HPV Vaccination Linked to Lower Preterm Birth Risk in Swedish Study

A nationwide Swedish study involving over 624,000 births suggests that quadrivalent human papillomavirus (HPV) vaccination before pregnancy is associated with a modest reduction in the risk of preterm birth. The findings, published in the BMJ, indicate that the protective association is most pronounced among women who received the vaccine before age 17.

Study Findings on Pregnancy Outcomes

The study examined nulliparous women aged 16 to 35 who gave birth to singleton infants in Sweden between 2006 and 2023. Approximately 15% of the participants had received at least one dose of the quadrivalent HPV vaccine before conception. After adjusting for factors such as maternal age, smoking, and body mass index, researchers found that vaccinated women had 5% lower odds of preterm birth and spontaneous preterm birth (sPTB) compared to unvaccinated women. The odds of very preterm birth were 15% lower.

When stratified by age at vaccination, the protective effect was stronger for those vaccinated before age 17, who showed 10% lower odds of preterm birth and 11% lower odds of sPTB. For women vaccinated at later ages, significant associations were observed only for severe small for gestational age (SGA) infants and stillbirth.

Biological Mechanisms and Study Limitations

Researchers propose two primary pathways for these protective associations: the prevention of HPV infection of the cervix and a reduction in the need for excisional procedures to treat high-grade cervical lesions. HPV infection has been linked to poor placental development, and surgical treatments for cervical lesions are known to increase the risk of adverse pregnancy outcomes. However, the study could not directly confirm which of these mechanisms was responsible for the observed results.

The study's strengths include its large population size and the use of high-quality nationwide registry data. Nevertheless, the authors noted limitations, including the observational nature of the research, which leaves the possibility of residual confounding. Factors such as health-seeking behavior and differences in obstetric interventions were not fully captured. While the findings are consistent with earlier studies from Finland and Denmark, the authors emphasize that this is an exploratory analysis and that further long-term monitoring is required to better understand the impact of HPV vaccination on pregnancy outcomes.

Understanding the Swedish study correctly

An observational finding that vaccinated women experienced fewer preterm births does not show that the HPV vaccine directly prevents premature delivery. Vaccination status may correlate with income, health-service use, screening or other characteristics that influence pregnancy outcomes. Researchers must account for these confounders and report absolute risks, not only relative percentages. The vaccine's established main purpose is to protect against infection with cancer-associated human papillomavirus types; a possible additional obstetric association requires independent confirmation. For readers, this distinction matters because pregnancy planning and immunisation decisions should follow current clinical guidance rather than a novel effect inferred from a single population dataset.