New Study Links Pfizer RSV Vaccine for Pregnant Women to Preterm Births
The first post-authorization safety analysis of Pfizer’s Abrysvo RSV vaccine found the average time between vaccination and preterm birth was three days. Two-thirds of reported cases occurred within a week.
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A new preprint study shows a statistically significant safety signal for preterm birth associated with Pfizer’s respiratory syncytial virus (RSV) vaccine, Abrysvo.
The first post-authorization safety analysis of Pfizer’s RSV vaccine — RSV prefusion F protein (RSVPreF) — found the average time between vaccination and preterm birth was three days.
Two-thirds of reported cases occurred within a week.
“This study highlights ongoing concern about preterm birth among pregnant individuals following RSVPreF vaccination,” the authors wrote.
Canadian researchers at the University of Ottawa School of Epidemiology and Public Health assessed all adverse events reported to the Vaccine Adverse Event Reporting System (VAERS) database following RSV immunization between Sept. 1, 2023, and Feb. 23, 2024.
Among the 77 reports submitted to the database — 55% of which were classified as serious — preterm birth was the most common pregnancy-specific adverse event, followed by preterm premature rupture of membranes, cesarean section, cervical dilatation, and hemorrhage during pregnancy.
Other non-pregnancy-specific adverse events included headache, injection site erythema and injection site pain.
60-year history of RSV vaccine issues
Dr. David Healy, a drug safety expert, told The Defender that the RSV vaccines developed so far have a 60-year history of causing problems.
“It looks like our latest efforts to overcome these problems have not helped and are leading to preterm births which have lifelong knock-on effects along with more serious RSV infections in children where these should be harmless,” said Healy, author of “Pharmageddon.”
“Sixty years ago we recognized the problems and stopped. But now we seem determined to press ahead regardless,” Healy said. “The intense push to get these vaccines means we will all have affected family members — this is not an abstract concern,” he added.
The U.S. Food and Drug Administration (FDA) approved Pfizer’s Abrysvo for pregnant women in August 2023. In September 2023, the CDC recommended it be administered to pregnant women during weeks 32-36 of their pregnancies to protect babies from RSV-associated lower respiratory tract disease after birth.
The American College of Obstetricians and Gynecologists also recommends a single dose of Pfizer’s RSV vaccine for pregnant women.
In a BMJ letter to the editor on Monday, Dr. Peter Selley, a retired U.K. general practitioner, said the preliminary findings by the University of Ottawa researchers provide “good reason” why the U.K.’s vaccine advisory committee “should not rush … into making recommendations on an immunisation policy until further safety studies have been reported.”
Selley told The Defender that although the study is not yet peer-reviewed, it is published on a highly regarded preprint server, medRxiv. “It provides yet another pointer to preterm births being a problem,” he said.
RSV vaccine rolled out despite known preterm birth risk
Pfizer’s own clinical trial data for Abrysvo showed elevated rates of preterm birth among vaccinated women, but the higher rates were not statistically significant, Pfizer said.
Still, the FDA limited approval of the vaccine for women in weeks 32-36 of their pregnancy to reduce risk and mandated post-market follow-up studies for both preterm birth and eclampsia.
The agency also labeled preterm birth as a potential risk associated with the vaccine.
Pfizer also observed more hypertensive disorders of pregnancy among RSVpreF vaccine recipients compared with placebo recipients in its Abrysvo trials. The drugmaker determined the rate of disorders was not statistically significant.
Some members of the FDA’s vaccine advisory committee said they had serious safety concerns based on the clinical trial data, and four members voted against approving the drug.
GSK stopped the development of its RSV vaccine for pregnant women when it found a safety signal for preterm births among vaccinated women. In that study, for every 54 infants born to women who received the vaccine, one additional preterm birth occurred.
Neonatal deaths — the death of an infant in the first 28 days of life — also were higher in the GSK vaccine group, occurring in 0.4% of the infants in the vaccine group (13 of 3,494) and 0.2% in the placebo group (3 of 1,739), which they also noted was not statistically significant.
In contrast, there were no deaths linked to RSV in the GSK study, which enrolled over 10,000 mothers and infants before it was halted.
Although Pfizer’s FDA-approved maternal RSV vaccine is bivalent and GSK’s RSVPreF3-Mat is monovalent, “the vaccines are otherwise similar,” according to an editorial in the New England Journal of Medicine by Dr. Sonja A. Rasmussen and Denise J. Jamieson.
The authors of the article said more data would be required to know whether the vaccine was responsible for the safety signal. “Whether the safety signal in the RSVPreF3-Mat trial is real or occurred by chance is unknown,” they wrote. And even if the signal is real, they argued, “It is essential to weigh this small risk against the proven benefits of maternal RSV vaccination.”
In March, shortly before the editorial was published, Jamieson was named to the CDC’s vaccine advisory committee.
Further research ‘urgently required’
The study adds to the body of evidence that there may be a real safety signal for preterm birth associated with Abrysvo.
To do the study, the researchers extracted all reports of adverse events related to the RSV vaccine from the VAERS database. They narrowed the scope of extracted reports to pregnant women.
They analyzed maternal age, gestational age at the time of vaccination, interval until the onset of the event and reported outcomes.
Using a statistical data mining technique commonly used to detect safety signals — the Bayesian confidence propagation neural network — they calculated the safety signal for adverse events related to the RSV vaccine.
The authors said the strength of their study was that it analyzed VAERS, which is “a comprehensive pharmacovigilance system, with a broad national scope, capacity for near-real time surveillance, and adeptness at detecting rare AEFI” — adverse events following immunization.
The limitations, they said, came from limitations inherent to the database itself, namely that it relies on participant-reported data with less comprehensive health information, and can have both over-reporting and underreporting biases.
Additionally, in VAERS, events temporally close to vaccination are more likely to be reported. Selly said that could explain why the preterm births happened very close to vaccination and other issues, like later preterm births or preeclampsia, didn’t appear in the analysis.
As a result, the study authors said they cannot establish causality, and further research is “urgently required.”
An RSV vaccine for children?
RSV is a common respiratory virus that usually causes mild cold-like symptoms, but in some cases can lead to hospitalization and death in infants and the elderly.
By age 2, 97% of all babies have been infected with the RSV virus, which confers partial immunity, making any subsequent episodes less severe.
But the disease burden for infants can be serious. In the U.S., RSV infection is the leading cause of infant hospitalization among those younger than 6 months, although a very small percentage of children with the virus will die.
According to a CDC study analyzing RSV deaths in infants between 2009 and 2021, there were only a total of 300 deaths in children under age 1, or 25 on average per year, Dr. Meryl Nass reported.
The RSV vaccine has not been approved for infants, but last year, the FDA approved and the CDC recommended a monoclonal antibody shot produced by pharma giants Sanofi and AstraZeneca, Beyfortus, for infants.
Several infant deaths — 12 in all — were reported during the clinical trial, which the FDA claimed were “unrelated” to the antibody.
The drug has been heavily promoted for infants globally, despite a lack of evidence for many of the safety and efficacy claims made by drug manufacturers, public health agencies and professional organizations like the Royal College of Paediatrics and Child Health and the Association of American Pediatricians.
There also have been no long-term studies involving Beyfortus and newborns.
Pfizer announced in a press release last month that it has begun a trial evaluating its RSV vaccine in children ages 2-18 who are at higher risk for RSV disease.
A Defender analysis of the VAERS database showed several instances of severe adverse events in newborns, including the death of a 27-day-old baby who was wrongly administered the vaccine.
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