Messenger ribonucleic acid (mRNA) as a vaccine platform carries genetic information exclusively for specific proteins, providing a higher safety grade due to the low probability of interference and modification of the genetic material of the host ^[1][74]. However, the likelihood of providing repeated doses to maintain immunity raises safety and efficiency issues regarding lipid aggregation ^[2][3][75,76]. Previous research in the mRNA technology field has significantly contributed to the rapid manufacturing process of the vaccines against SARS-CoV-2 ^[4][77]. The contribution of mRNA technology can be crucial to the development of vaccines in outbreaks of infectious diseases as well as in cancer treatments ^[5][6][78,79].

mRNA-1345 is a vaccine candidate currently in Phase III of development, manufactured by Moderna as an improved candidate of Merck’s mRNA-1777 vaccine, aiming to achieve a stabilized prefusion form of the F (preF) respiratory syncytial virus (RSV) protein ^[7][20]. The interim results of a Phase I trial (NCT04528719) of the candidate showed acceptable safety characteristics in younger adults and raised neutralizing antibody titers against both strains of RSV ^[8][9][27,28]. Women of child-bearing potential and children aged between 12 and 59 months were also tested in this researchtudy. ConquerRSV (NCT05127434) is an ongoing Phase II/III vaccine study in adults above 60 years of age. In February 2022, the company proceeded to the Phase III part after the preliminary review of the Phase II data, showing a vaccine efficacy of 82.4% for RSV-LRTD defined with ≥3 symptoms and 83.7% for RSV-LRTD defined with ≥2 symptoms with adequate safety analysis ^[10][80]. Supported by these results, the market approval of mRNA-1345 was requested by the company in July 2023 and addressed to EMA, Swissmedic, and Therapeutic Goods Administration (TGA) as a preventive medicine against LRTD and acute respiratory disease (ARD) related to RSV in an adult population ≥ 60 years of age. A Biologics License Application (BLA) to the FDA is also running ^[11][81]. ROSE (NCT05572658) is an additional prospective study based on data from the ConquerRSV aiming to assess the impact on the healthcare system and the economy. Another ongoing Phase III trial (NCT05330975) named RSVictory is testing the co-administration of mRNA-1345 with a seasonal quadrivalent influenza vaccine (Afluria Quadrivalent) or mRNA-1273.214 (against SARS-CoV-2).

Sanofi recently initiated a Phase I/II clinical trial (NCT05639894) in adults of a new mRNA vaccine candidate delivered via one of two different types of lipid nanoparticles (LNPs), named LNP CL-0059 and LNP CL-0137, which is expected to end on April 2025.

LAVs constitute an important milestone in the history of medicine. Edward Jenner’s observation regarding the smallpox field provided the impetus for the development of vaccination, and at present, a broad spectrum of LAVs is in clinical Phase IV of development ^[12][82]. The deletion of genetic information for specific proteins results in attenuated viruses or microbes promoting essential replication procedures within a living organism without being virulent. It has been proven that they induce higher efficacy rates regarding the immune responses relative to the other vaccine types without the obligatory addition of adjuvants by inducing an immune response similar to the response in the case of natural infection with the pathogen ^[13][83]. The vaccination scheme includes a single dose providing long-term immunity ^[14][84]. LAVs are generally avoided in immunocompromised people and pregnant women due to severe infection or congenital transmission risks, respectively ^[15][85]. Intranasally administered LAV RSV vaccines have not shown, to date, a correlation with enhanced RSV disease and can result in acceptable immunity induction, including mucosal immunity. The current vaccine studies show effective immunogenicity in RSV-naïve infants, although late-stage trials are still needed ^[16][86]. Reverse genetic methods have led to the ΔM2-2 deletion and ΔNS2 deletion of the genetic material of RSV. These modifications inactivate the viral replication and optimize the induction of innate immunity of the host, respectively. Chimeric vaccines include attenuated viruses of a related pathogen modified to express the specific genes of the virus of interest. Two chimeric vaccines that undergo development regarding RSV are the rBCG-N-hRSV and the SeV/RSV vaccine candidates ^[17][18][19][87,88,89].

A parainfluenza virus type 5 (PIV5) encoding the full-length RSV F protein is used to deliver the antigen through the nasal route in the BLB-201 vaccine candidate. BLB-201 was developed by Blue Lake Biotechnology and intends to prevent viral infection in the older adult population and young children under 2 years of age; it is currently in Phase II of development. The positive interim data of a Phase I trial (NCT05281263) shared by the company indicated a rise in antibody responses in 64% of the participants and a low replication rate without major safety concerns in adult participants ^[20][56].

CodaVax-RSV by Codagenix is based on hundreds of modifications in the genome that produce an attenuated, completely functional in terms of antigen production virus that is not virulent. The data from preclinical studies indicate that the virus was significantly less infectious than the wild-type of RSV because the process of new virion production in the host is decelerated and, at the same time, the induction of cellular and humoral immunity that provides protection after an RSV challenge is achieved ^[21][57]. A Phase I study (NCT04295070) was completed and reached the primary endpoint regarding the safety profile and induced cellular immunity, according to an announcement from the company. In addition, another Phase I study is planned to be completed in May 2024 ^[21][22][57,58].

In this Intravacc’s candidate, the genetic information for the G protein is deleted from the virus, as it is hypothesized that RSV virions without the G protein can still activate immunity while being inactive. Cotton rat models showed the safety and immunogenicity characteristics of the modified virus without the G protein through intranasal administration ^[23][90]. The results of the Phase I trial did not show a significant induction of immunity in the seropositive population that was tested, and as a result, a dose-escalation trial with seropositive and seronegative children is the next step of the company ^[24][59].

The Mycobacterium bovis bacillus Calmette–Guérin (BCG) is in clinical practice and has been proven effective in the induction of immunity. These characteristics, in combination with the fact that BCG can be safely administered in young children and infants, make BCG a suitable candidate for use as a recombinant vaccine vector. rBCG-N-hRSV, a candidate developed from Pontificia Universidad Catolica de Chile, consists of the live-attenuated BCG vaccine modified to express the N protein of RSV. The preclinical studies of recombinant BCG carrying genetic information for the N or M2 proteins of RSV in mice have indicated high effectivity rates ^[25][26][91,92]. Furthermore, this vaccine formulation was safe and capable of inducing protective immunity in cattle as a newborn animal model. Vaccination with rBCG-N-hRSV triggered a humoral and cellular response that protected cattle against bovine RSV, which is a natural pathogen for these animals ^[27][93]. The first human Phase I clinical trial (NCT03213405) revealed a safe candidate with no virulence evidence and increasing trends in IgG antibody values against both N-RSV and BCG (anti-PPD), with suggestions of further testing of the candidate in children and elderly ^[28][60]. To date, rBCG-N-hRSV is the only formulation that could be considered for use in newborns, particularly in countries where BCG is currently used as a vaccine for tuberculosis.

SeVRSV, supported by the National Institute of Allergy and Infectious Disease, expresses genetic information for the production of the F Protein from RSV and immunizes infants against human PIV-1 and RSV based on the Sendai virus, which is a parainfluenza virus-type 1 (PIV-1) that is not virulent in humans ^[29][30][61,94]. Despite the low levels of induced immune response in a group of seropositive adults tested in a Phase I clinical trial (NCT03473002), the safety characteristics with mild or moderate AEs suggest the further evaluation of the vaccine in seronegative children ^[29][61].

MV-012-968, a candidate of Meissa vaccines, is a LAV produced with the codon deoptimization of the NS1, NS2, and G genes and SH gene remotion, resulting in modified unusual codons and leading to incompetent translation ^[31][32][62,95]. Therefore, a high attenuation rate can be achieved ^[31][32][62,95]. An open-label Phase I trial (NCT04227210) in adults confirmed the attenuation of the vaccine and mucosal RSV-specific immunity induction. However, RSV-specific preF antibody titers in serum were not increased due to the initial seropositivity. An increasing trend was observed in RSV-specific mucosal IgA titers ^[31][33][62,63]. Seropositive children tested in a Phase Ib trial (NCT04444284) experienced specific mucosal immunity induction with a good safety profile and no signs of viral replication ^[34][64]. The interim data of a Phase I trial (NCT04909021) including seronegative children confirmed its safety and indicated the promotion of neutralizing antibody immunity in 78% of the study group population with the rate increasing to 89%, including the mucosal response ^[35][65]. The Phase II trial (NCT04690335) of the vaccine has been completed with no available results.

Sanofi Pasteur recently completed a Phase I/II trial (NCT04491877) study of VAD00001, a LAV, in children without published results. No further information in terms of the attenuation method has been provided.

The attenuation technique of this candidate includes a combination of the deletion of the NS2 gene from the genome, which is an RSV IFN antagonist gene, the deletion of Δ1313 codon in the L gene, and a stabilizing modification at codon 1314 ^[36][96]. The National Institute of Allergy and Infectious Diseases (NIAID) supports the development of RSV ΔNS2/Δ1313/I1314L, RSV LID/ΔM2–2/1030s, RSV 6120/ΔNS2/1030s, and RSV 6120/F1/G2/ΔNS1/RSV 6120/ΔNS1. A Phase I clinical trial showed an attenuated yet highly infectious candidate ^[37][66]. RSV ΔNS2/Δ1313/I1314L was tested together with RSV 276, another vaccine candidate, in a Phase I trial (NCT03227029) in children. In general, the immunogenicity parameters for both vaccines showed great infectivity, even though RSV ΔNS2/Δ1313/I1314L showed lower rates than anticipated, possibly due to previous RSV exposure. Regarding tolerability, the only concern arose with RSV/276-associated cough cases. RSV ΔNS2/Δ1313/I1314L will be assessed in other clinical trials ^[38][67]. A Phase I/II study (NCT03916185) is ongoing at present.

This candidate is based on an attenuation technique that deletes the M2-2 protein. This deletion was previously tested with other vaccine candidates and induced antigen expression with low levels of viral replication. An additional temperature-sensitive mutation in the polymerase protein L (1030s) completes the structure description of the vaccine candidate ^[39][68]. The vaccine was shown to be safe and stable in a Phase I clinical trial (NCT02794870 and NCT02952339) in seronegative children, and an increase in serum antibody assays and RSV F-specific immunoglobulin G (IgG) antibody titers were reported in 90% and 85% of the vaccine recipients, respectively.

An NS2 deletion mutation characterizes this candidate. Additionally, the modification in the polymerase L protein (1030s), as tested in the previous candidate RSV LID/ΔM2–2/1030s, is part of this vaccine candidate ^[40][97]. Seropositive and seronegative children participated in a Phase I trial (NCT03387137) of this vaccine candidate without available results.

RSV 6120/ΔNS1 contains a deletion in the NS1. In comparison, RSV 6120/F1/G2/ΔNS1 is similar to the previous candidate modified with codon optimization regarding the F gene and transport of both the F and G genes in other genome positions to optimize the translation ^[40][97]. The Phase I (NCT03596801) trial that intends to evaluate the vaccines is currently recruiting pediatric participants.

Subunit vaccines consist of purified fragments of the desired pathogen, which can be peptides, proteins, or polysaccharides, lacking the genome of the whole pathogen, resulting in a non-virulent vaccine with an increased level of vaccine safety ^[41][98]. The hepatitis B virus (HBV) vaccine was the first subunit vaccine available for evaluation in the market in 1986 ^[42][99]. A significant advantage of subunit vaccines is the high level of safety due to the lack of pathogenicity, making them an appropriate candidate for the immunization of immunosuppressed individuals ^[43][100]. At the same time, however, this specific characteristic may lead to reduced vaccine effectiveness and the need for adjuvant use ^[44][101]. An important limitation in the use of subunit RSV vaccines occurs in the pediatric population based on the results of the formalin-inactivated RSV vaccine. The researchers abandoned the plan of development of this class of vaccines for the pediatric population ^[45][102]. Booster dosages are usually needed to result in long-term immunity ^[46][103]. VLP vaccines are a subclass of subunit vaccines using virus-derived components that form a particle structure, which retains similarities to the parent virus without the ability of replication because they do not contain the whole viral genome ^[47][48][104,105]. This characteristic makes VLPs suitable for use in immunocompromised or older adults ^[49][106].

IVX-A12 is a bivalent candidate of Icosavax and consists of IVX-121, a candidate against RSV, and IVX-241, a candidate against human metapneumovirus (hMPV), aiming to prevent both infections. The vaccine technology is based on VLPs, specifically regarding the IVX-121. The VLP leads to a multivalent presentation of the RSV-F protein in its stable prefusion form. Positive feedback from the interim analysis of the Phase I study (NCT05664334) was shared by the company on May 2023 with no significant safety signals and increase in the geometric mean titers (GMTs) of neutralizing antibody titers against both RSV strains with a 4-fold and 3-fold geometric mean fold rise (GMFR) measured for RSV-A and RSV-B in all groups of seropositive participants. Similar results also emerged for hMPV measurements. The simultaneous administration of the vaccines did not seem to influence the effects of each ^[50][31]. In June 2023, the company initiated the Phase IIa (NCT05903183) clinical trial of the vaccine candidate in adult participants aged between 60 and 85 years.

V-306, funded by Virometix, is a self-assembling VLP candidate that presents on its surface multiple epitopes of the antigenic site II of the RSV F protein (FsII), which is the binding site of palivizumab (PVZ) and is common to both pre- and post-fusion F (postF) protein forms ^[51][32]. V-306 was tested in a Phase I clinical trial (NCT04519073) in 60 healthy women of 18–45 years of age. The vaccine was safe and did not show tolerability issues. Regarding epitope-specific FsII IgG antibodies, an increasing trend was recorded in the intermediate- and high-dose groups, with a minimal further increase after the second dose. PVZ-competing antibodies (PCA) increased in the intermediate- and high-dose groups, indicating a PVZ-like antibody response. However, the investigators believe improvements are needed to show better results in future studies ^[51][32].

DPX-RSV from Immunovaccine uses an oil-based system called DepoVax (DPX). The antigen of interest is a peptide found in the RSV-A SHe protein. The SH protein as a vaccine antigen platform for RSV is new ^[52][33]. In a Phase I trial (NCT02472548), the vaccine showed potency of antibody induction, as shown by the increase in SHe-specific antibody titers in the context of a safe vaccine candidate.

Developed by a Japanese company, this candidate uses VAGA-9001a as the antigen combined with MABH-9002b as an immunostimulator. No further biological information is provided regarding to the antigen target. At present, a Phase II clinical trial (NCT05547087) is active in Japan, but there are no results shared from the completed Phase I trial (NCT04914520).

Advaccine’s BARS13 uses the RSV G protein as the antigen of interest, produced in a bacterial (E. coli) platform. Additionally, cyclosporine A (CsA) is the factor that changes the immune responses and the solvent for reconstituting the RSV-G of the vaccine, and aims to prevent both RSV-A and RSV-B strains ^[53][54][34,107]. This combination intended to stimulate regulatory T cells (Treg) and the release of interleukin-10 (IL-10) to eliminate the chance of vaccine-enhanced disease (VED) development and significantly prevented VED ^[55][108]. BARS13 was clinically assessed in a Phase I study (NCT04851977), revealing a safe candidate with an increase in the measurements of RSV-G-specific IgG antibodies, which remained until day 60 after vaccination. After the second administration, the IgG antibody titers were escalated, indicating that the two-dose schedule induces better antibody-mediated immunity. The results suggested that the induced T-cell response is controlled and VED is not a possible development ^[53][56][34,35]. The company proceeded to the Phase II (NCT04681833) study conducted in Australia with an expected completion date of March 2024.

RSV preF vaccines are manufactured for administration in pregnant women and older adults ^[57][36]. DS-Cav1 is developed by the National Institute of Health and the National Institute of Allergy and Infectious Disease. It constitutes a protein subunit molecule, specifically the stable product of the preF conformation of the RSV F protein, which is produced through modifications ^{[58][59][60][61][62]}[109,110,111,112,113]. Specifically, the design of disulfide (DS) and cavity-filling alterations (Cav1) contribute to stabilization to the prefusion form of the F protein ^[63][114]. Modifications of the parent DS-Cav1 are being developed and tested in preclinical models. These techniques aim to optimize the induced immune activation ^{[64][65][66][67]}[115,116,117,118]. The results of the Phase I trial confirmed an elevation of the titers of neutralizing antibodies against both RSV strains. At the same time, a second vaccination does not add clinical effects on long-term immunity. In general, the vaccine had an acceptable safety profile with a long-term induction of immunity until week 44, overcoming an RSV season. The addition of aluminum hydroxide (AlOH) showed no significant changes in antibody induction, making the vaccine suitable for use during pregnancy ^[57][36].

Arexvy is a single-dose subunit vaccine, developed by GSK, resulting from the conjugation of the RSV-F protein stabilized in its prefusion form and adjuvant system 01 (AS01E) as immunostimulator and was the first preventive vaccine licensed by the FDA against RSV-mediated lower respiratory tract disease (LRTD) in subjects >60 years of age in the United States for market use on May 2023. The decision was predicated on the primary data of the ongoing Phase III clinical trial, which assessed the participants according to three RSV seasons. The FDA imposed post-marketing pharmacovigilance studies regarding evaluating Guillain–Barré syndrome and acute disseminated encephalomyelitis (ADEM) risks ^[68][119]. Additionally, the company is willing to assess the cases of atrial fibrillation in these studies. The vaccine is expected to be administered to the target population before the 2023/24 RSV season ^[69][70][44,120]. In June 2023, after an accelerated assessment of Arexvy by the EMA, the vaccine was approved for market release in Europe ^[71][121]. Several clinical trials evaluating the vaccine have already been completed. A Phase III clinical trial is currently ongoing that intends to add information related to the assessment of the immunity induction up to 3 years after a single dose of the vaccine and assessing the effect of the re-administration of the vaccine using different vaccination schemes. The primary safety results indicate a well-tolerated vaccine with a reported case of Guillain–Barré syndrome associated with the vaccine, according to the study investigator. Regarding immunological responses, the induction of specific immunity was demonstrated through the increase in the GMTs of neutralizing antibodies against RSV-A and RSV-B and geometric mean concentrations (GMCs) of RSVPreF3-specific IgG until one month after the vaccination, with a decrease until the month 6. Another Phase III trial (NCT04886596) is currently examining the prophylactic ability of RSVPreF3 OA against RSV-LRTD in older adults and has already enrolled 26,665 individuals. The interim data show high vaccine efficacy rates and protection against RSV acute respiratory infection (ARI) and RSV-LRTD in adults aged 60 years or older, even in the case of a chronic stable disease. Atrial fibrillation was experienced in 13 vaccinees and 15 placebo recipients at the 6-month follow-up ^{[69][72][73][74]}[41,42,43,44]. The company is currently conducting other Phase III clinical trials. RSVPreF3 presents antigenic sites crucial to the mediated response, and for this reason, it was selected as the antigenic part of the vaccine. The AS01 adjuvant was already associated with the activation of immunity, especially in older adults, and a compound of these two factors proceeded to further clinical evaluation as GSK3844766A ^[75][37]. The maternal vaccine was also under development and included the same antigen of RSVPreF3 without using the adjuvant part of the vaccine. In February 2022, the company shared the termination of enrolling and vaccinating participants in trials referring to the maternal RSV vaccine candidate in pregnant women under the guidance of the Independent Data Monitoring Committee, while the safety parameters of the conducted trials are examined ^[76][77][122,123]. It was demonstrated that the unadjuvanted vaccine in pregnant women was related to a very small increase in preterm births, while the placebo recipients did not show a similar effect ^[72][41]. Specifically, the rate of preterm births was 6.81% for the pregnant women receiving the vaccine and 4.95% for the placebo recipients. Arexvy is not indicated for the immunization of persons < 60 years of age ^[78][124].

Abrysvo is a bivalent subunit vaccine candidate based on developing the prefusion F protein as a stable molecule in this preF formulation administered as a single-dose regimen. The vaccine was developed by Pfizer and received authorization from the FDA against RSV-mediated lower respiratory tract disease for subjects 60 years of age or older after the approval of GSK’s Arexvy in May 2023. The results of an ongoing Phase III clinical trial named RENOIR (RSV vaccine efficacy study in older adults immunized against RSV disease) guided the decision of the FDA ^[79][125]. The interim safety results of the Phase III RENOIR study indicated a possible safety signal due to the occurrence of Guillain–Barré syndrome as a serious AE (SAE). However, other studies did not confirm evidence of Guillain–Barré syndrome or signals for other immune-mediated demyelinating conditions. The FDA announced on September 2023 a pharmacovigilance plan, which will be followed by the company. Potential safety signals that will be tested are risks for Guillain–Barré syndrome (GBS), allergic reactions, supraventricular arrhythmias, hypertensive disorders of pregnancy (HDP), and preterm births. Immunocompromised pregnant women and older adults will be included in further studies ^[80][81][53,126] The vaccine was also under accelerated assessment from the EMA with a pending decision for the marketing authorization application ^[82][127]. In July 2023, the Committee for Medicinal Products for Human Use (CHMP) stated a positive response regarding Abrysvo’s licensure, including both adult and maternal vaccine forms ^[83][128]. Recently, by August 2023, the FDA added the further indication of vaccination with unadjuvanted Abrysvo during the third trimester of pregnancy, especially between the 32nd and 36th week of gestational age, contributing to the prevention of LRTD and severe disease caused by RSV in neonates and infants until the first 6 months after birth ^[84][129]. Subsequently, the EMA licensed Abrysvo in the European Market as an immunizing agent against LRTD in the age group of adults 60 years of age or older and during the 24th and 36th weeks of gestational age for maternal use to provide infant protection. Abrysvo is currently the only market-approved immunizing agent in the field of RSV prevention, aiming at the target group of infants ^[85][130]. As stated in the prescribing information of Abrysvo, a not statistically significant disproportionate incidence of preterm births occurred between vaccinees and placebo recipients. It is recommended to use the vaccine according to the indication, because a causality between vaccination and preterm birth cannot be excluded based on the existing data ^[86][87][131,132].

The technology based on the crystal structure of the F protein announced by the National Institutes of Health (NIH) ^[62][113] is used in this vaccine candidate. The antigens that compose the vaccine are equal to 60 μg of the preF protein from both RSV-A and RSV-B strains of the virus. Phase I and II studies are already completed, while Phase III clinical trials are ongoing at present. MATISSE (MATernal Immunization Study for Safety and Efficacy) is a Phase III study (NCT04424316) in female pregnant subjects between 24 and 36 weeks of gestation testing the prevention of medically attended-lower respiratory tract infection (MA-LRTI) in infants through maternal vaccination. The study includes 14,750 participants and is expected to end in November 2023. Interim data were shared by the company in November 2022 indicating the tolerability and safety of the vaccine. The efficacy analysis presented a rate of 81.8% protection against severe MA-LRTI for infants within the first 90 days after birth. In the 6-month period, the efficacy reached a percentage of 69.4%. Moreover, no safety concerns for pregnant women or infants arose from the pre-review of the safety results ^[88][89][50,51]. RENOIR (NCT05035212) aims to assess the protective effect of the RSVpreF and the immune activation and safety parameters after a single administration of RSVpreF. From the interim analysis in August 2022, a vaccine efficacy of 66.7% for LRTI-RSV assessed as two or more symptoms and 85.7% using a definition of three or more symptoms emerged. The vaccine efficacy was 62.1% for the prevention of RSV-ARI. The safety results from a part of the population revealed an acceptable profile for the older adult group. Atrial fibrillation counted 10 cases in the vaccinees group compared to 4 in the placebo recipients, indicating a disproportion of this AE. SAEs that seem to be linked to the vaccine include hypersensitivity manifestation, Guillain–Barré syndrome, and Miller Fisher syndrome. In total, 2 cases of Guillain–Barré syndrome were recorded in a vaccinated number of 19.942 participants ^[80][90][91][52,53,54]. A Phase III study initiated in May 2023 named MONET constitutes a master protocol that will assess RSVpreF vaccine in adults aged 18 years and older with increased risk for developing severe RSV-mediated disease.