A live, cold-passaged (cp) candidate vaccine virus, designated respiratory syncytial virus (RSV) B1 cp-52͞ 2B5 (cp-52), replicated efficiently in Vero cells, but was found to be overattenuated for RSV-seronegative infants and children. Sequence analysis of reverse-transcription-PCRamplified fragments of this mutant revealed a large deletion spanning most of the coding sequences for the small hydrophobic (SH) and attachment (G) proteins. Northern blot analysis of cp-52 detected multiple unique read-through mRNAs containing SH and G sequences, consistent with a deletion mutation spanning the SH:G gene junction. Immunological studies confirmed that an intact G glycoprotein was not produced by the cp-52 virus. Nonetheless, cp-52 was infectious and replicated to high titer in tissue culture despite the absence of the viral surface SH and G glycoproteins. Thus, our characterization of this negative-strand RNA virus identified a novel replication-competent deletion mutant lacking two of its three surface glycoproteins. The requirement of SH and G for efficient replication in vivo suggests that selective deletion of one or both of these RSV genes may provide an alternative or additive strategy for developing an optimally attenuated vaccine candidate.Respiratory syncytial virus (RSV), the leading cause of severe viral respiratory illness in pediatric populations throughout the world (reviewed in ref. 1), accounts for approximately 90,000 hospitalizations in infants and children in the United States each year (2). The importance of RSV as a respiratory pathogen makes development of a safe and effective RSV vaccine a public health priority (3). Although a number of approaches to RSV vaccine development have been taken, live RSV vaccines may provide the best alternative for immunizing young infants, because a live vaccine would mimic natural infection, induce a balanced cellular and humoral immune response, and be unlikely to produce enhanced disease (4).RSV exists as two antigenically distinct subgroups, A and B, and both RSV A and RSV B infections are capable of inducing severe lower respiratory tract disease (5-7). For this reason, a bivalent live RSV vaccine containing attenuated RSV A and RSV B components would be most desirable. Recently, a live attenuated RSV A candidate vaccine has been identified that appears to be safe and immunogenic in infants and children over 6 months of age (8). In addition, a cold-passaged (cp) RSV B candidate vaccine, designated RSV B1 cp-52͞2B5 (cp-52), was derived by passage of the RSV B1 wild-type (wt) virus 52 times at low temperature (21-32°C) (9). Cp-52 was shown to be restricted in replication in vivo but still able to induce RSV serum-neutralizing antibody responses in cotton rats, African green monkeys, and chimpanzees (9). Also, it was found to be phenotypically stable after prolonged replication in cotton rats (9). Here, we describe the phase I evaluation of the cp-52 candidate vaccine in adults, children, and infants. Although this virus mutant grew to high titer (Ͼ10 7....

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.

A live-attenuated, intranasal respiratory syncytial virus (RSV) candidate vaccine, cpts-248/404, was tested in phase 1 trials in 114 children, including 37 1-2-month-old infants-a target age for RSV vaccines. The cpts-248/404 vaccine was infectious at 104 and 105 plaque-forming units in RSV-naive children and was broadly immunogenic in children >6 months old. Serum and nasal antibody responses in 1-2 month olds were restricted to IgA, had a dominant response to RSV G protein, and had no increase in neutralizing activity. Nevertheless, there was restricted virus shedding on challenge with a second vaccine dose and preliminary evidence for protection from symptomatic disease on natural reexposure. The cpts-248/404 vaccine candidate did not cause fever or lower respiratory tract illness. In the youngest infants, however, cpts-248/404 was unacceptable because of upper respiratory tract congestion associated with peak virus recovery. A live attenuated RSV vaccine for the youngest infant will use cpts-248/404 modified by additional attenuating mutations.