Flu shots only reduce the risk of infection by about half … which is still much better than nothing.

But there is no vaccine against the common cold, and no proven preventive measure for colds or the flu.

So it makes sense to adopt any diet or lifestyle measure that might reduce the risk or severity of an upper respiratory infection.
The U.S. Centers for Disease Control & Prevention stresses some obvious preventive steps:

• Avoid close contact with sick people.
• Avoid touching your eyes, nose and mouth.
• Wash your hands often with soap and water or an alcohol-based hand rub.
• Clean and disinfect surfaces and objects that may be contaminated with germs like the flu.