Depleting EP - II antibodies from double - positive serum samples increased 50 % inhibitory dose ( ID 50 ) neutralizing antibody titers ( up to 4 . 9 - fold ) in up to 72 % of samples ( P £ 0 . 0005 ) , contrasting with ID 50 neutralization titer increases in 2 of 70 double - negative samples ( 2 . 9 % ; P > 0 . 5 ) .