Catch-up vaccination against Haemophilus influenzae type b in human immunodeficiency virus-infected Thai children older than 2 years old.

Although most of Thai children older than 2 years are immune against Haemophilus influenzae type b (Hib) without prior vaccination, it may not be the case in HIV-infected children. Of 44 HIV-infected children tested before vaccination at the mean age of 36 months (range 24–84 months), 32 (73%) were susceptible (anti-PRP <0.15μg/ml). At 6 months after a single dose of tetanus-conjugated Hib vaccination, 67% developed anti-PRP ≥0.15μg/ml, however, only 33% developed titer of ≥1μg/ml. Four of seven (57%) with anti-PRP 0.15–0.99μg/ml at baseline were boosted to the titer of ≥1μg/ml after vaccination. Seroconversion rate and geometric mean titer (GMT) level in response to the vaccination did not correlate with HIV stage, but did correlate with viral load level of 100,000 copies/ml. HIV-infected children older than 2 years would benefit from Hib vaccination, although, one dose catch-up schedule is not sufficient in a third of these children. A second dose is needed in these children especially those with viral load of level of >100,000 copies/ml.