The study did not identify why some children were protected but say it may be because they are exposed to certain infections earlier, stimulating the immune system.
The research looked at 14 published studies involving 20,000 children worldwide.
The study did not identify why some children were protected but say it may be because they are exposed to certain infections earlier, stimulating the immune system.
The research looked at 14 published studies involving 20,000 children worldwide.