The presence of maternal antibodies can interfere with successful immunization of kittens even as late as 20 weeks of age. In two studies published this year, researchers concluded that vaccination of kittens should continue beyond 16 or even 20 weeks to ensure successful immunization.
From “Vaccination against Feline Panleukopenia: implications from a field study in kittens” (PDF of full study here):
Despite triple vaccination 36.7% of the kittens did not seroconvert. Even very low titres of MDA apparently inhibited the development of active immunity. The majority of kittens displayed significant titres of MDA at 8 and 12 weeks of age; in some animals MDA were still detected at 20 weeks of age. Interestingly, the vaccines tested differed significantly in their ability to overcome low levels of maternal immunity.
From “Effects of maternally-derived antibodies on serologic responses to vaccination in kittens” (abstract and article purchase information here):
A total of 15%, 44% and 4% of kittens had insufficient titers against FPV, FHV and FCV, respectively, at 17 weeks of age. Serologic response to vaccination of kittens varies based on vaccination type and MDA level. In most situations, MLV vaccination should be utilized and protocols continued beyond 14 weeks of age to optimize response by all kittens.
Looking for more information on effective vaccination in shelter animals? Check out these resources from Maddie’s InstituteSM.