Bordetella pertussis infection and antibody dynamics in household cohorts in two South African communities, 2016 - 2018: findings from the PHIRST study.

INTRODUCTION AND

METHODS

In a cohort study, enrolling new households annually during 2016-2018, we estimated the B. pertussis attack rate using serology and PCR, identified factors associated with seroconversion, and described antibody changes over time. Nasopharyngeal swabs were collected biweekly for 6-10 months annually, with cases defined as individuals testing PCR-positive at least once. Seroconversion was defined as a ≥4-fold increase in anti-pertussis toxin IgG concentration between consecutive blood draws. Logistic regression was used to identify factors associated with seroconversion among PCR-positive individuals.

RESULTS

Among 1509 participants, the serology attack rate was 5.8% (87/1509), 6.2% (94/1509) by PCR, and 9.6% (145/1509) combining both methods. Among PCR-positive cases, 38.3% (36/94) seroconverted, with a mean time to seroconversion of 2.9 months (range: 3 weeks-5.9 months). Younger participants (5-18 years) (adjusted odds ratio (aOR) 6.8, 95% confidence interval (CI) 1.3-35.1) and those with episode durations of ≥7 days (aOR 13.3, 95% CI 3.4-51.1) were more likely to seroconvert. Among the seroconverted, 75.0% (27/36) maintained antibody concentrations >40 IU/ml for at least 12 months post-infection.

DISCUSSION: Almost 10% of participants had B. pertussis infection during follow-up. Less than half of individuals with PCR-confirmed infection seroconverted but maintained elevated antibodies for at least one-year post-infection.