132 Vaccines

Efficacy and Effectiveness

Antibody response, determined by measuring haemagglutination inhibition titers, is used as a serological marker of the immunological response to the vaccine, or efficacy. In persons primed by previous exposure to viruses of the same subtype, antibody response is similar for the various types of vaccines. However, in persons without such previous exposure (either through vaccination or through natural infection), response is poorer in the split and subunit vaccines, where two doses are required.

In healthy primed adults, efficacy after one dose ranges from 80-100 %, while in unprimed adults, efficacy enters into this range after two doses. In other populations, efficacy is lower:

Table 1: Efficacy of influenza vaccination*

Population	Efficacy
Healthy adults and most children	80-100 %
Renal failure (chronic)	66 %
Renal transplant	18-93 %
Hemodialysis	25-100 %
Bone marrow transplant	24-71 %
Cancer	18-60 %
HIV infection	15-80 %

*adapted from Pirofzki 1998, Potter 2004, Musana 2004

Effectiveness, usually defined by prevention of illness, is generally slightly lower, with 70-90 % effectiveness in children and healthy adults under the age of 65. In those above 65 years of age, a lower rate of 30-40 % is seen. However, the vaccine is 20-80 % effective in preventing death from influenza in persons older than 65 years, with revaccination each year reducing mortality risk more than a single vaccination (Govaert 1994, Gross 1995, Nichol 1994, Partriarca 1985, Voordouw 2004). In patients with previous myocardial infarctions (MI), a study by Gurfinkel et al. (2004) showed a reduction in the one year risk of death (6 % in the vaccinated group, 13 % in the control group) and combination of death, repeat MI, or rehospitalisation (22 % versus 37 %), possibly due to a non-specific effect of immune responsiveness. Further studies are planned to evaluate the impact of influenza vaccination on acute coronary syndromes.

Vaccination of caregivers against influenza also reduces the exposure of vulnerable populations to influenza.

Studies have been done on effectiveness in terms of health benefits and cost in several healthy populations (Bridges 2000, Langley 2004, Monto 2000, Wilde 1999). They suggest that, while individual health benefits from vaccination certainly exist, as do reductions in days absent from work, vaccinating healthy working adults may not provide cost savings when compared to loss of productivity and days taken off due to illness. Vaccinating health care professionals is recommended, not only because of health benefits and reduced days absent from work, but because it is believed that hospital employees tend to report to work in spite of having an acute febrile illness. Previous studies have shown that vaccinating health care professionals reduces nursing home and hospital-acquired influenza infections (Pachuki 1989, Potter 1997).