that causing the “Asian” influenza), the population at large is considered more susceptible than usual to infection. In these cases, vaccination may be recommended for all persons except those who have conditions contraindicating vaccination. High-risk groups continue to have high priority, however.
If reliable evidence is forthcoming that an attenuated live influenza virus might reduce disease transmission within the population then reconsideration of the current high-priority target population may be desirable (to include other groups, e.g. schoolchildren). However, for the purposes of this evaluation, the current target populations have been adopted. This may underestimate the impact of vaccines if broader target populations are ultimately chosen.
Protection offered by current vaccines is limited in duration and by the antigenic shift and drift that occur in influenza strains. Thus, the vaccine needs to be reformulated and administered annually to match prevalent strains of virus. While antigenic shift and drift complicate vaccine control of influenza, the potential for epidemic spread and the dearth of prophylactic or therapeutic agents for the disease render vaccine prevention a desirable goal, particularly in view of the disease burden and the prospect that live vaccines might offer longer protection.
Defining the target population is the first step in calculating the benefits that could be produced by a vaccine candidate. This knowledge can be translated into an estimate for vaccine preventable illness (VPI). VPI is defined as the number of cases, complications, sequelae, and deaths that could be prevented by immunization of the entire target population with a hypothetical vaccine that is 100 percent effective.
The determination of VPI is complicated for influenza by lack of data on what proportion of illness actually falls within the target population. As noted above, the Immunization Practices Advisory Committee (IPAC) of the CDC recommends the administration of influenza vaccine to individuals whose health status places them at high risk (Centers for Disease Control, 1984). All individuals over age 65 are included in this definition, so all illness in that population is considered vaccine preventable, but no data exist on influenza-related illness in high-risk persons under age 65. To overcome this difficulty, the committee adopted figures from Glezen (personal communication, 1984), indicating the proportion of patients hospitalized for ARD whose discharge records describe relevant high-risk conditions listed by IPAC (see Table K.7). Unfortunately, there is no way to determine whether or not these chronic conditions were known to the individuals or their physicians prior to hospitalization.