TL;DR
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A Swedish register-based case-control study found lower odds of preterm, very preterm and spontaneous preterm birth among women who received a quadrivalent HPV vaccine before pregnancy. The associations were stronger for some outcomes when vaccination occurred before age 17, but the observational study cannot establish that vaccination caused the differences.
A study of 624,713 singleton births in Sweden found that women vaccinated against human papillomavirus (HPV) before pregnancy had lower odds of preterm, very preterm and spontaneous preterm birth than unvaccinated women. The observational findings, published in The BMJ, describe associations rather than proof that vaccination caused the lower risks.
Researchers led by Jiayao Lei of Karolinska Institute used nationwide registers to examine births from 2006 through 2023 among first-time mothers aged 16 to 35. The analysis focused on women who had received a quadrivalent HPV vaccine before pregnancy; 14.8% of eligible women had received it. The study excluded births where the first HPV vaccine was bivalent or nonavalent.
Compared with no recorded HPV vaccination, vaccination was associated with lower odds of preterm birth (adjusted odds ratio 0.95), very preterm birth (0.85) and spontaneous preterm birth (0.95). The researchers also reported lower odds of preterm prelabor rupture of membranes (0.93) and a severely small-for-gestational-age infant (0.92). These estimates are relative odds, not absolute reductions in the number of affected pregnancies.
For women vaccinated before age 17, the associations were stronger for preterm birth (adjusted odds ratio 0.90) and spontaneous preterm birth (0.89). The study found no significant differences for ordinary preterm rupture of membranes, stillbirth or neonatal mortality in the overall comparison. Some age-specific findings differed: those vaccinated at 17 or older had lower odds of severe small-for-gestational-age birth and stillbirth, but these results do not establish a causal effect.
Possible Benefits Beyond Cancer Prevention
HPV vaccines are used to prevent infection with HPV types associated with cervical and other cancers. The study raises the possibility that vaccination could also be associated with some pregnancy outcomes, adding a potential consideration to evaluations of vaccination programs. The authors said the differences for an individual pregnancy were modest, while suggesting that even modest changes could matter at population scale.
That interpretation needs caution. The research compared outcomes in groups whose vaccination status was recorded; it did not randomly assign vaccination. Differences in health care use, prior HPV exposure, sexual or reproductive behavior, and other factors could contribute to the observed associations. The results do not change the established role of HPV vaccination in cancer prevention or show that vaccination should be given as a pregnancy-risk intervention.
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How the Swedish Study Was Designed
The team conducted a national case-control analysis, matching each adverse pregnancy outcome case with as many as 10 controls without that outcome. Register data allowed researchers to compare vaccination histories and birth outcomes across a large population, but the design remains observational.
One possible pathway involves cervical disease: treatment for high-grade cervical lesions has been linked in previous research to higher risks of preterm birth and other complications. In this study, 0.6% of vaccinated women and 3.7% of unvaccinated women had a recorded high-grade cervical lesion before giving birth. When the researchers adjusted for these diagnoses, associations with preterm-birth outcomes weakened.
The study assessed the quadrivalent vaccine, whereas the U.S. routine vaccine is Gardasil 9, a nine-valent vaccine. The American Academy of Pediatrics recommends a two-dose series beginning between ages 9 and 12. Results from this Swedish analysis do not directly establish whether its findings apply to other vaccine formulations or schedules.
“Our findings highlight the importance of timely HPV vaccination and suggest that its potential benefits may extend beyond cancer prevention to reproductive health.”
— Jiayao Lei and Zhongsong Zhang, Karolinska Institute researchers, speaking to MedPage Today
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Causation and Mechanism Remain Unsettled
The study cannot show that HPV vaccination itself reduced pregnancy risks. Potential residual confounding includes catch-up or opportunistic vaccination, earlier HPV exposure, and sexual or reproductive behaviors, particularly among women vaccinated at age 17 or older. Researchers also could not fully assess differences in preventive interventions for preterm birth.
The weaker associations after accounting for high-grade cervical lesions are consistent with one possible explanation: vaccination may reduce lesions and subsequent cervical treatments that are associated with preterm birth. But the study lacked detailed information about cervical treatment, so it cannot confirm that pathway. The authors also discussed possible effects of HPV on embryonic development or placental function, but these explanations remain hypotheses.
The report gives adjusted odds ratios, but the supplied findings do not provide absolute risk differences for each outcome. Readers therefore cannot use these figures alone to estimate how many pregnancies might be affected or how much an individual’s risk changes.
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Further Studies Could Test the Link
The authors called for policy and cost-effectiveness analyses to examine HPV vaccination’s cancer-prevention effects alongside possible reductions in pregnancy complications and related health costs. Further research would need to assess whether the associations persist in other populations and with other vaccine formulations, and to distinguish the role of vaccination from cervical lesions, treatment and other differences between groups.
For now, the study adds observational evidence but does not establish a new pregnancy-related indication for HPV vaccination. The findings should be read alongside existing vaccination guidance and the limits of the Swedish study population and design.
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Key Questions
Did the study prove HPV vaccination prevents preterm birth?
No. It found associations between prior vaccination and lower odds of several preterm birth outcomes. Because it was observational, it cannot establish that vaccination caused those differences.
Which vaccine did the researchers study?
The analysis included women who received the quadrivalent HPV vaccine before pregnancy. It excluded births where the woman’s first HPV vaccination was bivalent or nonavalent.
Were the findings stronger for younger vaccine recipients?
For preterm and spontaneous preterm birth, the reported associations were stronger among women vaccinated before age 17. The study does not show that age alone explains the difference.
Did vaccination relate to every pregnancy outcome studied?
No. The researchers reported lower odds for several outcomes, including very preterm birth and severe small-for-gestational-age birth, but found no significant overall differences for ordinary preterm rupture of membranes, stillbirth or neonatal mortality.
Does this change HPV vaccine recommendations?
The study reports observational findings and does not establish a new pregnancy-related indication. The U.S. recommendation cited in the report is for the nine-valent vaccine, with a two-dose series starting between ages 9 and 12.
Source: rss
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