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Introduction: An important issue in
immunising older children and adults against diphtheria and tetanus is the
reactogenicity of the diphtheria toxoid, which may be overcome with low-dose
vaccine preparations. We evaluated whether substituting the paediatric dose
(DT, 25 Lf diphtheria/dose)
with the adult dose (dT, 2 Lf diphtheria/dose) for a booster administered to
pre-school children reduces reactogenicity without loss of immunogenicity.
Methods:
The study population consisted of 6-year-olds
vaccinated with 3 doses of DT in the first year of life. Children were
randomly assigned to the DT or dT group; the study was double-blinded. Side
reactions were actively surveyed in the 3 days following vaccination. A
sub-population underwent immunogenicity testing: capillary samples were taken
before vaccination and 30 days afterwards to measure diphtheria antibody
titres. Geometric mean titres
(GMT) and percentage of children with titres => 1 IU/ml (long-term
protection) were calculated.
Results:
The study included 754 children (372 DT, 382 dT);
143 underwent immunogenicity testing (76 DT, 67 dT). The two vaccine groups
were balanced in terms of age, gender and site of injection. Frequency of
side reactions was similar, except for local redness (31% DT, 16% dT, p< 0.001) and swelling (35% DT, 24% dT,
p<0.001). Of the children undergoing immunogenicity testing, 95% in the dT
group and 97% in the DT group had antibody titres => 1 IU/ml. The GMTs were 7.7 (dT) and 14.1 (DT)
(p<0.001).
Conclusions:
The use of the DT vaccine is associated with a
greater frequency of local side effects, though these are clinically minor.
Nonetheless, since the DT vaccine induced higher GMTs, it should be used for
the booster dose at 6 years of age, particularly if further boosters will not
be administered.
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