Why didn't the older boy catch chickenpox again when his younger brother had it two years later

This must have been based around earlier than the 2000'th century because now we do something call immunization which gives your baby medicine so that the baby does not get sick. So the most reasnable answer would be that the older boy either stayed his distance from his brother and or got a shot and or medication. If this is not the right answer to your homework/work then I am sorry.