Maternal Vaccine Promising for Invasive Group B Strep in Infants

A vaccine given during pregnancy produced antibodies against group B streptococcus that were transferred to infants at IgG thresholds associated with a reduced risk of invasive group B streptococcal disease, a phase II randomized placebo-controlled study from South Africa showed.

Among pregnant women who received a hexavalent capsular polysaccharide (CPS)-cross-reactive material glycoconjugate vaccine (GBS6), antibody responses to all serotypes were induced, with maternal-to-infant antibody ratios of about 0.4 to 1.3, depending on the dose received, resulting in concentrations associated with a reduced risk of invasive disease, reported Annaliesa Anderson, PhD, of Pfizer’s Vaccine Research and Development in Pearl River, New York, and colleagues.

In a parallel sero-epidemiologic study that was conducted in the same population, the researchers determined that IgG thresholds of 0.184 to 0.827 μg/mL are needed for a 75% to 95% reduction in the risk of disease, they noted in the New England Journal of Medicine.

“The percentage of infants with anti-CPS IgG concentrations above 0.184 μg per milliliter varied according to serotype and formulation, with 57 to 97% of the infants having a seroresponse to the most immunogenic formulation,” Anderson and colleagues explained.

Of note, there were no safety concerns for mothers or their infants, they said.

For newborns, the primary risk factor for invasive group B streptococcal disease is exposure to maternal rectovaginal group B streptococcal colonization during delivery, and ascending group B streptococcal infection in mothers can also affect the fetus before delivery, with risks of intra-amniotic infection, premature labor, or stillbirth.

Pregnant women who test positive for colonization of group B streptococcus in their third trimester are treated with intrapartum antibiotic prophylaxis, which is more than 80% effective in the prevention of early-onset disease in infants during the first 6 days after birth, but has not been shown to be effective against late-onset disease at 7 to 89 days.

Anderson and colleagues said that a maternal group B streptococcus vaccine given during pregnancy could potentially prevent both early- and late-onset disease, and “may mitigate the need for intrapartum antibiotic prophylaxis in otherwise healthy women.”

“Such a vaccine could be beneficial because intrapartum antibiotic prophylaxis may contribute to antimicrobial resistance and disrupt development of the infant microbiome,” they added. “It is important to note that this vaccine would provide a much-needed measure in the prevention of group B streptococcal infection worldwide and to a substantial percentage of pregnant women living in resource-limited community settings where microbiologic screening and intrapartum antibiotic prophylaxis are unavailable.”

In an editorial comment, Carol Baker, MD, of the McGovern Medical School at the University of Texas Health Science Center in Houston, noted that commercial apathy toward vaccinating pregnant women has slowed development of maternal vaccines, but that may be reaching a turning point.

“The protracted apathy was mitigated during the 2009 H1N1 influenza pandemic, when for the first time a substantial percentage of pregnant women received an influenza vaccine; then by the 2014 recommendation to give pregnant women a pertussis booster vaccine called Tdap (tetanus-diphtheria-acellular pertussis) to prevent death from pertussis in young infants; and finally during the coronavirus disease 2019 pandemic, when messenger RNA vaccines were recommended for pregnant women,” Baker wrote.

“At last, a maternal vaccine platform is moving toward widespread acceptance by women and their healthcare providers,” she added.

Although these results apply only to a South African population with a high disease burden among infants, she noted, “the road to group B streptococcal vaccine licensure could occur through an accelerated pathway based on an established serologic correlate of protection followed by confirmation of effectiveness.”

For this study, Anderson and colleagues first performed a case-control, longitudinal, observational cohort study involving 17,752 mothers. Infants were followed for the development of invasive group B streptococcal disease through 89 days after birth, and 28 cases were observed.

In the ongoing phase II trial conducted at three clinical research centers, 360 healthy pregnant women ages 18 to 40 were randomized to the GBS6 vaccine in three different dose groups — 5 μg, 10 μg, or 20 μg per serotype of GBS6 — with or without aluminum phosphate or placebo. Participants were tested for serotype Ia, Ib, II, and III anti-CPS IgG concentrations.

Solicited systemic events were similar among the vaccine groups and the placebo group, with most events mild or moderate. Severe systemic events were reported in four GBS6 recipients and four placebo recipients, with two GBS6 recipients having severe fever lasting 1 day.

Among the mothers, unsolicited adverse events occurred in 45% to 70% of the participants in the vaccine groups and in 61% of those in the placebo group. The most common adverse events and serious adverse events were in the system organ class of pregnancy, puerperium, and perinatal conditions, with the most common event being fetal distress syndrome. One stillbirth occurred in a GBS6 recipient that was deemed to be unrelated to the vaccine.

Among the infants, adverse events occurred in 62% to 75% of the participants in the GBS6 groups and in 74% of those in the placebo group, with the most common event being upper respiratory tract infection. There were three infant deaths (one in the group that received 5-μg GBS6 without aluminum phosphate and two in the placebo group), which were all deemed to be unrelated to the trial vaccine or placebo. The most common serious adverse events were minor congenital anomalies (umbilical hernia and congenital dermal melanocytosis).

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