Final answer:
The nurse should reply that there are other things the client can do to prevent the flu, such as hand washing.
Step-by-step explanation:
The nurse's best reply to the client's concern about the limited availability of the annual flu vaccine would be: "There are other things you can do to prevent the flu, such as hand washing." This response acknowledges the client's concern and provides a teaching opportunity to discuss alternative measures for preventing the flu. It is important to emphasize that hand washing is a simple yet effective way to reduce the spread of the flu virus.