A live respiratory syncytial virus (RSV) candidate vaccine, LIDΔM2-2, is attenuated by deletion of the RSV RNA regulatory protein M2-2, which results in upregulated viral gene transcription and antigen expression but reduced RNA replication. The authors report a study in which 20 RSV-seronegative children aged 6-24 months were given a single intranasal dose of 105 plaque forming units (PFU) of LIDΔM2-2 and compared to nine who received placebo. The authors found that vaccine virus was shed by 95% of vaccine recipients (median peak titres 3.8 log10 PFU/mL by quantitative culture and 6.3 log10 copies/mL by PCR) and 90% had ≥4-fold rise in serum neutralising antibodies. Respiratory symptoms and fever were common in both vaccine (95%) and placebo (78%) recipients. One vaccine recipient had grade 2 rhonchi associated with vaccine shedding, rhinovirus and enterovirus. Eight out of 19 vaccine recipients versus two of nine placebo recipients had substantially increased RSV antibody titres following the RSV season without medically-attended RSV disease, suggesting anamnestic vaccine responses to wild-type RSV. Therefore LIDΔM2-2 appears to be both infectious and immunogenic.
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