That was an anomaly in many other ways too. Keep in mind that a good chunk of the world was still recovering from WWI. Being exposed to some other disease earlier in life can give a similar effect to vaccination.

This is one of the reasons why HIV spread was not as bad as it could have been in Western Europe where exposure to a disease still gave some benefits many centuries later:

https://www.nature.com/news/2005/050307/full/news050307-15.h...