John wrote:
* In 1871-2, England, with 98% of the population aged between 2 and 50
vaccinated against smallpox,

Hilarious. Vaccination was only made compulsory in 1853, and then only
for newborns. Even by 1902 the level of vaccination in the population
was only 75% at best, and infant immunisation ran as low as 40% despite
the Act. See for instance "Surveillance and Control Measures during
Smallpox Outbreaks" published in Emerging Infectious Diseases 2005;
11(2):291 by the CDC.