High level of anti-PT IgG antibodies in subjects older than 7 years age are most likely due to contact of B.pertussis. Therefore, information about the sero-prevalence of high levels of anti-PT antibodies in combination with the post-infection antibody decline rate allows us to study B.pertussis infections in > 7 years age groups irrespective of clinical manifestation and reporting symptom. This evidence concerns the gene F2 and infection.