We vaccinate kittens with FVRCP twice starting at 8-9 weeks of age with the second, and final kitten vaccine, administered when the kitten is no younger than 16 weeks of age.
The AAFP guidelines state that you can start this vaccine when the kitten is as young as 6 weeks of age but, unless there is a very high index of risk, I would definitely not vaccinate a kitten this young.
We wait until the kitten is at least 16 weeks old to receive his last kitten shot because the antibodies he got from nursing on his mother will have decreased to a low enough level that his own body can respond to the vaccine in order to make his own antibodies. (Maternal antibodies within the kitten can ‘tie up’ the vaccine before his body has a chance to respond to it.)
The AAFP guidelines suggest giving the FVRCP every 3-4 weeks until the kitten is 16 weeks of age. This is done in an attempt to vaccinate the kitten the minute his maternal antibody level