Mozart and smallpox the imperial family were successful, and led to greater public acceptance for the procedure. Smallpox struck the Mozart family again in the next generation: Nannerl's eldest son Leopold and two of her stepchildren caught the disease during an outbreak in the Salzburg area in 1787. All three children survived. In 1796, the discovery of vaccination—the use of the related cowpox virus to immunize against smallpox—by Edward Jenner revolutionized the ability of medicine to deal with smallpox. Vaccination reached Vienna around 1800, when yet another local epidemic created impetus for its adoption. One of the doctors trained in the Vienna campaign,
Vaccination policy once vaccination rates reach a certain level, even if this level is below that optimal for the community. For example, a 2003 study found that a bioterrorist attack using smallpox would result in conditions where voluntary vaccination would be unlikely to reach the optimum level for the U.S. as a whole, and a 2007 study found that severe influenza epidemics cannot be prevented by voluntary vaccination without offering certain incentives. Governments often allow exemptions to mandatory vaccinations for religious or philosophical reasons, but decreased rates of vaccination may cause loss of herd immunity, substantially increasing risks even to vaccinated individuals.