Our vet has always said you can ride them provided you don't make them sweat. I usually give mine a day or two off - but then again I am a bit of a wuss when it comes to the horses!
You shouldn't make them sweat for 24hrs and bear in mind they may be feeling abit off colour after the injection because their body is reacting to it as if it were fighting an illness. Personally i wouldn't work them but if you do only do light work such as gentle hacking, it wouldn't be fair to ask them to work properly when they aren't feeling 100%!
I checked with my vet on this one last year and she said that the new vaccinations are fine to work with, though you wouldn't choose to work them very hard. But you also don't have to wrap them in cotton wool. It's more a chance of them feeling a bit shabby as some people do after jabs, rather than because it would be very bad for them.