In 1981, Hilleman and his team treated blood serum with pepsin, urea, and formaldehyde to obtain a safe vaccine before further improvements to the vaccine were made in 1986. As of 2003, over 150 countries and regions were using the vaccine. Hilleman believed that the hepatitis B vaccine was his company’s (Merck & Co) greatest achievement. Due to the possible complication of liver cancer from hepatitis B, Hilleman’s vaccine can be said to be the world’s first cancer vaccine. In Taiwan, the usage of said vaccine has lowered the prevalence rate of hepatitis among infants by 99%.