"Can flu shots cause the flu? Yep. Here's the mechanism how, explained by veterinarians. The article also describes how vaccines made outside the country may have no effect on local pathogens."
"The live virus vaccines, also known as attenuated, generate good immunity with just one dose, but are also problematic: if the attenuation of the virus (reduction of its virulence) has not been stable, it could find a complimentary virus and reverse its pathogenic capacity to the point where it could cause the illness", says the coordinator of the research group at the UN.