Inactivated flu vaccines cannot cause the flu in a vaccinated person because the virus in these vaccines is "inactivated" or killed by processes such as heat or chemicals. This means that the virus is no longer alive and cannot infect cells or replicate in the body. Instead, the vaccine works by exposing the immune system to parts of the virus (antigens), which stimulates the body to produce protective antibodies without causing illness. Any symptoms experienced after vaccination are typically immune reactions or side effects, not the flu itself, which would require an active, replicating virus to cause infection.