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Repeated Vaccination of Man Against the Virus of Equine Encephalomyelitis¹

Department of Surgery, Duke University School of Medicine, Durham, N. C.

Abstract

The content of neutralizing antibodies has been studied in the serum of man following primary vaccination against the Eastern and Western strains of virus of equine encephalomyelitis. The titer of antibodies effective against the Eastern strain in most instances diminishes so rapidly that within 6 to 9 months the serum possesses little or no more immunizing capacity than that taken before vaccination. In no instance was the titer maintained for this period at the level observed 2 weeks following initial vaccination. In contrast, antibodies for the Western strain, though diminishing markedly in a few instances, were maintained in general at a relatively high level for the same period. One instance of four was found in which antibodies induced by subclinical infection before initial vaccination and augmented by the primary vaccination were lost at about the same rate as those produced in normal individuals by vaccination alone.

Response to revaccination with a single dose resulted in every case in the induction of antibodies such that serum taken after 7 days neutralized 100,000 to 1,000,000 infectious units of virus of either strain. This reaction with respect to the Eastern strain was much more rapid than that following the first vaccination. The degree of response to both antigens was markedly greater than that seen at the initial vaccination.

General reactions to revaccination were more marked than to first vaccination, though still relatively mild.

The findings constitute evidence of the transient nature of at least one immunological factor in resistance to the virus. This finding in connection with results of direct investigation on the duration of immunity in vaccinated animals indicates the advisability of a second course of vaccination within one year under conditions of continued exposure to the virus. The height of the response to this second vaccination indicates the possibility that the interval between the second and subsequent vaccinations may safely be much longer than that between the first and second.

Footnotes

¹ This work was aided by a grant from Lederle Laboratories, Pearl River, N. Y. and by the Dorothy Beard Research Fund.