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Background In people over 65, there are on average 177,000 hospitalizations and 14,000 deaths because of respiratory syncytial virus (RSV) each year. Elderly patients infected with RSV can suffer serious infections leading to pneumonia and congestive heart failure. RSV vaccines have failed in the elderly in part because they have been unable to mount a robust cellular immune response. Methods RSV-MinL4.0 is a live-attenuated intranasal vaccine candidate that was generated by codon pair deoptimization of the L gene followed by the addition of four stabilizing mutations found via stress passaging. Four African Green Monkeys (AGMs) per group were vaccinated with RSV-MinL4.0 or wild-type (WT) RSV at 2 × 106 PFU, boosted on day 28 and challenged with wild-type (WT) RSV on day 104. Oropharyngeal swabs and tracheal lavage were collected daily and every other day, respectively, to evaluate virus shedding (qPCR) and blood was drawn on days 1, 14, 21, 28, and 49 for antibody titers (PRNT50), and PBMC activation (IFNγ ELISPOT with whole inactivated virus). Results MinL4.0 was 2 to 3 log10 attenuated when compared with WT RSV in AGMs. Despite the presence of antibodies on day 28, there was a “take” of the boost indicating the potential for this vaccine to be immunogenic in the elderly with pre-existing circulating antibodies (Figure 1A). MinL4.0 led to robust activation of PBMCs comparable to WT RSV (> 2,000 spots per 106 total cells, Figure 1B). Shedding of the vaccine and challenge viruses was minimal (data not shown). Conclusion MinL4.0 led to robust activation of cellular and humoral immune responses, which are critical for induction of protective immunity in the elderly. Animals were protected from WT challenge. Preliminary data in AGMs with pre-existing antibodies to RSV indicate that circulating antibodies do not prevent vaccine “take,” critical for a vaccine targeting sero-positive elderly individuals. Disclosures All authors: No reported disclosures.

2777. Live-Attenuated Vaccine Against RSV Generates Robust Cellular and Humoral Immune Responses