Establishing Correlates of Protection for Vaccine Development: Considerations for the Respiratory Syncytial Virus Vaccine Field

Abstract

Correlates of protection (CoPs) can play a significant role in vaccine development by assisting the selection of vaccine candidates for clinical trials, supporting clinical trial design and implementation, and simplifying tests of vaccine modifications. Because of this important role in vaccine development, it is essential that CoPs be defined by well-designed immunogenicity and efficacy studies, with attention paid to benefits and limitations. The respiratory syncytial virus (RSV) field is unique in that a great deal of information about the humoral response is available from basic research and clinical studies. Polyclonal and monoclonal antibodies have been used routinely in the clinic to protect vulnerable infants from infection, providing a wealth of information about correlations between neutralizing antibodies and disease prevention. Considerations for the establishment of future CoPs to support RSV vaccine development in different populations are therefore discussed.

What Is a Correlate of Protection and What Are the Benefits and Risks of Establishing a Correlate of Protection?

What is a correlate of protection?

There is no standard definition of a correlate of protection (CoP) that is universally accepted by global regulatory agencies. This is perhaps an inevitable consequence of immune complexity, and the cognizance by regulators that no one immune factor stands alone in control of an infectious disease. In 2005, the FDA described four levels of CoPs, ranked 1–4 by the strength of correlations between a given factor and a clinical endpoint, with level 1 being the strongest (28). The best CoPs may be established when there is a direct, mechanistic, cause/effect relationship between a factor and an endpoint. As an example, a high virus-specific neutralizing antibody response might afford protection against an acute lower respiratory infection (ALRI) caused by a virus. If this is the case, when the neutralizing antibodies are high, the protection will usually follow. Weaker correlates may exist when there is a second, potentially unrelated outcome when the causative factor is present. For example, on upregulation of neutralizing antibodies, there may be Fc-receptor-induced cytokine expression by macrophages. These cytokines might not be protective, but their upregulation may nonetheless correlate with protection. The situation is rarely simple, because a wide variety of interactive factors usually lead to, and are a consequence of, a clinical endpoint.

The terms “correlates” and “surrogates” have been used by researchers and regulatory agencies, but definitions are again variable (1). There are also further designations of CoPs; mCoP refers to a mechanistic CoP (a CoP that is mechanistically causal of the protective, clinical outcome) and nCoP refers to a nonmechanistic CoP (a CoP that is apparently not causally related to a clinical outcome) (84). As examples, bactericidal antibodies induced by the meningococcus vaccine are labeled as mCoP, because they are known to clear pathogen directly (84); antibodies that bind meningococcus antigens without neutralization are designated nCoP, because protective mechanisms associated with these factors have not been defined.

The benefits and risks of establishing a CoP

The development of vaccines is a lengthy and expensive process. The entire course of vaccine development, including pivotal phase III clinical trials, can span decades of research, require tens of thousands of study participants, and cost billions of U.S. dollars (25,26). The testing of every promising vaccine candidate is therefore not feasible due to the overbearing time and cost requirements. Establishing a standard CoP may save time and money, perhaps enabling tests of more candidate vaccines and improving the design and interpretation of clinical trials. Furthermore, if a CoP has been established and accepted by regulatory boards, improvements to a vaccine and testing can be accomplished at lower cost without repeating efficacy studies. An established CoP will additionally assist manufacturers who wish to produce a previously tested vaccine, to ensure that the new product is superior or noninferior to the old (28,81,84).

The benefits and risks of selecting a CoP obviously depend on the CoP accuracy. If the correlate is strong, money and time will be saved. If the correlate is weak, poor gatekeeper choices will be made, potentially advancing poor vaccine candidates rather than good ones, thus escalating the time and money required for successful vaccine development.

Even when there is a long-standing acceptance of a given CoP, researchers appreciate that no correlate is absolute. This is because (i) multiple innate and adaptive immune system components influence protection and (ii) there is inherent variability of protective immune responses among individuals (88).

Examples of CoPs

In some cases, as with the development of the rabies vaccine, the conduct of an efficacy trial was not possible, because the frequencies of infections were too low. In this case, the CoP was critical for the support of vaccine development and licensure (63,112).

The use of CoPs to improve vaccines is well demonstrated in the meningococcal vaccine field. Vaccine development for Neisseria meningitidis, an important cause of bacterial meningitis and sepsis, began in the 1960s. Vaccines comprising high-molecular-weight polysaccharides were developed at Walter Reed Army Institute and were proven efficacious in military recruits (4,10,32,34,35). Advisory committees then recommended that vaccine efficacy be predicted based on immune responses. For meningococcal C vaccines, a titer of 1:8 in a serum bactericidal assay supplemented with rabbit complement, or a titer of 1:4 in an assay supplemented with human complement, was predicted to indicate protection. The recommendation streamlined vaccine production and the development of new products, including new conjugate vaccines, without a requirement for large-scale phase III clinical trials (7,15,27,60,68,77,91,105).

Other fields with established CoPs include the influenza virus and poliovirus fields (80,83,110). For the influenza virus field, the acquisition of a hemagglutination inhibition antibody titer of at least 1:40 is often used as an indicator of protection. The CoP assists modifications of influenza virus vaccines from year to year. However, the 1:40 cutoff value does not always associate with protection and is therefore a topic of ongoing debate (21,61). For poliovirus, antibody neutralization at a titer of 1:4 or 1:8 is desired (11,81,82). Plotkin has tabulated more than 20 CoPs used currently for the evaluation of vaccine products (81).

We note that although antibodies are often viewed as immune correlates, these are not the only factors associated with protection. Other factors, including cytokines or micro-RNAs (miRNAs), may serve as both positive and negative regulators (19,106). Antigen expression patterns (e.g., peptides presented by major histocompatibility complex proteins) and epigenetics will further influence outcome (39,45,48,98). Each situation differs from the next, and some correlates prove particularly difficult to define (2,8).

Respiratory syncytial virus as a test case

Here we will examine the potential establishment of a standard CoP in the respiratory syncytial virus (RSV) field. RSV is the most common cause of childhood ALRI worldwide. Based on data from 2015, it has been estimated that 33.1 million [uncertainty range 21.6–50.3] episodes of RSV ALRI occur in children younger than 5 years resulting in 3.2 million [2.7–3.8] hospitalizations annually, with 59,600 [48,000–75,500] hospital deaths (71,102). If community deaths are considered, annual mortality is as high as 118,200 [94,600–149,400] (102). For these reasons, the development of a successful RSV vaccine has been declared an urgent priority by healthcare agencies in the United States and worldwide (71,102).

The RSV field is unique in that passive transfer of protective antibody has been frequently studied, both in small animal models and humans, providing a wealth of information about antibody-mediated protection. Although no consensus protective threshold is currently accepted by regulatory agencies, the establishment of an accepted threshold may be in sight. Below, we explore how a CoP might be established for the RSV vaccine field based on the extensive data available in published literature. We focus on antibody-mediated protection, but appreciate that CD4⁺ T cells, CD8⁺ T cells, and innate cell responses can also serve as co-CoPs.

Previously Defined Antibody Correlates in the RSV Field

Several studies have demonstrated that naturally induced RSV-specific polyclonal antibodies or monoclonal antibodies with neutralizing activities correlate with protection. This phenomenon has been demonstrated by analyses of maternal sera, cord blood, and infant sera, and by passive antibody transfer studies. Examples of each are described below.

Maternal sera and cord blood: binding and neutralizing antibodies reveal correlates

• In 1981, Ogilvie et al. described the testing of maternal antenatal sera for RSV-specific antibodies by an indirect membrane fluorescence technique. Authors found that serum antibodies correlated with protection from infection for infants within the first 6 months of life (74).

• Similarly, in 1981, Glezen et al. reported a study of infants in the United States who were <6 months of age with RSV infections (confirmed by virus replication in tissue culture). RSV-specific neutralizing antibody titers in the cord blood of these infants were lower than for randomly selected cord blood samples (31).

• In 1983, Ward et al. described an examination of serum antibody responses to RSV glycoproteins (using RSV-infected and RSV-uninfected, cultured cells as targets in radioimmunoprecipitation analyses) among the mothers of young infants at the time of a local RSV epidemic. Researchers found that protection from infection among infants ≤6 months of age was correlated with the mother's RSV-specific antibody levels (116).

• In 1993, de Sierra et al. compared cord blood from premature babies born at ≤28 weeks of gestation to term babies. They measured RSV F- and RSV G-specific binding antibodies and neutralization, demonstrating that antibody levels were significantly lower in the preterm babies compared to controls (22). This result was used as a potential explanation for the enhanced vulnerability of preterm babies to RSV infections.

• In 2009, Stensballe et al. provided a more quantitative correlate. They studied children in Denmark who were hospitalized with RSV. They found a temporal association between neutralizing antibody levels in cord blood and RSV hospitalizations, in that RSV incidence peaked soon after mean antibody levels reached a nadir (or dropped below a titer of 1:181 [2^7.5]). Neutralizing antibody levels in cord blood were inversely associated with RSV hospitalizations in infants <6 months of age, and inversely associated with recurrent wheeze following RSV-associated hospitalizations (107,108).

Infant sera: neutralizing and binding antibody levels identify CoPs

• In 1975, Lamprecht et al. described an examination of 15 infants who were younger than 9 months with pneumonia caused by RSV. Researchers found that the levels of circulating neutralizing antibodies in these infants were inversely correlated with the severity of pneumonia (56).

• In 1981, Glezen reported a similar finding in that infants with severe illness caused by RSV exhibited lower serum levels of RSV-specific neutralizing antibodies compared with infants with less severe disease (31).

• In 2002, Roca et al. reported results from Mozambique, in which sera from infants with or without RSV infection were compared using a membrane fluorescent antibody test for IgG and neutralization assays. These investigators found that RSV-specific IgG antibodies and neutralization functions were each at lower levels in the RSV-infected group compared to controls (92).

• A study reported in 2003 provided quantitative correlates. This U.S. study included children and adults hospitalized with ALRI. When geometric mean RSV-specific antibody neutralization titers or geometric mean RSV F-specific antibody binding titers were examined, they were found to be lower in patients with RSV-associated hospitalizations compared with patients who were hospitalized for other reasons. Specifically, when comparing RSV-associated versus RSV-unassociated hospitalizations, neutralization titers against RSV A were 1:69 [2^6.1] versus 1:239 [2^7.9]; neutralization titers against RSV B were 1:158 [2^7.3] versus 1:676 [2^9.4]; RSV F specific-binding titers were 1:6,208 [2^12.6] versus 1:15,287 [2^13.9]. The study allowed establishment of a minimal protective threshold. Specifically, titers of ≥1:64 [2⁶] and 1:256 [2⁸] were determined to afford protection (∼70%) against RSV A- and RSV B-associated hospitalizations, respectively (79).

Passive transfer of RSV-specific antibodies in cotton rats and humans identifies CoPs

RSV immune globulin (RSVIG), a mixture of immunoglobulins from human adult sera with high-titered RSV-specific antibodies, was previously used as an effective prophylaxis against RSV infections in vulnerable children, and was then replaced by the monoclonal antibody palivizumab. Sample studies are described below.

• In one passive transfer study, sera from RSV-experienced cotton rats were transferred to infant cotton rats. When a serum antibody titer of 1:380 was achieved, there was near-complete protection of the host from lung infection after RSV challenge, whereas a 10 × higher titer was required for protection of the upper respiratory tract. Passive antibody transfers were also protective in nonhuman primates, and in one study, topical administration of antibody in cotton rats appeared superior to parenteral administration (85,86,96).

• A study of infants and young children who were vulnerable to RSV infection due to prematurity and/or congenital heart disease was later conducted in which children received RSVIG. Two different doses of RSVIG were tested. RSV infections were of lower frequency among children who received prophylactic antibodies compared to controls. Among children who were RSV infected, when the higher dose group and control group were compared, the lengths of hospital stays as well as the rates of ALRI, hospital admittance, ICU admittance, and ribavirin use were all lower in the group that received RSVIG (36,37).

• In cotton rats, it was shown that when a concentration of ≥40 μg/mL palivizumab was achieved in sera, pulmonary RSV replication was reduced by more than 100-fold (47,104).

• The same concentration of palivizumab in humans (≥40 μg/mL) with a neutralizing antibody titer of at least 1:64 [2⁶] defined a minimum protective threshold and reduced the likelihood of RSV-related hospitalizations to ≤30% (17,38,55,95). Preventing RSV ALRI with palivizumab additionally reduced recurrent wheezing in premature infants (13,69,103,104,107,114,120), and for every twofold increase in neutralizing antibody titers, there was a further reduction of ∼20% in RSV-related hospitalizations (57,78,79).

Altogether, these compelling results demonstrated that antibodies correlated with protection against infection and disease caused by RSV.

What Are the Targets of Neutralizing Antibodies Toward RSV?

The main targets of neutralizing antibodies are the G (attachment) and F (fusion) proteins, as these are the two most prominent viral membrane molecules. Small hydrophobic (SH) protein is also on the surface, but has received little attention, and is thought to induce binding, but not neutralizing antibodies (99).

RSV G-specific antibodies have long been recognized as potential protective correlates (41,90). However, only some of the protein epitopes that are targeted by RSV G-specific antibodies have been characterized. As an example, there is a conserved CX3C motif that binds CX3CR1 (the receptor for the CX3C chemokine fractalkine) and adversely affects T cell responses (40). The induction of antibodies that bind the CX3C-mimic on RSV G and block RSV G protein CX3C-CX3CR1 interactions is reported to reduce RSV lung titers, prevent weight loss, and prevent pulmonary inflammation in mice. Neutralizing antibodies can also be observed when vaccines encompassing the CX3C mimic are used (49,50,111,121).

Antibodies that target the RSV F protein rather than the G protein have received the greatest attention recently, in part, because RSV F is a relatively well-conserved membrane protein compared with RSV G, and because palivizumab, an efficacious monoclonal antibody in preventing RSV-related hospitalization, is specific for antigenic site II on the F protein (70).

The F protein is responsible for fusion of host and virus membranes. F proteins assume numerous configurations from their first appearance on the infected cell surface to the completion of the fusion process. These include prefusogenic, prefusion, and postfusion conformations [defined by analyses of RSV and related viruses (94)]. During the infection process, the prefusion F protein (a metastable, spring-loaded protein) on the virion surface refolds into a postfusion F, a stable, six-helix bundle, hairpin structure that supports juxtaposition of membranes from the virus and host (38 –40).

There is currently a debate concerning the conformation of the F protein found on the surface of the virus, prefusogenic versus prefusion. RSV is unique among the viruses in the Paramyxoviridae and Pneumoviridae families (3,5) in that it has two furin cleavage sites (33). Cleavage at both furin sites is required to establish the prefusion conformation. Cleavage is often described as a post-translational process (67), but data generated by Krzyzaniak et al. now challenge this concept. These authors described the second furin site as undergoing cleavage after the virus enters the cell, internalized by macropinocytosis. Then, the virus is fully infectious (54). If this RSV entry mechanism is correct, it indicates that a prefusogenic F is present on the respiratory epithelial cell surface as the virus is budding and being released into the respiratory secretions. The prefusogenic F is an intermediate F that retains p27 (the 27 amino acid peptide located between the two furin cleavage sites), suggesting that virions with prefusogenic F and p27 can stimulate the host immune response. Fuentes et al. recently demonstrated p27 to be a dominant antigenic site recognized by sera from children and adults infected with RSV (29). Researchers suggested that among young children, there was a significantly greater binding activity in sera for the p27 epitope than other antigenic sites on the F protein.

Several antigenic sites have been described that exist on “postfusion F,” or that are shared by prefusion and postfusion F (e.g., sites II and IV). Among these, site II is of particular interest, because this is bound by palivizumab. Vaccine researchers often conduct assays for palivizumab competing antibodies. To compete, antibodies might share specificity with palivizumab. They might also inhibit palivizumab by binding a similar protein region without shared specificity or by altering the protein's three-dimensional structure (64,65,70).

A monoclonal antibody similar to palivizumab, motavizumab, was created to increase potency by substituting residues in the complementarity-determining regions (44). This antibody was further characterized and its structure was defined using a 24-residue peptide. Models then suggested that binding was influenced by the quaternary configuration of the F glycoprotein, emphasizing the importance of a protein's native structure (64).

Ongoing debates concern the importance of maintaining the prefusion F configuration in an RSV vaccine. Some studies suggest that the bulk of neutralizing antibodies responsive to RSV recognize epitopes on the prefusion form of F. For example, studies were conducted by adsorbing human adult sera with prefusion F epitopes of interest, and examining the percentage of residual neutralizing function. One such study suggested that adsorption of individual sera with prefusion F protein removed more than 90% of neutralizing activity and that a single Ø site accounted for ∼35% of all activity (72).

Adsorption studies identify important epitopes on viral antigens, but can also miss targets. This was illustrated in the HIV-1 field when a study defined the crown of the envelope V3 loop as the “principal neutralizing determinant” for HIV-1 (46,87,101), a description that is rarely used in the HIV-1 field today. Adsorption study results are dependent on numerous factors such as the choice of neutralization assay, target virus, antibody sample, and method of antigen preparation (14,42,73,97). A change in any one of these parameters can influence outcome.

The full repertoire of neutralizing antibodies responsive to RSV among humans is yet to be discovered. An extraordinary number of RSV-specific antibodies and targets likely exist, due to the sophisticated mechanisms of mammalian B cell development. Each developing B cell undergoes unique V-D-J rearrangements at the heavy-chain locus and V-J rearrangements at the light-chain locus that define antibody specificity. Due to this enormous combinatorial diversity (supplemented by n-region and p-region diversity and somatic hypermutation), there may be more than 10¹⁰ distinct antibody specificities in a human adult (18,23,59).

Added to the complexity of antibody binding sites are complexities of mechanisms by which antibodies inhibit virus amplification. Mechanisms of virus inhibition include virus aggregation, Fc-receptor-mediated destruction of virus, removal of the infected cells (e.g., by antibody-dependent cellular cytotoxicity), prevention of virus binding to cell receptors, prevention of virus entry into the host, and (for at least for some viruses) inhibition of the virus after it has been internalized (e.g., TRIM21-associated mechanisms) (9,58,62). When considering the plethora of inhibitory mechanisms, it makes sense that antibodies directed toward a variety of antigenic determinants could inhibit RSV.

Future research, including analyses of unselected RSV-specific monoclonal antibodies from RSV-seropositive persons, is warranted to discover new RSV neutralizing epitopes, and to illustrate the complexity of antibodies that appear in any one individual over time, or between individuals within a population. A better understanding of epitope complexity, while potentially complicating the definition of CoPs, will surely improve CoP accuracy.

Standardizing the Serum Neutralization Assay and Establishing Phase III Study Design

Serum neutralizing antibodies as future CoPs

For future analyses, researchers will likely use neutralization assays to measure CoPs. To simplify studies, microneutralization assays are often used, scored by plaque reduction or ELISA. Neutralization assays rather than binding assays are preferred, because binding antibodies need not be protective. Serum is the preferred source of antibodies, because even though there have been indications that mucosal antibodies correlate with protection (114), antibody levels in the mucosa are relatively low, and collection methods yield variable results (6).

If regulatory agencies endorse a particular method for CoP analyses, standardization of that method will then be critical. Piedra et al. have considered these concepts and have described methodologies, standard operating procedures, and training, to ensure that microneutralization assays are performed with rigor (78,79). Researchers must contemplate precisely which viruses will be tested, what will be the readout, and what will be considered a positive score. Once assays are selected, these may be used to assist vaccine selection and clinical study design.

There is currently a wide variety of neutralization assays. A recent exercise was spearheaded by PATH to understand the overall agreement of 12 different RSV neutralization assays (43). Although precision was high among the assays, overall agreement varied. The level of agreement between the assays was significantly improved using a standard to harmonize the neutralizing antibody titers (43). The demonstrated feasibility of harmonizing across a broad array of assay formats supports the development of an international standard.

How to design a clinical study to establish a CoP

Late-phase clinical trials provide a unique opportunity to identify vaccine-induced and infection-induced immune responses that correlate with protection. These should be randomized, blinded, conducted with a large number of participants, and designed with well-defined clinical endpoints. A well-designed phase III trial should minimize uneven assignments of at-risk factors and disease exposures, to ensure consistency in the measurement of clinical endpoints. If disease prevention is observed, it can then be attributed to the vaccine and the associated immune response. Researchers realize that protective immune responses induced by various RSV vaccines (e.g., using prefusogenic, prefusion, or postfusion F protein conformations) may differ from one another and from natural RSV infection.

In a well-designed phase III trial, the immune response in both the placebo and the vaccine groups must be evaluated statistically. Determination of a CoP is preceded by an assessment of a correlate of risk (CoR), which is based on the immune markers in those infected or not infected during a phase III trial. The CoR is in effect the immune marker lacking in those infected. If the CoR is influenced by vaccination, the CoP in vaccinees is the amount of the immune marker induced by vaccination necessary for protection (53,84,89).

RSV-specific neutralization assays are favored for use in clinical studies, because they define pathogen-specific responses, because antibody titers or concentrations can be determined in a qualified biological assay, and because virus-specific neutralizing antibodies have a causal relationship to protection against severe disease. The CoP may be relative (when levels of responses of antibodies are variably correlated with protection) or absolute (when a specific antibody level correlates with protection).

Plotkin reminds us of several important points to be considered when establishing a CoP (80,81,83,84,110). First, infection with a large inoculum may overwhelm immune protection. It should be noted that a human exposure to an RSV-infected infant is likely to result in a greater viral inoculum than exposure to an RSV-infected older adult. Thus, volunteers in placebo and vaccine groups of a clinical study should be matched for household composition, to minimize disparities in RSV exposures. Second, the mechanism of protection may not be the same as the mechanism of recovery from infection. This point is demonstrated by the finding that palivizumab prevents severe RSV infection when administered prophylactically, but does not significantly alter the course of disease when administered after infection (66). Neutralizing antibodies will likely serve as a CoP in infants and young children but in older adults, a CoP associated with virus clearance might be a better target. Indeed, a CoP may vary depending on age and other host factors. Thus, clinical trials conducted in different populations are needed to establish a CoP that is general to all populations or specific to each population studied.

Concerns in the RSV field about immune-mediated enhancement

RSV vaccine development is particularly difficult, because of the experience in the 1960s in which a formalin-inactivated vaccine (FI-RSV) caused enhanced disease when children were naturally infected with RSV (16,30,51). A recent webinar was held by the FDA CBER Vaccines and Related Biological Products Advisory Committee (VRBPAC, 5-17-17, https://collaboration.fda.gov/rsvvaccine0517) during which discussions addressed concerns about identifying immune-mediated enhancement in infants. The precise explanation for the vaccine result in the 1960s remains unknown, and debates continue as to the influence of T cells, eosinophils, and cytokines (12,20,24,75,76,93,109,113,115,117 –119). It has been recently reported that the prefusion F is absent on the surface of the FI-RSV vaccine, suggesting that it was not involved in the FI-RSV-associated pathology (52). However, studies in the cotton rat suggest that immunization with suboptimal, low doses of recombinant prefusion or postfusion RSV F primes for vaccine enhanced disease, even in the presence of neutralizing antibody (100). These results emphasize the complexity of viral/immune interactions that lead to enhanced pathology.

To avoid immunopathology in infants in future vaccine studies, careful and long-term surveillance of vaccinated individuals must be conducted. Given that a vaccine can exacerbate RSV disease, the definition of a CoP in the RSV field may be unusually complex. Despite such obstacles, continued attention to the establishment of standardized assays and thresholds for the measurement of neutralizing antibodies is likely to simplify and expedite RSV vaccine development.

Conclusion

In a situation such as that of the rabies vaccine field, when low disease frequency precluded a conventional phase III study, the CoP played a prominent role in vaccine development and licensure. CoPs are also used to select vaccines for clinical development, to design clinical trials, to assist modification of vaccines, and to transfer manufacture from one company to another. Because of the critical role CoPs play in vaccine development, it is essential that CoPs be identified carefully, and that the limitations of CoPs are recognized. What may suffice as a CoP in one situation may not apply to another. Also, when a vaccine appears to fail based on one CoP, it may nonetheless be successful based on an alternative method of protection. For the RSV field, the path to CoP development is instructed by previous basic and clinical research, including passive transfer experiments. Polyclonal and monoclonal neutralizing antibodies have been used routinely and effectively in vulnerable infants to prevent disease caused by RSV, emphasizing the mechanistic correlate between neutralization and disease prevention. The next phase III study of an RSV vaccine, if carefully designed (and if the vaccine proves to be efficacious), will likely define a CoP for future use in the RSV field.

Footnotes

Acknowledgments

This work was supported by the NIH NCI P30 CA21765 and ALSAC.

Author Disclosure Statement

Julia L. Hurwitz is an author of a patent describing a Sendai virus-based vaccine vector. Prasad S. Kulkarni is employed by Serum Institute of India Pvt Ltd, which is developing a respiratory syncitial virus vaccine.

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