Actually there is some evidence that norovirus has a hard time infecting people with other blood types, but I know plenty of people with O here and my dad in real life...that don't often get sick. If you think about it it woud make sense that norovirus would preferentially have adapted to infect type O hosts if it is the most common blood type. Also, my understanding is that current strains of noro cannot infect people whose blood type cannot be detected in other bodily fluids. You would know more about this than I would given your background. I wouldn't assume the other types are immune, because as an RNA virus noro is mutating all the time, and these mutations could render it harmless or way worse or have little or no effect. For now my understanding is that type B and AB blood confers some protection. This is a risk factor you cannot control, so not one I'd get obsessively hung up on. I don't even know what my blood type actually is though to be honest. I suppose I should.