During the first year of a child's life they are exposed to vaccinations and infections which can challenge their immune system. Researchers from the University of London have suggested that early life exposure to vaccinations and infections may influence allergic disease development, of particular note, hayfever. Although there appeared to be no relationship between the MMR vaccine and hayfever, there is increasing evidence to suggest a relationship between the DPT vaccine and the development of hayfever later in life.
Bremner, S.A., Carey, I.M., DeWilde, S., Richards, N., Maier, W.C., Hilton, S.R., Strachan, D.P. & Cook, D.G. 2007. Vaccinations, infections and antibacterials in the first grass pollen season of life and risk of later hayfever. Clinical & Experimental Allergy, 37, 512-517.