Research has shown that transmission of CJD can occur in animals when infected blood is injected directly into the brain. It can also occur when infected brain matter, containing a high concentration of infectious particles, is injected into blood. However, there is no definitive proof that CJD cannot be transmitted by blood transfusions. Researchers will only say that the low infectivity and the low efficiency of transmission by intravenous or intramuscular injection make CJD transmission very unlikely. There is also some evidence that infection with prions alone is not enough to cause disease.
Epidemiological data is information on transmissible diseases in the population.
To date, epidemiologists have found no evidence that CJD is transmitted through blood transfusions. The number of cases of CJD is not increasing. Many countries report between 0.5 and 1.5 cases per 1 million people per year, a rate that is unchanged. Epidemiologists argue that, if CJD were transmissible through transfusions, we would see an increasing number of cases in Western countries, where transfusions have been common for 50 years. We saw this in the 1980's with human growth hormone causing CJD in children and young people.