In-wound kittens receive antibodies from their mother through the placenta and after they are born, through the colostrums, which is the first milk. The age at which kittens can effectively be immunized is proportional to the amount of antibody protection the young kittens received from their mother.
Antibodies are small proteins with disease-fighting role and they are produced by certain types of cells called 'B cells.' The proteins are made in response to 'foreign' particles – i.e. bacteria or viruses. These antibodies make team with certain proteins called antigens to fight against foreign particles like bacteria, in order to inactivate them.
High levels of maternal antibodies present in a kitten's bloodstream will block the effectiveness of a taken vaccine. The immunization by a commercial vaccine will work when maternal antibodies fall to a low enough level in the young kitten.
The antibodies from the mother generally circulate in the newborn's blood for a few weeks. The fact that represents a complicating factor is that there is a period of time from several days to weeks in which the antibodies received from mother are too low to provide protection against diseases, but too high to allow the vaccine to work and produce immunity.
This period is called the 'window of susceptibility'. This is the time when a kitten can contract a disease although it have been vaccinated. This window of susceptibility can vary a lot. The length and timing of the 'window of susceptibility' is different from litter to litter and between animals in the same litter. Since the timing and the length of this window of susceptibility varies so widely, it is impossible to determine the best time to administer the vaccine to each kitten. There are a lot of variables. For this reason, kittens are given a series of vaccinations in hope that we will vaccinate the kitten as soon as it is leaving the 'window of susceptibility.'
What is the earliest age at which a kitten can be safely vaccinated?
In general, 6 to 8 weeks is the earliest age at which a scheduled vaccination program could and should be started. Exceptions will occur for those animals that are living in environments which would increase the danger to contact a disease, e.g., animal shelters, breeding facilities etc. These kittens may benefit from earlier vaccinations. Vaccinating kittens less than 4-5 weeks from their birth, especially with those modified live vaccines, can cause severe complications.
