Considering just how many people from outside of the US Google employes it's quite possible that they simply came from a country that historically didn't have a good vaccine coverage for example India.
In 1980 even Germany had 25% vaccination coverage but I'm guessing it could include East Germany too so a developer in their mid 30's simply might not been vaccinated at a young age and if they didn't travel to high risk countries, joined the military or worked with at risk groups it was never caught.
Heck according to this data https://data.worldbank.org/indicator/sh.imm.meas
In 1980 even Germany had 25% vaccination coverage but I'm guessing it could include East Germany too so a developer in their mid 30's simply might not been vaccinated at a young age and if they didn't travel to high risk countries, joined the military or worked with at risk groups it was never caught.