Prevalence of antibodies associated with herd immunity: a new indicator to evaluate the establishment of herd immunity and to decide immunization strategies.

OBJECTIVE

To develop a method to estimate the critical prevalence of antibodies associated with herd immunity (p(c)) and to assess the establishment of herd immunity against measles, mumps, rubella, varicella, and pertussis in Catalonia (Spain) using this method.

METHODS

The herd immunity is established when the prevalence of antibodies (positive serological results) is higher than p(c) = I(c) Se/PV, where I(c) is the prevalence of protected individuals necessary to block the transmission of an infection in the population, Se is the sensitivity of the serological test, and PV is the predictive value of a positive serological result. The establishment of herd immunity was assessed comparing the age-group specific p(c) with the prevalence of antibodies (p) observed in Catalonia in 2002 in schoolchildren (6-9 years) and individuals aged 14 to 29 years and 45 to 54 years.

RESULTS

The herd immunity was not established (p < p(c)) against measles, mumps, and varicella in schoolchildren aged 6 to 9 years, against measles in individuals aged 15 to 24 years, and against pertussis in all age groups. Based on these results, the measles-mumps-rubella (MMR) vaccine should be given to schoolchildren and individuals aged 15 to 24 years, the varicella vaccine to schoolchildren aged 6 to 9 years, and the diphtheria-tetanus-pertussis (DTaP/dTaP) vaccine to all age groups in order to establish the necessary herd immunity in the population.

CONCLUSION

Serological surveys and the critical prevalence of antibodies can be used to assess the establishment of herd immunity for vaccine-preventable diseases in different population groups, indicating to which of them immunization activities should be prioritized.