Appendix B. —Safety and Efficacy of Inactivated Influenza Vaccine Description of Inactivated Influenza Virus Vaccine

Influenza vaccines can be categorized in two basic groups—live, which contain small amounts of live, attenuated (weakened) influenza viruses, and inactivated, which contain either whole influenza viruses or subunits of viruses. Only inactivated influenza viruses are licensed for general use in the United States; therefore, this discussion of vaccine safety and efficacy pertains exclusively to inactivated vaccines. Inactivated influenza virus vaccines have been manufactured and used in the United States since the 1940’s (64). Early development of influenza vaccines was spearheaded by the Armed Forces Epidemiological Board and its Commission on Influenza (12). The early vaccines have undergone several improvements. Influenza virus vaccine production procedures have been described elsewhere (33,71). Today, there are two types of inactivated influenza virus vaccine commercially available in the United States, whole virus and subunit. The antigenicity of whole virus vaccine has been demonstrated more extensivel y than that of subunit vaccines (91). On the basis of evidence generated from clinical trials conducted in 1976, however, subunit vaccines are now considered by some researchers to be equally antigenic with whole virus vaccines in adults with prior influenza virus antibody production and less antigenic in either children or adults with no prior influenza virus antibody production (51). In some studies, administration of a booster injection of subunit vaccine yielded antibody levels comparable to those produced by whole virus vaccines (21,44). In other studies, however, a second injection yielded no “booster” effect among those responding poorly to an initial injection (25). Subunit influenza vaccines tend to cause fewer adverse reactions than whole virus vaccines (30,51,65).