Critical value of prevalence for vaccination programmes. The case of hepatitis A vaccination in Spain

Programmes using screening and vaccination are efficient in populations with higher levels of antibodies, while in those with lower levels is more efficient to vaccine the whole target population. The prevalence that makes the cost-effectiveness of vaccination programmes equal to that obtained for programmes using screening and vaccination is defined as the critical value of prevalence p*. In this study, a mathematical procedure to obtain the critical value of prevalence is developed. The formula obtained is used to decide the best vaccination strategy against hepatitis A in Spain. If V is the vaccination cost, S the screening cost, PV the predictive value of a positive test result, D the mean disease cost, A the attack rate in susceptible individuals, E the vaccine efficacy and C the vaccination compliance, the critical value of prevalence is equal to: [formula: see text] The critical value of prevalence obtained for vaccination against hepatitis A in Spain with three doses of vaccine Havrix 720 is 22%. This result show that the optimal decision is to implement vaccination programmes without screening for immunity in individuals aged < 15 years and with screening in those aged > 15 years.