The attenuated Chinese equine infectious anemia virus (EIAV) vaccine is the first lentiviral vaccine that provides solid protective immunities to vaccinated horses. To investigate properties of EIAV vaccine, especially the relationship between its replication and the immunity, viral plasma loads of an EIAV vaccine strain EIAV_FDDV in immune suppressed horses were detected. Three horses, which were immunized with EIAV_FDDV for 16 months, were treated with dexamethasone for 14 days to suppress their immunities. Reduced immune response in these animals was confirmed by significantly declined lymphocyte proliferation rate detected after 10 days of the drug treatment. The plasma viral loads of EIAV_FDDV, which was indicated by the genomic RNA copy numbers, in horses before and after the treatment of dexamethasone were monitored by real time RT-PCR. Results revealed that the viral plasma loads in two of three immune-suppressed horses were kept a steady low level around 10^{3～104 copies/ml. The load was increased by 10 folds in the third horse, but was still among the standard levels for EIAV_FDDV vaccinated horses. As a positive control, the viral copy number of an asymptomatic carrier of EIAV virulent strain EIAV_Liao was jumped nearly 25 000-fold higher after being treated with dexamethasone. The typical clinical symptoms of EIA, characterized by febrile episodes and thrombocytopenia, were also appeared in this horse. These results clearly indicate that it is the unique biological feature of the attenuated EIAV vaccine, but not the immunity, resulted in EIAV_FDDV remaining in low levels in the body harmlessly. In addition, the steady low level of viremia and the inability to cause clinical symptoms of EIAV_FDDV in immune-suppressed hosts further demonstrated the safety of attenuated Chinese EIAV vaccines. The data provide a new sight for studies on the immunity to lentiviruses.}