Don't expect a kid in school to answer 9.
Most of them can't use an analog clock to tell time.
That was his point, which I suppose was lost on you like:...

Smallpox and Chickenpox, Measles and Polio give natural immunity and you can only develop each disease "once".

Covid can make a person ill an indefinite number of times
because it offers no natural resistance. Therefor, a vaccine developed from the Covid virus of any strain does not offer any resistance (and certainly no immunity) to Covid.

"SCIENCE": Learn some.

Don't expect a kid in school to answer 9.
Most of them can't use an analog clock to tell time.