The time that one has to spend in quarantine after a Covid-19 infection has been getting shorter and shorter in Germany. Those who had no symptoms for 48 hours can now leave isolation after just five days. The situation is similar in an international comparison: the US Centers for Disease Control and Prevention (CDC) already reduced the recommended isolation period for people with Covid-19 from ten to five days in December 2021. The reason for this change was based on scientific findings at the time: most Sars-CoV-2 transmissions would take place early in the course of the disease, namely in the one to two days before the onset of symptoms and two to three days after, the CDC explained.
This decision was already controversial among researchers - and it is still today. Their skepticism is underpinned by a number of studies that show that many patients with Covid-19 to well into the second week after the first symptoms occur remain infectious. The shortening of the recommended insulation time - which is now common worldwide - is more politically motivated than by new, soothing data.
"The facts about how long people have been infectious haven't really changed," says Amy Barczak, an infectious disease specialist at Massachusetts General Hospital in Boston. "There is no data to support an isolation of less than ten days." According to Barczak's own research results, published on the medRxiv preprint server, a quarter of people infected with the Omikron variant of Sars-CoV-2 could still be infectious after eight days.
What infectia is dependent on
Although the question "How long has someone been contagious with Covid-19?" it's simple, the answer is complicated. "We like to look at it in black and white: either someone is contagious or not. In reality, however, it is about probabilities. It's a numbers game," says Benjamin Meyer, a virologist at the University of Geneva in Switzerland.
The rules and starting values change again and again. New variants, vaccinations and different natural immunity, which is caused by previous infections - all of these things influence how quickly someone can remove the virus from their system, says Meyer. And that ultimately determines when this person stops being infectious. Behavior also plays a role in this. People who feel uncomfortable tend to avoid others. In this way, the severity of the symptoms can influence how likely it is to infect others.
Most scientists are sure that PCR tests can still give a positive result even when a person is no longer infectious. This is presumably due to the fact that the tests that detect viral RNA also pick up non-infectious remnants that remain after most of the living virus has already been eliminated. In contrast, rapid antigen tests provide better indications of infectivity, as they detect proteins produced by actively multiplying viruses. If the antigen rapid test is positive, you should therefore stay away from people you don't want to infect, summarizes microbiologist and molecular geneticist Emily Bruce of the University of Vermont in Burlington.
But what about someone whose anti -test has been negative for a few days, but they still have cough and fever? According to Bruce, persistent symptoms do not necessarily indicate a further existing infection. This could be related to the fact that many symptoms arise from the immune reaction and not by the virus itself.
Most people are probably no longer contagious after ten days.
Now not only the quarantine time has been shortened in many countries-at the same time, the range of free antigen fast tests has also been reduced. In the assumption that many of the people who adhere to the new recommendations end their isolation after five days without a test, researchers are now interested in how many patients are still contagious at this point. Since the further distribution of the virus is difficult to track over many people, scientists are dependent on auxiliary variables when looking for an answer to this question.
For example, researchers who have access to a high–security level 3 laboratory – such as Barczak - are investigating whether live Sars-CoV-2 viruses can still be grown from samples taken from patients on several consecutive days. "If they are still secreting viruses that we can cultivate from their nose, there is at least a real chance that they are still contagious to other people," says Barczak. Meanwhile, there is a certain consensus among experts that people only in rare cases still give off reproducible viruses after ten days. The fact that someone is still contagious ten days after infection is therefore rather unusual, concludes the infectious disease specialist.
Other studies are trying to conclude from the amount of Viral RNA, which can be measured with PCR tests. With this approach, it is easier to work with large samples: a project by the Crick Institute and the University College Hospital, both in London, can use PCR tests, for example, which were carried out for more than 700 participants than the symptoms occurred. It was shown that many people still have a viral load after seven to ten days that is high enough to trigger further infections - regardless of the virus variant and the vaccination status of a person.
"We don't measure living viruses, but there's a lot of work in the literature now that gives a pretty good idea of what constitutes a viral load that is likely to produce infectious viruses," says David Bauer, a virologist at the Crick Institute who contributed to the study. »It's not a perfect picture, but it's a plausible one.«
Antiviral treatments may alter reality.
Yonatan degree from the Harvard T.H. Chan School of Public Health in Boston, Massachusetts, which has worked on similar PCR-based studies on contagability, agrees that ten days are a useful rule of thumb when people should no longer be contagious. A small number of people, he points out, but could also infect others afterwards.
Some such cases in the United States have been linked to the common antiviral drug paxlovid, he says. "There is a rebound phenomenon in which the symptoms seem to subside and a rapid test may even turn out negative, but a few days later the complaints return and the virus is back.«
Barczak explains how such funds affect infection ability to contagability is one of the key questions that researchers are currently investigating. »Antiviral agents change the dynamics of the symptoms, the immune response and virus excretion. People go out into the world and think that they are no longer infectious after ten days. But if you have a Paxlovid rebound, you may be. "
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