Table 5. Anti-OPS IgG2a Titers in the Sera of the Four Groups of Mice 2 Weeks After the First, Second, and Third Vaccinations^a.

Group	Titer of Sera (OD Unit) (Mean ± SD)
	2 Weeks After the First Vaccination	2 Weeks After the Second Vaccination	2 Weeks After the Third Vaccination
OPS-vaccinated	18 *	39 *	56 *
OPS-DT-vaccinated	33 **	63 **	100/2 **
DT-vaccinated (control I)	0	0	0
Neg. control (control II)	0	0	0

^a* and ** denotes a significant difference in comparison to the control groups (P < 0.001).