some cases, first throat swabs and later lung secretions (coughed up secretions) or stool were tested in the patients mentioned.

"That is my explanation for this phenomenon," said Drosten. According to Brinkmann and Weber, a certain flare-up of the virus could also play a role in isolated cases. Brinkmann emphasizes that there is no such thing as latency - hiding the virus in body cells and reactivating it in the event of stress, for example - as is the case with herpes viruses. The process of latency is extremely complicated, the herpes viruses would only have perfected this in millions of years of coevolution with humans.

Direct re-infection unlikely

Experts consider a direct new infection not to be completely ruled out, but very unlikely. In the patients tested so far, antibodies against Sars-CoV-2 were usually detected in the blood about 10 to 14 days after the onset of symptoms, which should in principle protect recovered people from re-infection for at least some time. How good and how long? "Nobody can make a sure assessment at the moment," explain Brinkmann and Weber.

But what does it mean for the risk of infection if those who have recovered carry the virus for a few days? The most important point: The PCR tests used all over the world by no means show that the viruses are active. Brinkmann and Weber explain that they also work on remaining residues, since they only demonstrated the presence of the genetic molecule RNA. This does not make it possible to tell whether the virus material detected is still infectious or not.

After the recovery, there is probably no risk of infection

According to the current state of knowledge, the virus material detected after recovery is unlikely to be infectious to others, it is very likely mainly dead, excreted material, explains Drosten. There have already been initial analyzes on this. "We were never able to isolate infectious virus."