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Vaccination against human papillomavirus (HPV) during pregnancy did not increase the risk of maternal or fetal complications, a retrospective, matched, case-control study showed.

Comparison of 1,800 vaccine-exposed pregnancies and 7,000 unexposed pregnancies showed no significant differences in the frequency of six adverse outcomes. Unadjusted analyses did show higher rates of low birth weight, preterm birth, and major birth defects, but none of the differences remained significant in the propensity-matched analyses, reported Nikolai M. Scheller, MD, of the Statens Serum Institut in Copenhagen, and colleagues.

"Our results are consistent with other evidence that does not indicate that the vaccination of pregnant women with inactivated virus, bacterial, or toxoid vaccines generally confers a higher risk of adverse pregnancy outcomes than no such vaccination," they wrote in the . "Our results also confirm and considerably expand on results from previous studies of the quadrivalent HPV vaccine."

Five phase III, randomized clinical trials of the quadrivalent HPV vaccine, , showed no increase in the risk of spontaneous abortion, stillbirth, or birth defects. However, most pregnancies occurred more than 6 months after exposure to the vaccine, precluding any direct assessment of the vaccine's risk during pregnancy, Scheller's group noted.

Given the global recommendation for vaccination of girls and women, ages 9 to 26 years, against HPV, some women inevitably will have inadvertent exposure to the vaccine during early pregnancy. As the authors suggested, limited data have accumulated regarding the safety of the quadrivalent HPV vaccine during pregnancy.

In an effort to help fill the data void, investigators conducted a study that included identification of all pregnancies occurring in Denmark from Oct. 1, 2006 through Nov. 30, 2013. Using nationwide registry data, researchers collected information on vaccination, adverse pregnancy outcomes, and potential confounding factors among women included in data analysis.

Scheller's group identified women who had exposure to the quadrivalent HPV vaccine (women who received the bivalent vaccine were excluded) during prespecified time windows that varied according to the type of pregnancy complication: first trimester for birth defects; weeks 7 through 22 for spontaneous abortion; week 7 until birth for stillbirth; before 37 weeks of gestation for preterm birth; and any time during pregnancy for low birth weight and small for gestational age.

Subsequent data analysis included 463 vaccine-exposed women for spontaneous abortion; 1,665 for for major birth defects; 501 for stillbirth; 1,774 for preterm birth; and 1,768 for low birth weight and small for gestational age. For propensity analyses, each vaccinated woman was matched with four unvaccinated women.

The data showed that 65 birth defects in vaccine-exposed pregnancies versus 220 in unexposed pregnancies; 20 spontaneous abortions with vaccine exposure versus 131 without; 116 preterm births versus 407; 76 cases of low birth weight versus 277; 171 cases of small size for gestational age versus 783; and two stillbirths versus four. The absolute numbers translated into the following odds ratios for vaccine-exposed versus unexposed pregnancies:

• Major birth defect: OR 1.19 (95% CI 0.90-1.58)
• Spontaneous abortion: OR 0.71 (95% CI 0.45-1.14)
• Preterm birth: OR 1.15 (95% CI 0.93-1.42)
• Low birth weight: OR 1.10 (95% CI 0.85-1.43)
• Small for gestational age: OR 0.86 (95% CI 0.72-1.02)
• Stillbirth: OR 2.43 (95% CI 0.45-13.21)

Unadjusted analyses performed prior to propensity-score matching showed significantly increased risk associated with the quadrivalent HPV vaccine for low birth weight (OR 1.26, 95% CI 1.00-1.59), preterm birth (OR 1.38, 95% CI 1.14-1.67), and major birth defect (OR 1.36, 95% CI 1.06-1.75). Nonsignificant hazard ratios emerged from unadjusted analyses of spontaneous abortion (HR 0.93, 95% CI 0.60-1.44), small for gestational age (HR 0.98, 95% CI 0.83-1.14), and still birth (HR 1.90, 95% CI 0.48-7.61).

The authors acknowledged several limitations of their study: potential for misclassification of physician-assigned pregnancy outcomes, possible unidentified confounding factors, and limited statistical power for some analyses because of small numbers of events.

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