Over the last 50 years, seasonal flu vaccines have had very good safety track records. Over the years, hundreds of millions of Americans have received seasonal flu vaccines. Millions of people have also safely received the 2009 H1N1 flu vaccine. The most common side effects following flu vaccinations are mild. The Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) closely monitor for any signs that flu vaccines are causing unexpected adverse events and are working with state and local health officials to investigate any unusual events.