The epidemiological and etiological importance of respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) in lower respiratory tract infections, both in children and adults, dictates the need for a combined vaccine that protects against both viruses. Both viruses have a single-stranded RNA genome of 15.2 kb for RSV and 13.3 kb for hMPV. Serotypes or subgroups that cocirculate each season have been described for both viruses. Of the different proteins contained in both viruses, the F proteins have a high degree of similarity in structure and conformation, sharing between 30 and 35% of their sequence; therefore, they have been used to develop combined vaccines. Attenuated combined vaccines use a strain of hMPV deleted in the SH gene, which is replaced by the RSV F protein gene. This strain replicated in cell cultures and mice allowing for the initiation of a trial in mice. The humoral and cellular immunity induced by this vaccine was heterologous and encompassed different subtypes of both viruses, and also protected animals from exogenous infection. Preliminary data have been reported on a combined mRNA vaccine with the F protein in its preF form. The results showed that both vaccines increased neutralizing antibody titers against RSV-A and RSV-B, and hMPV in children aged 5–8 months and 8–23 months who were both seronegative and previously seronegative against this virus, meaning that the immune response is independent of the prior presence of antibodies. A chimeric vaccine consisting of the globular head of the RSV F protein and the stem of hMPV is also in the very preliminary phase. Definitive data are not yet available, yet it is quite possible that a combined vaccine that could prevent RSV and hMPV infections will be available in the near future.
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