Do you know any who suffered from the shots?
Actually yes. My first purebred Siberian had a reaction to her 1st set of shots. Called the vet right off but she recovered pretty quick.
If you want reaction data though, check out the monthly poison treat people are giving out. Or the liquid you put on their backs. Dogs are dying or really bad reactions. I've used natural products but they honestly don't work as well. Baths and flea combs, regular brushings etc are alternatives. Last year though, I had to give our 3 a poison treat because we had a big prob with fleas. Could not get rid of them to save our lives. Each pup (3) got one and the fleas cleared up quick and nothing since and none of them had a reaction. But I gave the stuff out over 3 days in case one of them did react.
There are groups on FB that discuss these treatments and alot of people are fed up with the drugs drugs drugs. Don't get me started on kibble and certainly not 'science diet'.