The «primer» pandemic influenza vaccine — made from live but weakened virus — introduces the immune system to H7N9 influenza virus, and
subsequent vaccination with the «booster» inactivated virus vaccine recalls a more robust immune response.
After
that subsequent vaccination boosters are given every 1 to 3 years or based on antibody levels depending on the policy of the supervising animal hospital.
Subsequent vaccinations are good for 3 years, as long as proof of a prior rabies vaccination is provided.
Subsequent vaccinations should be based on risk of exposure (e.g., a single housed indoor cat is at no appreciable risk of exposure to FeLV so vaccination may not be warranted, whereas it may be important in an indoor - outdoor cat.