BOOKS OF RtfiDIfGS - PAHO/WHO

- 95 -
R. F'RERICItS'AND J. PRAWDA
data, of what each vaccination campaign would cost if the experiment were done in
the rcal world.
Rather than presenting frequency distributions derived froni multiple epidemic
trials per experiment, we will deal only with mean values. The policy of no vaccination
and the policy of 70 percent initial vaccination were the only ones which
consisited of 100 epideic trials per experiment. The results from all other tested
policies are based on 10 epidemic trials per experiment.
3a. A Single 70 Percent Vaccination Catmipaigtn
Looking only at the niean nuiiber of ca;nilti rabies cases per year, Figure 3
indicates that the policy of 70 percent initial vaccination was able to reduce the yearly
incidence of rabies for approximately five years. Thereafter, the yearly incidence of
rabies was greater than ii vwe had followed the policy of doing nothing at all (no
vaccination). With rabies absent froni a barrio. the dcnsity of susceptible dogs and the
concurrent likelihood of a rabies outbreak increased. All that was required was for an
infectious rabid dog to wander by chance into the barrio and trigger a new epidemic.
This same plhenomenon of a disease being kept out of an area and then suddJenly
appearing in an epidemic fornm lhas bcen frequently tobserved iii the real world with
rabies as well as with other infectious diseases [161, [24].
Comparing after ten years the cumulative mean nuniber of rabid dogs in the
absence of any control policy and the value with an initial 70 percent vaccination
campaign, we see in Table I that thc immunization program prevented 2.985 rabid
dogs (21.588 minus 18.603). The theoretical vaccination campaign, however. took 85
daily iterations to complete and required the operator to assign 48 vaccinators
traveling in 8 trucks to all barrios of the city. During tcse three months. 59.323 dogs
were vaccinated. Taking into accounti the cost of wages. the use of the trucks. ihe
4000 - 70% ni/h/v. va
40001
/70% initol'tl iac.
o 3000- 70% revac. 5J
No vaccinat/on
2000- 1
%I0 0 ,Preferred ¡
vac.
',~:~ _ .'opo/, y(V4-va-7O YJ
I 2 3 4 5 6 7 8 9 10
Year
FiLGUR 3. Compuier Simulation Experiments: Comparison of the mean number of rabid dogs per yecar
,t.ri;nc a ten-year period with policies of (I) no vaccination (mean, 100 epidemic triais). (2) initial 70 percent
vacciration (mean, 100 epidemic trials), (3) two 70 percent vaccination campaigns. one a tihe beginning and
the other after 5 years (mean, 10 epidemnic trials), and (4) the Preferred Vaccination Pulicy with VA sei al 70
percent (mean, 10 epide.a.: irials).