Identification of a recombinant live attenuated respiratory syncytial virus vaccine candidate that is highly attenuated in infants

BACKGROUND: Recombination technology can be used to create live attenuated respiratory syncytial virus (RSV) vaccines that contain combinations of known attenuating mutations. METHODS: Two live attenuated, recombinantly derived RSV vaccine candidates, rA2cp248/404 Delta SH and rA2cp248/404/1030 Delta SH, were evaluated in 31 adults and in 95 children >/=6 months old. rA2cp248/404/1030 Delta SH was subsequently evaluated in 44 infants 1-2 months old. These vaccine candidates share 4 attenuating genetic elements and differ only in a missense mutation (1030) in the polymerase gene. RESULTS: Both vaccines were highly attenuated in adults and RSV-seropositive children and were well tolerated and immunogenic in RSV-seronegative children. Compared with that of rA2cp248/404 Delta SH, replication of rA2cp248/404/1030 Delta SH was restricted in RSV-seronegative children (mean peak titer, 10(4.3) vs. 10(2.5) plaque-forming units [pfu]/mL), indicating that the 1030 mutation had a potent attenuating effect. Although rA2cp248/404/1030 Delta SH was well tolerated in infants, only 44% of infants who received two 10(5.3)-pfu doses of vaccine had detectable antibody responses. However, replication after administration of the second dose was highly restricted, indicating that protective immunity was induced. At least 4 of 5 attenuating genetic elements were retained in recovered vaccine viruses. CONCLUSIONS: rA2cp248/404/1030 Delta SH is the first RSV vaccine candidate to be sufficiently attenuated in young infants. Additional studies are needed to determine whether rA2cp248/404/1030 Delta SH can induce protective immunity against wild-type RSV.