Discussion
Aprevious reviewof studies on Engerix-
Bhad shown that this vaccinewas very safe in
neonates and adults.3 In the present study,
Engerix-Bwas also safe in preadolescents, with
a low but significant frequency of headaches
and gastrointestinal problems attributable to the
vaccine. Even if parents were instructed to record
every health problem, the informationwas
dependent on their subjective appraisal ofwhat
constitutes a health problem and on how diligently
they recorded data.They probably had a
greater sensitivity for adverse events after vaccination,
a bias thatwould overestimate the risk
due to vaccination. In spite of that,we observed
only a minimal increase in the incidence of adverse
events during theweek after vaccination
compared with that of the week preceding it,
confirming the safety of this vaccine.
In this study, 47.5% of the children experienced
at least 1 adverse event in the 4 weeks
following each of their 3 doses of hepatitis B
vaccine; while this figure is remarkably high,
less than a quarter of the adverse events were
attributable to the vaccine. This result clearly
demonstrates the overestimation of the risk of
vaccine that occurs when only postvaccination
data are collected. Paradoxically, the safer the
vaccine, the greater will be this overestima