No, not his immune system per se, but the antibodies against the ChinaVirus.
You know, a big deal was made early on about how different this virus is from others and how little is known about it, and then there were the vids of the Chinese falling down dead in the street from it (propaganda, we now know), and some people are still going on about the supposedly long-lasting bad effects from this flu, but basic biology is still in effect. By September, the docs should (there's that word "should" again) have figured out the antibody issue.