Vaccination
The people who are in the 5% are there for many reasons. Some got the vaccine but did not react to it. Their immune system did not learn how to fight it well. Some of them are too sick to get the vaccine. It can be children who are too sick with other diseases to get vaccines. It can be a pregnant woman who cannot get the vaccine because it could hurt her baby. It can be a person with cancer who does not have a strong immune system. It can be an older person who has a weak immune system.