SIMIAN virus 40 (SV40) is a natural infection of rhesus and some other Asiatic macaques1 and has no known natural host in the United States. It is therefore difficult to account for our finding of antibodies neutralizing SV40 in the sera of some elderly persons in the United States who do not have a history of immunization with any vaccine prepared in rhesus kidney cultures. The only recognized exposure of the US population to this virus occurred when poliovirus and adenovirus vaccines prepared in rhesus kidney cultures and administered to a large number of people were later found to have been contaminated with SV40 (ref. 1). The chief exposure occurred between 1955 and 1961 through the use of Salk vaccine, and it is known that subcutaneous administration of SV40 gives a long-lasting antibody response2.
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