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Preterm infants have been shown to respond with lower antibody levels of protective anti‐polyribisolribitolphosphate (PRP) to immunisation against Haemophilus influenzae type b (Hib) than term infants. This may be particularly relevant in the UK where fourth doses of Hib vaccine were not given until September 2006, and preterm infants were at increased risk of Hib disease. Other measures such as antibody avidity may give valuable information.
Having previously reported the anti‐PRP responses to primary immunisation in a large group of UK preterm infants,1 we went on to measure the geometric mean avidity index (GMAI) where the antibody level was sufficiently high to allow this (0.7 μg/ml) by modification of the assay as previously described.2,3 Results of a comparison with contemporaneously immunised term infants are shown in table 11;; no significant differences were found.
Infants with post primary anti‐PRP level of <1.0 μg/ml were offered a fourth dose of vaccine at median 6.9 (9.2) months (term (preterm)) (interquartile range 6.7–8.1 (8.2–12.8)) of age, and we again measured the avidity. These infants' serum antibody had a similar level of avidity to those who had responded to their primary immunisations (table 11).
These data suggest that if an anti‐PRP level of >0.7 μg/ml to priming with three doses of Hib vaccine is achieved in a preterm infant, then the avidity of antibody is not likely to be significantly reduced compared to term infants similarly immunised. If an anti‐PRP level of <1.0 μg/ml is seen after primary immunisation, a fourth vaccine dose produces high antibody levels with similar avidity indices in both pre‐term and term infants. These data suggest that the introduction of the new UK schedule, including a fourth dose of Hib vaccine in the second year of life, should boost the protection that the standard schedule offers to those infants born prematurely who respond poorly to priming doses and render it comparable to that seen in term infants, at least from 1 year of age.
Competing interests: None.