My email correspondence started with a very interesting video by someone who is neither an Epidemiologist nor any kind of Medic. Titled 80% Cannot Catch Covid, the video, now sadly taken down, draws its conclusions from what happened on a US Aircraft Carrier and a Cruise Ship that suffered infections. I do not believe the assertion in the title of the video, but it was clear that it made a point that needed an answer.
After some back and forth, I asked Knut Wittkowski for his exact opinion. His reply, simply, “I think that Sunetra Gupta was right: We already have some background immunity from previous infections with coronaviruses that cause some of the common colds.”