…[Relapse] doesn’t happen with these respiratory viruses. The symptoms that drag on are your body’s response to the virus, but the virus is gone after a few days. I take great umbrage at the lengths of time you are meant to be infectious for because it is just not true. Nine days is nonsense. You don’t excrete a live virus that long.
Those studies are not checking for live virus, they are checking for genome. They do something called a PCR test (polymerase chain reaction), which is the test we are using to diagnose patients. It doesn’t tell you that you have live virus in your nose, it tells you have had it. For about 72 hours of a viral infection you have a live virus. In children it can last for longer – four or five days have been observed in flu.
So, there’s a big difference between how long we can detect the virus and how long they can infect someone else. With this coronavirus the only way you can say, yes, they are still shedding live virus – which is the only thing that will infect someone else, is if you take that sample from the patient and extract it and put it on tissue culture cells and then see it growing. That is done very rarely. There are not a lot of studies that look at live viruses. It is very easy to do PCR tests. It is harder to do live virus studies.
Source: Cambridge virologist explains what we do and don’t know about Covid-19