Estimation of the herd protection of Haemophilus influenzae type b conjugate vaccine against radiologically confirmed pneumonia in children under 2 years old in Dhaka, Bangladesh.

BACKGROUND

Herd protection of Haemophilus influenzae type b (Hib) conjugate vaccine has been associated with excessive decrease of invasive Hib diseases, i.e., pneumonia and meningitis, with increased national or regional Hib vaccine coverage. Only a few studies have examined herd protection at the individual level and even less evidence is available from Asia. We examined Hib vaccine herd protection against radiologically confirmed pneumonia among children less than 2 years old.

METHODS

We incorporated data from a matched case-control study and a vaccine coverage survey in Dhaka, Bangladesh. Pneumonia cases (n=343) were confirmed by radiology. For each case, two controls with conditions other than pneumonia or meningitis were selected from the same hospital. Hib vaccine coverage was calculated as percentages of children who received at least 2 doses of Hib vaccine from a survey in the neighborhood centered on each case and control. Conditional logistic regression was fit to examine the association between vaccine coverage and risk of radiologically confirmed pneumonia.

RESULTS

Neighborhood Hib vaccine coverage varied from 0% to 63.5% for cases and from 8.7% to 61.5% for controls, respectively. Cases were less likely to have neighborhood coverage higher than 20% (OR=0.49, 0.52, 0.55, and 0.69 for coverage 20-29%, 30-39%, 40-49%, and ≥50%, respectively) than coverage <20%, compared to controls, although the estimates for coverage 40-49% and ≥50% were not statistically significant.

CONCLUSIONS

The study indicates that Hib vaccine may provide herd protection, even when the coverage is as low as 20-39%, in a low-income country. Asian countries should consider herd protection in implementing effective vaccine policy with limited resources.