Several substances other than antigens are included in vaccines. These are classified as follows: stabilizers, preservatives, buffer, adjuvants, antibiotics etc. Some of these substances, for example gelatin and thimerosal, have been removed from vaccines because of their undesirable side effects. However, stability and sterility are very important conditions for vaccines. Therefore, removal of excipients from vaccines should be investigated very carefully. Impurities derived from culture substrates for bacteria and viruses should be reduced as much as possible. Japanese vaccines are highly purified. For example, influenza vaccines contain only a few ng/dose of OVA which are significantly lower than the WHO standards (5 microg/dose).