LIVE POLIO IRUS VACCINES

Use of Attenuated Live Poliovirus Vaccine in Cuban Children
369
APPENDIX
THE SERONEGATIVE INDEX: A METHOD OF EXPRESSING
POLIOVIRUS IMMUNE STATUS
Observed total of type specific seronegatives
Number in population sample X3
X 100 = Seronegative Index
Theoretically, each individual is a potential susceptibles. Conversion rates, on the other hand,
victim for paralytic attack by each of the three do n( nt by themselves reveal the relationship of
types of poliovirus. Hence, the maximum num- the seronegatives to the immune status of the
ber of chances of paralytic disease in a popula- comirtunity as a whole.
tion is three times the number of persons in the Th4e seronegative index affords a simple means
group. The actual number of seronegatives ob- of c< omparing populations, age or economic
served in the population sample is a measure of groupis and methods of immunization with one
the extent to which the maximum hazard has another. It may also prove useful in determining
been reduced by natural exposure and infection. when and where to initiate immunization meas-
Thus, in dividing the observed number of type- ures. For example, in Dr. Cox's report 3 on 933
specific seronegatives by the theoretical maximum Leder -le employees who received the trivalent
and multiplying the result by 100, one obtains the vacciiie, there were 742 type-specific seronegain
the pre- and 88 in the post-vaccination
per cent of the theoretical number of paralytic tives
chances which still remain in a community that specir
mens. The pre- and post-vaccivnation inbased
on these data are 26.5 and 3.14.
has been adequately sampled. A convenient dices
term for the figure thus obtained is the seronega-
Thesee indices may be compared with those based
tive index.
Epidemiologically, such an index conveys
on th
more
ie pre-vaccination serologic data reported
information about the population under investi- from
Medellín, Colombia in 1959 by Dr. Abad
gation than the geometric mean serologic titer Gómeez and his co-workers 4 at the First Interna-
which varies with antibody levels, but does not Conference on Live Poliovirus Vaccines.
tionall
indicate concisely the changes in the number of The d[ata and indices are tabulated by age below.
PRE-VACCINATION
Poliovirus Seronegative Indices by Age in
Inter-epidemic Medellín, Colombia, 1959
AGE IN
YEARS
NUMBER
PERSONS
SERONEGATIVES
1 2
TOTAL
3 NEGATIVES
SERONEGATIVE
INDEX