VACCINATING against bronchiolitis during pregnancy may increase the risk of pre-natal complications and lead to premature birth and infection, according to a new study published in JAMA Network Open.
Researchers investigated Pfizer’s Abrysvo respiratory syncytial virus (RSV) vaccine, the only RSV shot approved for pregnant women in the US and which is meant to protect babies of up to six months old against illness.
According to the study, women who received the vaccine were 29 per cent more likely to experience hypertensive disorders, including preeclampsia, eclampsia and HELLP syndrome (Hemolysis, Elevated Liver enzymes and Low Platelets), a life-threatening pregnancy complication considered a variant of pre-eclampsia.
The study also identified statistically significant signals for pre-labour rupture of membranes (PROM) and preterm PROM.
Although the researchers emphasised that their findings don’t establish causation, they acknowledged the signals were strong enough to warrant further investigation.
The findings, published this week, add to post-marketing studies that have since found links to preterm births and Guillian-Barré syndrome.
Dr Peter Selley told the Defender that although the study claims that globally RSV causes ‘substantial morbidity and mortality in the first year of life’, the authors fail to note that more than 97 per cent of childhood deaths from RSV occur in middle and lower-income countries.
In the 2024-2025 RSV season, 31 children and adolescents died from the illness in the US. The vaccination status of their mothers during pregnancy is not known.
The US Food and Drug Administration (FDA) approved Abrysvo in August 2023, even though members of the FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) raised concerns about premature births identified during Pfizer’s clinical trials.
In September 2023, the Centers for Disease Control and Prevention (CDC) recommended the vaccine for pregnant women at 32-36 weeks of gestation.
The authors of the JAMA study tracked 13,619 pregnant women who took the RSV shot during pregnancy. To assess the safety of Pfizer’s RSV vaccine, the researchers analysed the post-marketing data using insurance and health system data from five sources to compare the women who received the vaccine with pregnant women who received the flu, covid or combined tetanus, diphtheria and whooping cough shots.
They also compared the post-marketing data with historical data from women vaccinated between 2018 and 2023, before the RSV vaccines were available.
The researchers tracked ten predefined safety outcomes, including preterm birth, hypertension, stillbirth and complications involving the amniotic sac. Despite their expectations that the vaccine would prove safe, they found elevated safety signals for several pregnancy complications.
The study was designed as an early ‘sequential surveillance’ analysis — a rapid monitoring system intended to detect potential safety issues soon after a vaccine is introduced.
As a result, the authors noted that their ability to control for confounding factors was limited and that the findings should be interpreted as preliminary rather than definitive.
The study found no increase in preterm births among women who took the shot. The researchers — and media coverage of the study — highlighted this finding as evidence to quell concerns about potential links between Pfizer’s RSV vaccine and preterm birth.
During Pfizer’s clinical trials for Abrysvo, researchers found that preterm birth and hypertensive disorders of pregnancy occurred more frequently among women who received the shot, although they said the differences were not statistically significant.
In 2022, GSK stopped development of its maternal RSV vaccine, which was nearly identical to Pfizer’s, when it detected a similar increase in the number of preterm births among pregnant mothers.
Prescribing information for Abrysvo warns of a potential risk for preterm birth and Guillian-Barré syndrome, which was identified in another post-marketing study.
A different post-marketing study published last year in BMJ Open reported pregnant women vaccinated with Pfizer’s Abrysvo RSV vaccine were more likely to experience preterm births.
The authors of the new JAMA study also downplayed the findings of increased risk and underscored that they are not conclusive. They noted that this is ‘only one analysis in a multiphase approach’ and said more studies are planned.
‘This early monitoring did what it was designed to do: it ruled out some concerns and raised other possible safety questions quickly,’ study co-author Ashley Michnick told MedPage Today. ‘The purpose of early rapid-cycle analyses like those described in this paper is to guide more robust studies, not necessarily to draw firm conclusions about risk.’
Children’s Health Defense senior research scientist Karl Jablonowski said there was a contradiction in the way the authors represented their findings.
‘With confidence, the authors declare no increased risk of preterm birth. With timidity, under the same conditions, they show a risk for pregnancy-related hypertensive disorder and PROM,’ he said. ‘The same grain of salt that comes with a found signal also applies to the lack of a signal.’
The study was sponsored by Pfizer. In a long conflict of interest statement, the researchers reported receiving grants from and holding stock options in the company.
This article appeared in the Defender on April 24, 2026, and is republished by kind permission.
