Currently available TIVs are prepared from viruses that are grown in embryonated hens' eggs. Alternative substrates for vaccine production are desirable in order to reduce the vulnerability of and to expand influenza vaccine supply. Recombinant DNA techniques allow for expression of the influenza hemagglutinin (rHA) by baculovirus vectors in insect cell cultures. Advantages of this technique include speed of production, absence of egg protein, and a highly purified product. Previous studies among healthy younger and older adults have confirmed that rHA vaccines are safe, well tolerated and immunogenic at dosages up to nine times higher than those contained in TIV. Dose-related increases in serum antibody levels after immunization also were observed.