First, flu travels person to person (sneezing, coughing) and does not survive well otherwise.
Second, copper is a well-known antibacterial agent, but will not kill viruses. Copper is toxic to most bacteria at high amounts
Third, washing your hands with soap and water is the #1 way to prevent any type of infection. Certainly, wash your hands after handling coins.
I agree that coins contain very low (if any) infectious disease counts on them, because metals are toxic to most bacteria. Paper currency contains much higher bacterial counts than metal coins.