Name three epidemiologic features of smallpox that made it a good candidate for eradication

Name three epidemiologic features of smallpox that made it a good candidate for eradication.

My understanding were these three from NIH article:

1) cases could be identified because smallpox is a clinically evident disease and there is no subclinical illness;

2) the disease moves relatively slowly— transmission does not occur during prodrome and maximum transmission is at the time of substantial illness;

3) the vaccine is highly effective.

I was wondering if anyone else could help me understand if there were other epidemiological features that assisted with the eradication for smallpox. Your assistance is greatly appreciated.