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Combined active and passive prophylactic
immunisation is designed to obtain both the
immediate but temporary benefit of protection:
afforded by serum and the more remote but
lasting benefit afforded by vaccine injection. It has
been used chiefly in diphtheria. If, for example,
this disease breaks out in a school, a small dose
of serum, such as 500 antitoxic units, may be
injected at once into all the children, and a first
dose of diphtheria prophylactic vaccine given
simultaneously. A second dose of vaccine is
administered four weeks later. The serum confers
passive protection during the time that active
immunity is developing. Since the main effect of
the serum passes off in 3-4 weeks and active
immunity is not established for 5-6 weeks, there
may be a short intermediate period of relative
susceptibility, but it has been found in practice
that, provided the children are protected torn
infection by temporary segregation of carriers,
there is very little risk of diphtheria breaking out
again. An outbreak can thus be brought to an
abrupt end.