Group B Strep in Newborn Babies: What Reading the Germs' Genes Tells Us About Premature Babies and Future Vaccines

Two national studies from the Netherlands and Denmark on why these infections are still rising, why very premature babies tend to fall ill later, and how well vaccines given in pregnancy might match the germs

Group B Strep is a common bacterium that can cause rare but serious infections in young babies. Two large studies from the Netherlands and Denmark found that these infections have become more common in Dutch babies despite antibiotics given during labour, that babies born very early tend to fall ill later and with a wider mix of strains, and that two vaccines being tested in pregnancy target the strains behind almost every infection.

This article explains what the Dutch study [1] and the Danish study [2] found, how they fit together, and what they may mean for families of newborn and premature babies.

Why a common bacterium still matters for newborns

Group B Streptococcus, usually shortened to GBS or "Group B Strep", is a bacterium that many healthy women carry in the gut or vagina without any symptoms. Worldwide, an estimated 19.7 million pregnant women were carrying it in 2020 [3]. Most babies exposed to it stay perfectly well. A small number, however, develop an infection of the bloodstream (called sepsis) or of the lining around the brain and spinal cord (called meningitis). Doctors divide these infections into two kinds: "early-onset" infections that start in the first week after birth, and "late-onset" infections that start later, up to about three months of age. Because GBS infections can have lasting effects, researchers want to understand exactly which strains cause them and which babies are most at risk.

How doctors have tried to protect babies until now

The story of protecting babies from GBS has two threads. The first began in 1976, when researchers found that mothers whose babies became seriously ill with one common type of GBS lacked protective antibodies, the proteins the immune system makes to fight germs, while most mothers who carried the same type but had healthy babies had them [4]. That suggested a mother's antibodies, passed to her baby through the placenta, can protect the baby. It planted the idea of a vaccine given in pregnancy, but no such vaccine has yet been approved [5].

The second thread became everyday practice: giving antibiotics through a drip to mothers during labour. In the United States this helped reduce early-onset infections, but late-onset infections did not fall [6], and by 2015 they had become more common than early-onset ones [7]. Countries also choose differently who receives the antibiotics. Some test all pregnant women for GBS; others, including the Netherlands and Denmark, offer antibiotics only when a mother or her labour has certain warning signs, known as risk factors. Combined results from many studies found fewer early-onset infections where women were tested than where risk factors were used [8]. In the United Kingdom and Ireland, early-onset infections had not decreased despite national prevention guidelines [9]. In the Netherlands, an earlier genetic study had already shown that infections were becoming more common, driven by particular bacterial families [10]. What nobody knew was whether this rise had continued, and whether babies born early are infected by the same strains, at the same ages, as babies born at full term.

What the two research teams did

Both teams used a powerful laboratory method: reading the complete genetic code of each GBS sample, which works rather like a family tree. Bacteria with very similar code belong to the same "family", which scientists call a clonal complex and label with a number, such as CC17. The genetic code also shows which surface features each germ carries, including the features that vaccines are designed to recognise, so the same data can show whether a future vaccine would "match" the germs making babies ill.

The Dutch team, Groenveld and colleagues, studied 2,212 infections in 2,180 babies up to three months old whose samples were sent to the national reference laboratory in Amsterdam over 37 years, from mid-1987 to mid-2024 [1]. The Danish team, Murra and colleagues, collected samples from 664 babies under one year old with GBS infection between 2000 and 2023, read the genetic code of 529 of them, and linked 510 to national birth records showing how many weeks of pregnancy each baby had completed [2]. This let them compare babies born at full term (37 weeks or later), moderately early (32 to 36 weeks) and very early (before 32 weeks). The Danish team counted infections in the first three days of life separately from those starting between day 4 and three months. Neither study recorded whether each mother had received antibiotics in labour. The two studies were carried out separately, in neighbouring countries that both give antibiotics in labour based on risk factors; the Dutch one shows how the infections have changed over almost four decades, and the Danish one shows how they differ depending on how early a baby is born.

Infections in Dutch babies have roughly tripled

In 1987, about 19 in every 100,000 Dutch babies born developed a GBS infection. By 2023 this had risen to about 57 in every 100,000, with a peak of 84 per 100,000 in 2020 [1]. The increase came from bloodstream infections; meningitis stayed at a similar level throughout. The researchers checked that this rise was real, and not simply the result of more samples from older babies being sent in after the rules changed in 2016: among babies aged four weeks or younger, the rate also more than doubled. Because only infections confirmed by a laboratory test were counted, the true number of affected babies is probably somewhat higher. Importantly, the researchers found no sign that early-onset infections fell after the Netherlands introduced risk-based antibiotics in labour in 1998. One bacterial family, CC17, grew from causing about 29% of infections in the first decade to about 49% in the last, and it was especially linked to meningitis.

Babies born very early tend to fall ill later, and with a wider mix of strains

The Danish study adds something new. About six in ten full-term babies with GBS (60%) and about half of moderately early babies (48%) became ill in the first three days of life. Among the 88 babies born before 32 weeks, only 8 (9%) did; 70 of them (80%) fell ill between day 4 and three months [2]. The very early babies were also infected by a wider range of bacterial families. Among babies who fell ill between day 4 and three months, the CC17 family caused about 71% of infections in full-term and moderately early babies combined, but only 31% in very premature babies, where two other families caused 27% and 19%. The researchers think that antibiotics routinely given in labour before 35 weeks might delay, rather than prevent, some infections in very premature babies, but this idea still needs testing, because the researchers did not know which mothers had received antibiotics.

What this means for vaccines given in pregnancy

Two vaccines for pregnant women are being tested. One, called GBS6, targets the sugary coat of six common GBS types. The other, called AlpN, targets four proteins on the bacterial surface. Both studies checked whether the strains causing infection carry these targets. In the Netherlands, 97% of infections were caused by strains matching GBS6 and 99% by strains matching AlpN [1]. In Denmark, every one of the 529 strains matched AlpN, and 516 matched GBS6 [2]. Families should know that three of the Danish researchers receive research support from MinervaX, the company developing AlpN, and one advises the company, although the authors state it had no role in the study. In a company-funded, mid-stage (phase 2) trial involving 272 pregnant women at ten clinics in South Africa, Denmark and the United Kingdom, AlpN had an acceptable safety record and produced antibodies that were found in the babies' cord blood at birth [11]. You can read more about GBS6 in our article on the GBS6 maternal vaccine.

A good match on paper is not the same as protection. How many of a mother's antibodies cross the placenta depends partly on how far the pregnancy has progressed [12], and the Danish team notes that most of this transfer happens late in pregnancy, increasing a great deal after about 36 weeks. Babies born very early may therefore receive less protection from a vaccine given in pregnancy, and whether they are protected is not yet known.

What this means for your family

If your baby was born early or is in a neonatal unit, these findings are not a reason for alarm. Serious GBS infection remains uncommon, it can be treated with standard antibiotics, and the bacterium has stayed fully sensitive to penicillin [1]. What the studies show is that the risk does not end in the first few days, especially for babies born very early, which is why neonatal teams keep watching for infection throughout a premature baby's stay. Babies born prematurely are known to be at greater risk of late-onset GBS infection [13]. Antibiotics in labour still help reduce early infections, especially where all mothers are tested [8], but they cannot prevent all of them, and they do not prevent later ones. If you are worried that your baby seems unwell, the right step is always to tell your care team promptly.

Some babies who have had GBS infection need extra check-ups as they grow, and a large study from Denmark and the Netherlands found higher rates of developmental difficulties years later after these infections [14]. Your team can explain what follow-up suits your child.

What researchers are working on next

Researchers now want to find out whether vaccines given in pregnancy protect babies, including those born very early, and when in pregnancy vaccination would work best. The Dutch team believes vaccination should be urgently considered because the current antibiotic approach has not stopped the rise in infections. The Danish team recommends that future tracking of GBS should always look at premature babies separately, so that any vaccine is judged on how well it protects the babies who need it most.

References

  1. Groenveld MA, Jamrozy D, Visser DH, et al. Group B streptococcal disease in infants 0-3 months in The Netherlands, 1987-2023: a nationwide genomic and epidemiological surveillance study. Lancet Reg Health Eur. 2026;66:101711. doi:10.1016/j.lanepe.2026.101711 ↩
  2. Murra M, Henriksen TB, Andersen M, et al. Gestational age modifies associations between Group B Streptococcus genomic characteristics and postnatal age at disease onset. Nat Commun. 2026;17(1):10246. doi:10.1038/s41467-026-76999-y ↩
  3. Gonçalves BP, Procter SR, Paul P, et al. Group B streptococcus infection during pregnancy and infancy: estimates of regional and global burden. Lancet Glob Health. 2022;10(6):e807-e819. doi:10.1016/S2214-109X(22)00093-600093-6) ↩
  4. Baker CJ, Kasper DL. Correlation of maternal antibody deficiency with susceptibility to neonatal group B streptococcal infection. N Engl J Med. 1976;294(14):753-756. doi:10.1056/NEJM197604012941404 ↩
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  7. Nanduri SA, Petit S, Smelser C, et al. Epidemiology of Invasive Early-Onset and Late-Onset Group B Streptococcal Disease in the United States, 2006 to 2015: Multistate Laboratory and Population-Based Surveillance. JAMA Pediatr. 2019;173(3):224-233. doi:10.1001/jamapediatrics.2018.4826 ↩
  8. Hasperhoven GF, Al-Nasiry S, Bekker V, Villamor E, Kramer B. Universal screening versus risk-based protocols for antibiotic prophylaxis during childbirth to prevent early-onset group B streptococcal disease: a systematic review and meta-analysis. BJOG. 2020;127(6):680-691. doi:10.1111/1471-0528.16085 ↩
  9. O'Sullivan CP, Lamagni T, Patel D, et al. Group B streptococcal disease in UK and Irish infants younger than 90 days, 2014-15: a prospective surveillance study. Lancet Infect Dis. 2019;19(1):83-90. doi:10.1016/S1473-3099(18)30555-330555-3) ↩
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  11. Heath PT, Zuma-Gwala N, Helmig RB, et al. Immunogenicity and safety of a group B Streptococcus vaccine (GBS-AlpN) in pregnant women and their infants: a phase 2, multicentre, observer-blind, randomised, placebo-controlled study. Lancet Infect Dis. 2026;26(5):486-496. doi:10.1016/S1473-3099(25)00659-000659-0) ↩
  12. Palmeira P, Quinello C, Silveira-Lessa AL, Zago CA, Carneiro-Sampaio M. IgG placental transfer in healthy and pathological pregnancies. Clin Dev Immunol. 2012;2012:985646. doi:10.1155/2012/985646 ↩
  13. Karampatsas K, Davies H, Mynarek M, Andrews N, Heath PT, Le Doare K. Clinical Risk Factors Associated With Late-Onset Invasive Group B Streptococcal Disease: Systematic Review and Meta-Analyses. Clin Infect Dis. 2022;75(7):1255-1264. doi:10.1093/cid/ciac206 ↩
  14. Horváth-Puhó E, van Kassel MN, Gonçalves BP, et al. Mortality, neurodevelopmental impairments, and economic outcomes after invasive group B streptococcal disease in early infancy in Denmark and the Netherlands: a national matched cohort study. Lancet Child Adolesc Health. 2021;5(6):398-407. doi:10.1016/S2352-4642(21)00022-500022-5) ↩