Development of antibody titres against FPV in kittens during primary vaccination measured by SNT B. The inverse titre is shown. The vertical dotted line indicates the arbitrary titre of 1:10 below which interference of MDA with vaccination might not be expected. All kittens were vaccinated three times with 8, 12 and 16 weeks of age. Allocation to the respective group (maternal, infection, vaccination antibodies) was based on the antibody course in the individual animal as explained in the text. In some kittens differentiation between active and passive immunity at 8 weeks was unclear and the antibodies titled as “undefined”.