Many of the studies on the vaccines were done (finished) about 20 years ago. The titres of parvo and a couple other vaccines were found to remain in the blood for at least 7 years, which was the lenght of the studies. Shortly thereafter, the funds were raised to do a 7-year study at the University of Wisconsin on the rabies vaccine. Same results, The titres remained in the blood for at least 7 years.
The state of Massachusetts passed a law requiring veterinarians to inform their clients of the study results on parvo. Most pet owners have said they were. never informed. Can we all say 'money money money'? I remember when vets insisted Dobermans and Rottweilers NEEDED parvo shots every 6 months. Not even remotely true.