I suspect it is at least a bit lower, just due to the fact that people are outside more, therefore not passing germs around in a closed inside environment as much.. Noro passes around most often in places like cruise ships, retirement homes, schools etc.. and the thing all those have in common is that people are in a contained environment, giving lots of opportunity for exposure. Summer time we tend to be outside a lot more, less contained so to speakI don't think it ever goes away, but definitely should lessen during those months!
But the last point I agree with fully.. as an emet, you have SUCH a slim chance of getting sick. We are already super careful with germs, washing hands, etc and that's what will keep you healthy! Just keep remembering, tons of people take way less care and don't get sick.. so how much better off are we!
For what it's worth, it might be worth your time to look into ways to diminish stress.. Physical activity, anti anxiety meds, yoga, these things all help our bodies cope with the stress and less stress (for me at least) means less phobic. The more stress I'm under, the greater need for control, which then kicks in the phobia to a greater extent.