Cannot say -- only know that my brother was told specifically that he was not to donate for convalescent plasma again because and this is a direct quote: "You have very little antibiodies in your blood and we want you to keep everything that you have to protect you from another COVID-19 occurrence." I believe I posted a link sometime ago that the severity of your occurence has a direct correlation to how much antibiodies you have in your system. So with being asymptomatic, do I have any antibiodies in my system-- I don't know -- I would have to be tested to know. I believe that the link said that asymptomatic people have little or no antibiodies in their systems but that was several months ago so I'm not going to fall on my sword over it
I also know that Fauci said that even after getting your COVIDl-19 booster you still need to wear your mask. Partially because it is possible for those with the COVID-19 vaccines to infect those who have not yet had the vaccine. I don't understand the why and wherefore of that statement but that was what I read after Fauci's briefing yesterday.
This is just a guess, but after reading some of the stuff that came out of Japan a couple of months ago I'd bet that he probably had some cross immunity from being exposed to a different coronavirus recently.