There have been a few stories in the press of people apparently being re-infected by the coronavirus SARS-CoV-2. These people reportedly became infected and hospitalised, and then were sent home once they’d tested negative for the virus. Then, days or weeks later, they tested positive again.
But this doesn’t necessarily mean that they caught the coronavirus twice.
First, during recovery from infection, a person may have very low amounts of the virus remaining in their body – low enough that our tests can’t accurately detect it. In this case, the person may be sent home on the assumption that they’re virus-free. However, their body may still be fighting the virus, and a resurgence of the virus (and symptoms) can occur, resulting in a positive test. In this case, it would just be one protracted infection, not a re-infection.
Second, we know that in most people, SARS-CoV-2 generates a strong response from the immune system. With the related coronavirus SARS-CoV, this response creates an immune memory of the virus that prevents re-infection for one to two years, and it’s likely that this is also the case for the new virus. SARS-CoV-2 also has a fairly low mutation rate, which means that it (hopefully) won’t change enough that our immune system no longer remembers it (this is what the flu virus does and why we need a new jab every year).
If this all turns out to be true, then it would suggest that re-infections are unlikely and that the cases in the news reflect testing sensitivity. However, SARS-CoV-2 is so new that we won’t know for sure until we’ve found out just how protective our immune response to the virus is, and how long it lasts.