I guess it is like yellow fever vaccine where one shot of attenuated yellow fever virus. A great amout of work has been done to investigate how yellow fever vaccine elicits complete protection with one immunization. Your case might be similar to this. YFV elicits long lasting nuetralizing antibodies which ciruclate in the body and tissue residents. It also elicit CTLs (cytotoxic T cells) which are active effector cells with PD-1 controlled expression and long lasting memory CTLs whice resident in various tissues.

I think that colleagues more experienced in basic immunology may complete my answer that aims to be as simple as possible. It is considered that, while antibodies may be also involved,  cell immune response plays a major role in the protection induced by strain 19 in cattle but also by strain Rev.1 in small ruminants. It is also considered that in most vaccinated animals these live attenuated vaccine strains persist in lymphoid organs, i.e. not only (and not necessarily) the spleen but also lymph nodes. This long persistence would be sufficient to maintain a certain lmevel of immunity. For detailed information see the attached publication-link.

http://www.veterinaryresearch.org/content/43/1/29