Prior to his death Keating complained of sore throat and fatigue. So, while it is possible he made a mistake of downplaying his condition...
Perhaps, he thought that those were the side effects of the vaccination. I am sorry for him. I won't have the third dose as far as it goes.
Possibly - and, sadly, you can not ask him anymore. But I suspect it was just male stoicism at play - the "it's no big deal, you are a man, you can take it" kind of thought process. It is really big in your culture, AFAIK.
Plus - and that is entirely possible - he may have never had any significant pain or discomfort until his heat just stopped.