Three cytopathic strains of subacute sclerosing panencephalitis (SSPE) virus were isolated from brain biopsies of three patients. These strains were isolated and maintained by cocultivation of infected brain cells with fresh Vero cells. The biological characteristics of two strains were studied. It was found that these strains remain cell-associated after repeated cocultivations with Vero cells and produce plaques under fluid medium or traga-canth overlay. The correlation with measles virus was demonstrated by the plaque reduction test as well as by the immunofluorescence test. Large numbers of nucleocapsids were observed in the cytoplasm of infected cells but none in nuclei. Intracerebral inoculation of monkeys, adult guinea pigs, newborn and adult hamsters or mice was followed by acute encephalitis and death.

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The results of immunization of 523 children from 9 months to over 5 years of age with two attenuated measles vaccines, ;Denken' and ;Biken', are described. The clinical reactions following vaccination with both vaccines were mild, but Denken vaccine produced more rashes than the Biken vaccine. The serological conversion was satisfactory for both vaccines but a higher titre of neutralizing and haemagglutination-inhibiting antibodies was found in children immunized with Denken vaccine. The incidence of severe reactions or complications was negligible in this trial. The immunity to diphtheria or tetanus was not altered by vaccination with live measles vaccine.

After encouraging results of the mass vaccination programme in Iran, in which 5 million children in rural areas were vaccinated with the Japanese Sugiyama strain at its 82nd passage in baby calf kidney, and a progressive decrease in the incidence of measles as well as a reduction of excessive infant mortality, a further attenuated vaccine, produced with the same strain, cloned in Japan, was compared in a field trial with the parent vaccine. The new strain caused fewer reactions than the original strain. Seroconversion with a geometric mean antibody titre of 6.1 was observed in 95% of susceptible children.

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