When a cat is vaccinated, the vaccine stimulates a response in certain cells, which produce antibodies to a specific disease. Should the cat then come into contact with that disease at a later date, the antibodies neutralize the bacteria or virus causing the disease and prevent the infection taking hold.
Vaccination, therefore, sets up a protective screen against disease. How efficiently this is achieved largely depends on the individual animals ability to respond to the vaccine.
Animals can vary in their capacity to respond to vaccination. It is not possible, therefore, to certify that a cat has been protected against a specific disease - only that it has been vaccinated. Nevertheless, the vast majority of cats will be protected once they have had time to develop the antibodies and, provided they receive booster vaccinations at the recommended time intervals, will maintain that protection for many years. Your Veterinary Surgeon will advise you on all these points.