Current situation of intranasal attenuated vaccines against respiratory syncytial virus in the child population

The respiratory syncytial virus (RSV) is the cause of acute respiratory pathologies (bronchiolitis and pneumonia), which occur preferably in the paediatric population in an epidemic manner. The only way to prevent these infections is through prior vaccination >6 months. Of the different existing vaccines, attenuated vaccines, defined as those that contain an altered or modified virus to minimise its pathogenic capacity while maintaining its immunological response capacity, seem to represent an important advance. In recent years, 2 vaccine candidates based on the M2-2 gene deletion have been evaluated, those designated MEDI/ΔM2 and LID/ΔM2-2/1030s. In the study carried out in 21 seronegative children (6–24 months), they received an intranasal dose (105 PFU) of the new vaccine compared to a placebo group. 85% of those vaccinated developed a neutralising antibody titre >4 times the previous one. The combination of the Δ1313 deletion with the NS2 deletion was shown to be better for the development of an attenuated RSV recombinant vaccine candidate (ΔNS2/Δ1313), with good protective results. A new vaccine candidate designated RSV/6120/ΔNS2/1030s was developed that is identical to the previous virus RSV/ΔNS2/Δ1313/I1314L but with additional mutations. Finally, a VRS with a single replicative cycle has been developed by eliminating the gene encoding the matrix (M) protein (VRS-M-null). Most of these vaccines are still in phase 2/3 and very good results are expected.