In Sri Lanka pertussis continues to circulate in the community and cases among adolescents and adults have been reported despite 95% coverage of the four dose pertussis vaccination during early childhood. Waning of immunity following natural infection or immunization may contribute to the persistent circulation. An adolescent booster dose is not included in the national immunization schedule of Sri Lanka, although this is routine practice in many countries. Therefore information on immunity to pertussis in the adolescent group is needed prior to considering vaccination schedule changes.
The quantitative determination of specific Immunoglobulin G antibodies to Bordetella pertussis toxin was done using a commercially available validated ELISA method. The antibody values were categorized into groups according to the interpretive criteria provided by the manufacturer. The values were <55 IU/mL, negative; 55-<60 IU/mL, borderline; 60-125 IU/mL, positive; >125, strongly positive respectively. Sera of 385 asymptomatic individuals aged 4 to 24 years admitted to surgical units of Lady Ridgeway Hospital, Colombo and Colombo South Teaching Hospital were used for the study. Mann-Whitney U and Kruskal-Wallis tests were used in analysis of results and p ≤0.05 was considered as statistically significant. Details of epidemiological variables were collected using a questionnaire and correlation with significant levels of pertussis antibodies was determined.
Median age of the study population was 12 years with 212 (55.1%) females. The median anti PT antibody level was 3.31 IU/mL and 352 (91%) had anti PT levels ≤55 IU/mL. Median of anti PT levels were 3.18 IU/mL for 4-7 years, 1.43 IU/mL (IQR 0.336-6.27) for 8-11 years, 4.28 IU/mL (IQR 0.978-13.39) for 12-15 years, 6.14 IU/mL for 16-19 years and 4.89 IU/mL for 20-24 years and the differences were statistically significant (p = 0.000). Females (p < 0.003) and those having a sibling aged ≥12 years (p = 0.017) had significantly higher anti PT levels.
The majority of the study population, especially 8 to 11 year age group had low anti PT IgG levels. The higher antibody titers in the 12-15 year age group seem to indicate infection in early adolescence. A booster dose of acellular pertussis vaccine need to be considered.