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GBS6 Vaccine Shows Comparable Safety and Immunogenicity in HIV-Positive and HIV-Negative Pregnant Women
In a double-blind, placebo-controlled phase 2 trial of 300 pregnant women in Uganda, the hexavalent GBS conjugate vaccine GBS6 induced antibody responses against all six group B streptococcus serotypes with no statistically significant differences between women living with and without HIV — for example, serotype Ia geometric mean concentrations of 12.02 versus 9.38 (p=0.575) — and all serious adverse events were deemed unrelated to the vaccine. Vaccine-induced antibodies were transferred across the placenta to infants regardless of HIV exposure status, with HIV-exposed infants showing responses similar to those of HIV-unexposed infants through the follow-up period.
What Was Studied
This trial evaluated whether a hexavalent capsular polysaccharide conjugate vaccine against group B streptococcus (GBS6, targeting serotypes Ia through V) was safe, tolerable, and capable of inducing protective antibody concentrations in pregnant women living with HIV — a population rarely included in maternal vaccine trials — and whether vaccine-induced antibodies were transferred to their newborns. The central question was whether HIV status would meaningfully compromise vaccine safety or immunogenicity, and whether placental antibody transfer would be sufficient to potentially protect HIV-exposed infants from early-life GBS disease.
How It Was Studied
This was a randomised, double-blind, placebo-controlled phase 2 trial conducted across five antenatal care facilities in Kampala, Uganda. A total of 300 pregnant women aged 18 to 40 years, at between 27 and 35 weeks and 6 days of gestation with low-risk singleton pregnancies, were enrolled: 150 living with HIV and 150 without HIV. Participants were allocated 1:1 to receive a single intramuscular injection of either 20 μg GBS6 or normal saline placebo, using stratified block randomisation by HIV status. The investigational product was masked in opaque-labelled syringes by an unblinded site pharmacist, preserving blinding for both participants and clinical staff. Mother-infant pairs were followed for 12 months after delivery, with safety and immunogenicity assessments at seven pre-specified timepoints.
What Was Observed
- Comparable antibody responses regardless of HIV status: GBS6 induced measurable serotype-specific IgG concentrations against all six serotypes in both groups. At one month post-vaccination, geometric mean concentrations for serotype Ia were 12.02 (95% CI 6.42–22.50) in women with HIV versus 9.38 (95% CI 5.06–17.41) in women without HIV (p=0.575), and for serotype II were 19.50 (95% CI 12.93–29.41) versus 17.92 (95% CI 11.76–27.31) (p=0.776), with no statistically significant differences across any serotype.
- Durable but declining maternal antibody concentrations: Vaccine-induced antibodies remained detectable and above placebo levels at 12 months after delivery, though concentrations declined substantially over time from their post-vaccination peak.
- Placental antibody transfer confirmed across HIV groups: GBS6-induced antibodies were transferred to infants via the placenta in both HIV-exposed and HIV-unexposed groups, with HIV-exposed infants showing similar immunological trajectories to HIV-unexposed infants through 18 weeks after delivery.
- Serious adverse events were balanced and unrelated to vaccine: A total of 86 serious adverse events were recorded across all groups, distributed roughly equally between vaccine and placebo recipients; two infant deaths and one maternal death occurred, none attributed to the study vaccine. Five stillbirths were observed among 298 births (approximately 2%), also deemed unrelated to vaccination.
Why This Matters
Maternal immunisation against group B streptococcus is a candidate strategy for protecting neonates during the period of highest GBS disease risk, yet pregnant women living with HIV have historically been excluded from vaccine trials, leaving a critical evidence gap. This trial directly demonstrates that GBS6 achieves equivalent immunogenicity in HIV-positive women and transfers antibodies to their infants, supporting the feasibility of including this high-risk population in maternal GBS vaccination programmes. The findings address an important question about whether immune responses sufficient for placental transfer can be generated in women whose immune status may be altered by HIV infection.
How to Read This Result
These are encouraging phase 2 findings from a well-designed randomised trial, but because the study was not powered to detect differences in clinical GBS disease incidence, whether the observed antibody concentrations translate into protection against neonatal GBS infection — and what antibody thresholds may be protective — remains to be established in larger efficacy trials.
Limitations
The abstract does not explicitly report study limitations.