One hundred and ninety-two lots of aqueous-base (serum-free) yellow fever vaccine were tested in 200 rhesus monkeys (1) to evaluate the behavior characteristics of the vaccine virus in producing clinical and pathological signs and symptoms, (2) to determine the ability of the vaccine virus to multiply and appear in the blood stream, and (3) to test the power of the vaccine to stimulate specific yellow fever immunity. The monkeys constitute a strictly consecutive series. All lots of vaccine employed likewise were of a consecutive series broken only by the absence of 1 lot which was not tested in a monkey because of possible bacterial contamination.
Each monkey was inoculated intracerebrally with 0.25 ml. of yellow fever vaccine from a single homogeneous lot. The inoculums contained from 200 to 2,230,000 minimum lethal mouse doses (m.l.d.) of virus each. The animals were observed for a median period of 30 days subsequent to inoculation. For comparative purposes the 200 monkeys were divided into two groups, viz., a “low virus” group of 102 animals, each of which received 200 to 199,000 m.l.d. and a “high virus” group of 98, each of which received 202,000 to 2,230,000 m.l.d.
No monkey died of trauma or from intercurrent disease.
Each monkey was bled on the 2nd, 3rd, and 4th postinoculation days to determine if vaccine virus was present in the circulation. Evidence of its presence was found in 192 animals (96 per cent). Results indicated that during these days more virus was present in the blood of the “high virus” group of monkeys than in the “low virus” group.
Daily rectal temperature determinations were taken throughout the postinoculation observation period. A temperature of 40°C. or higher was regarded as fever. One hundred and sixty-seven of the monkeys (83½ per cent) showed fever. The very great majority of febrile temperatures occurred on the 8th, 9th, and 10th days subsequent to the injection of vaccine. Median duration of fever was 2 days.
The pre- and postinoculation yellow fever immunity status of 199 of the 200 monkeys was studied by the protection test. All 199 animals proved to be nonimmune immediately prior to vaccination and all proved to be immune 12 to 33 days later.
Of the 102 animals in the “low virus” group, each of which received 200 to 199,000 m.l.d. of virus, 18 (17.6 per cent) showed transient weakness with complete recovery and 4 (3.9 per cent) developed signs of severe encephalitis characterized by extreme weakness and/or paralysis. Of these 4, 2 were sacrificed, 1 recovered, and 1 was left with a residual paralysis of the right hind leg. Of the 98 monkeys in the “high virus” group, each of which received 202,000 to 2,230,000 m.l.d. of virus, 24 (24.5 per cent) showed transient weakness with complete recovery and 8 (8.2 per cent) developed signs of severe encephalitis. One of these 8 died and the other 7 were sacrificed. Histopathological study of the brains of the 10 animals coming to autopsy showed encephalitis in each instance.
The 2 most important factors in producing encephalitis in these monkeys are believed to be the inherent susceptibility of the individual animal and the amount of virus injected.
Twenty-two of the monkeys studied had been previously used in studies of poliomyelitis, mumps, measles, or Rocky Mountain spotted fever while 178 had not been previously subjected to experimentation. No significant differences in reaction were observed between the 2 groups.
Of the 192 vaccines studied, 174 were released for human vaccination, principally to the United States Army. No icterogenic or other untoward reactions indicating faulty vaccine have been encountered.