Just tucking on the end but the page linked to below suggests that some of the surveys used to link rotties to increased incidence of parvo didn't take into account how many dogs they saw of each breed;
"Early literature on parvovirus identifies black and tan breeds with a possible predisposition to canine parvovirus and I believe that there
was a concern that the breed may have not been as responsive to early vaccines against this disease. Data from clinical trials specifically
looking at serological responses shows no significant difference in responsiveness in face of maternally-derived antibodies. Against this
pharmacovigilance data still seems to show two breeds which consistently seem make up an increased proportion of the few cases
reported as suspected lack of efficacy- namely the rottweiler and the staffordshire bull terrier. Recent work looking at PDSA cases of the
disease sponsored by us and published in the veterinary record whilst showing more cases with both these breeds but when compared to
the proportion of these breeds that visited these premises the breed effect disappeared. My explanation for this is that the so-called
predisposed breeds are more common in areas with a higher incidence of disease. On this basis a different protocol specifically for
these breeds seems counter-intuitive."
http://www.aritaur.co.uk/Vaccination%20page.html Right at the bottom of the page.