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group—the test group—were inoculated with the pertussis
vaccine under trial, while those in the other group—the control
group—were inoculated with an ' 'anti-catarrhal'' vaccine
containing no H. pertussis. After inoculation the children in
both groups were kept under close observation for a period of at
least two years, by monthly home visits made by specially
appointed nurse-visitors. Those children who came into
known contact with whooping cough or who developed symptoms
which might be those of early whooping cough, received special
visits in order that a bacteriological diagnosis might be made.
As it was essential that observers should not be influenced in
their clinical assessment of the disease, efforts were made to
ensure that neither the doctor, the nurse-visitor nor the parents
should know whether the child under observation was in the
test or the control group.
Seven of the early trials comprising a total of 4,691 test and
control children have now been in progress for over two years,
and from a preliminary analysis of the results the following
general conclusions may be drawn:—
(a) In each trial the random allocation to test and control
produced two equivalent groups of children, comparable in age,
sex, duration of breast-feeding, size of family and incidence of
infections other than whooping cough.
(b) In six of the trials the incidence and severity of
whooping cough in the test groups were appreciably less than
in the control groups. In the seventh trial the results were
only slightly more favourable in the test group.
(c) There was considerable variation in the prophylactic
potency of vaccines from different sources, and there was
probably also some variation in the potency of different batches
of vaccine from the same source.
(d) One particular type of vaccine, not at present readily
available, was found to give decidedly better protection against
the disease than any of the others tested.
The reason for variations in the prophylactic potency of
these vaccines is not clear, nor is there yet available a conclusive
laboratory method of assay by which this property of a vaccine
can be accurately assessed, but an investigation is in progress
in which the prophylactic value in children of a number of