I am going to try to get out ahead of this and hopefully you can all help spread accurate information. Supposedly a legitimate case of “reinfection” with CV-19 has occurred in Hong Kong. (HK Article) The person had a mild infection several weeks ago and a new “infection” was detected on re-entry to the country after traveling, but the person was completely asymptomatic. The genetic sequence was different, so a true reinfection was likely. Now the purveyors of panic are already seizing on this as proof that there is no lasting immunity and we are all doomed. (We are all doomed, but not because of coronavirus, but because that is the nature of life and death, and, well, because of the apparent incredible irrationality of a large portion of humanity.)
Okay, it took me a while to realize I needed to explain this better, which I did a couple of weeks ago, but will repeat here. Adaptive immunity is what develops after an acute infection with a pathogen the body hasn’t encountered before. It is comprised largely of memory B cells, and their associated antibodies, and memory T cells, which can be present in blood and mucosal tissues. Adaptive immunity doesn’t encase us in a plastic bubble that keeps a pathogen from ever entering the body. What it does is provide a rapid response when the pathogen, or a related pathogen, is recognized as having re-entered the body, and that rapid response almost always will clear the pathogen very quickly, without you ever even noticing anything. Happens all the time, every day. While this process is occurring, you are almost certainly not infectious.
So, let’s take the example of a seasonal coronavirus. You wander around and encounter them every day. Because you have been infected with them before, your body has memory B cells and T cells that keep the virus from getting any foothold and beginning to replicate. But if you were getting tested every day for seasonal coronavirus, at some point a test might be positive because there was virus in your nasal or throat passages. This would just randomly occur at some point. Means nothing, because you are not infectious, you just got detected in the process of having virus in your upper respiratory tract before it got cleared by the adaptive immune response. So this poor guy was traveling, picked up some virus, had no idea he did, and got tested before it was cleared.
This is back to my ruminations on what it means to be “infected”. I think what is important is are you “infectious”. Once you have developed adaptive immunity, it is highly, highly, highly unlikely that you are going to be infectious again, even though you may encounter the pathogen and even breath it in.