Neutralizing-antibody-combining (NAC) test for measuring antigenic potency of poliomyelitis vaccine.

An in vitro test for the quantitative determination of the antigenic potency of poliomyelitis vaccine is described. This test-termed the neutralizing-antibody-combining (NAC) test-is based on the measurement of the capacity of an inactivated virus in tissue culture to bind neutralizing antibody. Details of the experimental design and technique are given, the authors stressing the importance of using small and precisely known dosages of virus and antiserum and of incubating the test mixtures for a sufficiently long period. In preparing the test mixtures, it is essential to incubate first the vaccine + antiserum, and then add the virus. No vaccine activity will be demonstrable if the test is set in the reverse order.EXPERIMENTS CARRIED OUT WITH THE NAC TEST HAVE SHOWN THAT: (a) the test is type-specific; (b) the antigen-antibody union in the system is irreversible; (c) vaccines containing preservative yield the same potency titres as untreated vaccines; (d) the test gives reproducible results; (e) the results of the NAC test did not always agree with those obtained in animal potency tests, probably because of vagaries in the latter; (f) dried vaccines retain their complement-fixing activity, but lose their power to combine with neutralizing antibody in vitro and to produce neutralizing antibodies in vivo-a finding which suggests that the NAC test measures in vitro the same antigen responsible for neutralizing-antibody production in vivo.