Transversal sero-epidemiological study of Bordetella pertussis in Tehran, Iran

Abstract

Objectives: Pertussis remains endemic despite high vaccine coverage in infants and toddlers. Pertussis vaccines confer protection but immunity wanes overtime and boosters are needed in a lifetime. Iran, eligible for the Expanded Program on Immunization that includes the primary immunization, implemented two additional booster doses using a whole-cell vaccine (wPV) at 18 months-old and about 6 years-old. Duration of protection induced by the wPVs currently in use and their impact as pre-school booster are not well documented. This study aimed at assessing vaccination compliance and at estimating the duration of protection conferred by vaccination with wPV in children aged < 15 years in Tehran, Iran.

Methods: Detailed information on vaccination history and capillary blood samples were obtained from 1047 children aged 3-15 years who completed the 3 doses-primary pertussis immunization, in Tehran. Anti-pertussis toxin IgG levels were quantified by ELISA.

Results: Compliance was very high with 93.3% of children who received the three primary and 1st booster doses in a timely manner. Timeliness of the 2nd booster was lower (63.3%). Rate of seropositive samples continuously and significantly increased from 1-2 to 5-6 years after 1st booster attaining 30.4% of children exhibiting serological sign of recent contact with B. pertussis. Second booster dating back 1 or 2 years was associated with high antibody titers, which significantly decreased within 3 years from injection. Among children who received 2nd booster injection more than 2 years before serum analysis, seroprevalence of pertussis infection was 8.4% and seropositivity rate was higher from the 10 years-old group.

Conclusion: Seropositivity in children aged 6-7 years with no 2nd booster supports the need for a vaccination at that age. Adolescent booster may also be considered.

Fig 1. Compliance with Iranian pertussis immunization schedule.

Numbers (n) and proportions (%) of children who received early (light grey), on time (white) or late (dark grey) their pertussis vaccine dose as compared to Iranian recommendations are shown for each injection. Respect to intervals in between 1^st and 2^nd, and 2^nd and 3^rd are shown with respect to compliance with 1^st and 2^nd dose, respectively. Days (d); weeks (w); years (y).

Fig 2. Distribution of anti-PT IgG levels by timespan since 1^st booster among individuals who received 4 injections.

Timespans were categorized by year.

Fig 3. Distribution of anti-PT IgG levels by age groups among fully vaccinated individuals since >2 years.