Why GBS matters in pregnancy
WHO reports that on average about 15% of pregnant women worldwide carry GBS in the vagina, usually without symptoms. The bacterium can pass to the baby in the womb or during labour. A 2021 report from WHO and the London School of Hygiene and Tropical Medicine linked GBS to about 91,000 newborn deaths, at least 46,000 stillbirths and more than 500,000 preterm births each year. In pregnant women, the CDC lists bloodstream infection, intra-amniotic infection, urinary tract infection, preterm delivery and stillbirth among GBS-related complications. In newborns, GBS causes early-onset disease in the first 7 days of life and late-onset disease from 7 to 89 days.
How GBS causes disease
The review describes several virulence factors that allow GBS to cause disease. The capsular polysaccharide helps the bacterium avoid the immune system by inhibiting complement deposition and resisting phagocytosis. Surface proteins and pili help it attach to host cells. One adhesin, HvgA, is specific to the highly virulent CC17 lineage and is closely associated with late-onset disease such as neonatal meningitis. Several of these structures are also being studied as vaccine targets.
Antibiotic resistance
Penicillin remains the preferred drug for preventing GBS infection during labour, and GBS remains highly sensitive to it. The review notes, however, that strains with reduced penicillin susceptibility have been reported sporadically since 1994, linked to changes in penicillin-binding proteins, with a high isolation rate reported in Japan. Resistance to clindamycin, an alternative for some women allergic to penicillin, has risen worldwide; erythromycin is no longer recommended for prophylaxis during labour, and the choice of alternative depends on the severity of the allergy and on susceptibility testing. One study cited in the review found multidrug resistance in 59.19% of GBS isolates from pregnant women in Vietnam. These findings support susceptibility testing when an alternative to penicillin is needed, and continued local surveillance of resistance.
Prevention by screening and antibiotics in labour
Swabs of the vagina and rectum taken late in pregnancy identify carriers so that intravenous antibiotics can be given during labour. The CDC advises screening during the 36th or 37th week of each pregnancy. The review cites American Academy of Pediatrics guidance that antibiotics given at least four hours before delivery are optimal for reaching adequate levels, although delivery should not be delayed only to complete four hours. The CDC notes that this approach has reduced disease in the first week of life, but no effective strategy has yet been identified to prevent late-onset disease.
Progress towards a vaccine
Because screening and antibiotics do not prevent all GBS disease, maternal vaccination is a research priority. Candidates include capsular polysaccharide conjugate vaccines, among them a hexavalent vaccine covering serotypes Ia, Ib, II, III, IV and V, and protein vaccines built from the Alpha C and Rib surface proteins. The review reports that two candidates have entered phase II or III clinical trials. Several vaccine candidates, including one in a global phase 3 trial, remain investigational, and none is yet licensed.