Young adults:

The AAFP guidelines suggest giving a FVRCP booster 1 year after the last kitten vaccine – i.e. – when the cat is ~ 16 months of age. However, if the kitten responds as he should to the kitten series, this booster should not be needed. The rationale behind the 1 year booster shot is to cover any kitten that did not properly respond to the kitten series.

Reasons why a kitten may not fully respond to a series of vaccines as a kitten and would benefit from a 1 year booster are:

1) The last kitten shot was given when he was younger than 16 weeks of age.

2) Maternal antibodies hung on longer than 16 weeks and interfered with his immune system’s ability to respond to the last vaccine. (We have good studies showing that the maternal antibody levels are low enough in most kittens to allow them to respond to a vaccine by the time they are 8-12 weeks of age so this is an improbable scenario.)
3) The kitten was in poor health when vaccinated and did not respond properly. (Vaccines should never be administered to sick animals but, unfortunately, unhealthy animals are vaccinated more often than you may think.)

4) The vaccine was of inadequate immunogenicity which means that the vaccine was damaged in terms of its efficacy. This could happen due to a problem within the manufacturing process or because of poor handling of the vaccine after it left the manufacturing plant.

Note that some cats are genetically ‘non-responders’ and never will respond to a vaccine no matter how many you give them. In these rare cases, giving a booster vaccine 1 year after the last kitten vaccine would be of no benefit.

Deciding to give a booster vaccine 1 year after the last kitten vaccine, or not, is a judgment call.

The older a kitten is (past 16 weeks of age) when he receives his last kitten shot, the less inclined I would be to give a booster shot 1 year later.

This is because the older he is, the more mature his immune system is – and better able to respond – and the less chance there will be for the maternal antibodies to be at a high enough level to interfere with his ability to respond to a vaccine.

Another option would be to test his titer (antibody level) to panleukopenia (not herpes or calici) to help you make a decision. (More on titer testing below.) This is what I would personally opt for.