Live Respiratory Syncytial Virus Attenuated by M2-2 Deletion and Stabilized Temperature Sensitivity Mutation 1030s Is a Promising Vaccine Candidate in Children

Abstract

Background: The safety and immunogenicity of live respiratory syncytial virus (RSV) candidate vaccine, LID/ΔM2-2/1030s, with deletion of RSV ribonucleic acid synthesis regulatory protein M2-2 and genetically stabilized temperature-sensitivity mutation 1030s in the RSV polymerase protein was evaluated in RSV-seronegative children.

Methods: Respiratory syncytial virus-seronegative children ages 6-24 months received 1 intranasal dose of 105 plaque-forming units (PFU) of LID/ΔM2-2/1030s (n = 21) or placebo (n = 11). The RSV serum antibodies, vaccine shedding, and reactogenicity were assessed. During the following RSV season, medically attended acute respiratory illness (MAARI) and pre- and postsurveillance serum antibody titers were monitored.

Results: Eighty-five percent of vaccinees shed LID/ΔM2-2/1030s vaccine (median peak nasal wash titers: 3.1 log10 PFU/mL by immunoplaque assay; 5.1 log10 copies/mL by reverse-transcription quantitative polymerase chain reaction) and had ≥4-fold rise in serum-neutralizing antibodies. Respiratory symptoms and fever were common (60% vaccinees and 27% placebo recipients). One vaccinee had grade 2 wheezing with rhinovirus but without concurrent LID/ΔM2-2/1030s shedding. Five of 19 vaccinees had ≥4-fold increases in antibody titers postsurveillance without RSV-MAARI, indicating anamnestic responses without significant illness after infection with community-acquired RSV.

Conclusions: LID/ΔM2-2/1030s had excellent infectivity without evidence of genetic instability, induced durable immunity, and primed for anamnestic antibody responses, making it an attractive candidate for further evaluation.

Keywords: RNA regulatory protein M2-2; live-attenuated viral vaccine; neutralizing antibodies; pediatric RSV vaccine; respiratory syncytial virus (RSV).

Figure 1.

Vaccine virus shed in nasal washes (NWs) in vaccinees. Titers from individual participants (closed circles and open diamonds) and median titers (solid line) of vaccine virus detected in NWs collected from vaccinees during study visits (indicated study day ±1 day) after inoculation on day 0, determined by immunoplaque assay ([A] log₁₀ plaque-forming units [PFU] per mL) and reverse-transcription quantitative polymerase chain reaction (RT-qPCR) ([B] log₁₀ copies per mL). Peak titers for each participant are indicated by open diamonds; nonpeak titers are indicated by closed circles. Lower limits of detection indicated by dashed lines were 0.5 log₁₀ PFU/mL and 1.7 log₁₀ copies/mL for immunoplaque assay and RT-qPCR, respectively.

Figure 2.

Serum respiratory syncytial virus (RSV) antibody titers in vaccine and placebo recipients. Serum RSV 60% plaque reduction neutralizing antibody titers (PRNT₆₀) (A) and anti-RSV F immunoglobulin (Ig)G enzyme-linked immunosorbent assay titers (ELISA) (B) were determined by complement-enhanced 60% plaque reduction neutralization assay and IgG-specific ELISA against purified RSV F protein, respectively, for vaccine (open circles) and placebo (x) recipients in sera collected at preinoculation (screening), postinoculation (study day 56), presurveillance (October 1–31), and postsurveillance (April 1–30, after the RSV season). Two vaccinees who did not shed vaccine virus and did not have a ≥4-fold increase in either antibody response are indicated by asterisks instead of open circles. The lines indicate median (solid line) and mean (dashed line) values. Serum antibody titers are expressed as the reciprocal log₂. P values were determined by Wilcoxon rank-sum test. Postsurveillance data are missing for 1 vaccine recipient, and pre- and postsurveillance data are missing for 1 placebo recipient.

Figure 3.

Rises in serum respiratory syncytial virus (RSV)-neutralizing antibody titers, and incidences of medically attended acute RSV illness (RSV-MAARI), during the RSV season surveillance. Serum RSV-neutralizing antibody titer (PRNT₆₀) in sera collected pre- and postsurveillance are shown for vaccine (left) and placebo (right) recipients who had a postsurveillance ≥4-fold increase in either serum RSV PRNT₆₀ or anti-RSV F immunoglobulin G enzyme-linked immunosorbent assay titer. Dashed and solid lines indicate participants with and without RSV-MAARI during surveillance, respectively. Titers are expressed as the reciprocal log₂, but for ease of interpretation, titers corresponding to the arithmetic values are indicated for several participants.