To keep a kitten healthy, vaccines are given as a series at specific intervals beginning around 7 - 8 weeks old until they have developed sufficient immunity, typically around 16 weeks old. (catfriendly.com)
However, they were unable to provide sufficient immunity to the disease. (sciencedaily.com)
If the mother has sufficient immunity in her own system, some of this protection is passed to the unborn pet. (speedwayvet.com)