This just happened to us. If in doubt, always call the dr. but what they told us is that I should wait about an hour and a half before giving even fluids. then try sips of pedialite. continue breastfeeding as normal if you are breastfeeding, although it would be good to wait an hour and half on that too. If he holds down fluids for I believe 4 or 5 hours, you can try some rice cereal or something bland like that. If he goes more than about 6 hours without a wet diaper he is dehydrated and you should call your dr. right away and possibly take him to the emergency room. I think the flu shot mostly covers that achy/respitory/cold like flu, not the little stomache bugs that come and go. And watch out because my husband and I both caught it after being thrown up on a few times and it was miserable. I felt like I was dying. Once again, I don't know if I remember everything perfectly so you may still want to call the Dr.