evidence: What to know - Flu vaccines have a good safety record. Hundreds of millions of Americans have safely received flu vaccines over the past 50 years, and there has been extensive research supporting the safety of flu vaccines - Flu vaccines cannot cause flu. [...] Flu vaccine cannot cause flu The vaccines either contain inactivated virus, meaning the viruses are no longer infectious, or a particle designed to look like an influenza virus to your immune system. While the nasal spray flu vaccine does contain a live virus, the viruses are changed so that they cannot give you the flu.