How Does rVSV-ZEBOV Vaccination Affect Pregnant Women?

The 2014-16 West Africa Ebola outbreak was the largest outbreak of the blood-borne virus on record. The introduction of vaccines trialed during the outbreak has been a major development in the outbreak response strategy currently being deployed in the Democratic Republic of the Congo.

rVSV-ZEBOV is a recombinant, replication-competent vaccine consisting of a vesicular stomatitis virus modified to express a glycoprotein which catalyzes a neutralizing immune response to Ebola virus.

The Sierra Leone Trial to Introduce a Vaccine Against Ebola (STRIVE) took place during 2 periods in 2015 in order to test the safety and efficacy of rVSV-ZEBOV. A new article in the US Centers for Disease Control and Prevention’s Emerging Infectious Diseases journal provided detail on outcomes among women who became pregnant following vaccination.

STRIVE was a phase 2/3, unblinded clinical trial. Adult health care and Ebola frontline workers were individually randomized to receive either immediate or deferred vaccination with an intramuscular dose of rVSV-ZEBOV.

The immediate group was vaccinated between April and August of 2015. The deferred group was vaccinated from September through December of 2015.

Out of 8651 participants enrolled in STRIVE, 3101 were women between 18-49 years of age.

Pregnancy was an exclusion criterion for the study. During pre-enrollment screening, women between 18-49 years of age were asked if they were pregnant and required to take a urine pregnancy test. This process was repeated for women in the deferred vaccination group. After vaccination, women were advised to avoid pregnancy for 60 days.

During monthly follow-up calls, women were asked about pregnancy. A total of 84 women were found to be inadvertently vaccinated early in pregnancy or to have become pregnant within 60 days after vaccination.

The 84 pregnancies led to 51 live births, 30 pregnancy losses, and 3 cases in which pregnancy outcome was unknown. There were no ectopic pregnancies or neonatal deaths reported.

Out of the 51 live births, 29 occurred in vaccinated women and 22 were in women from the deferred group who had not yet been vaccinated.

A total of 7 serious adverse events were reported among pregnant participants. There were 2 hospitalizations for gestational hypertension, 2 for prolonged labor, and 1 for postpartum hemorrhage that led to a maternal death. The other 2 serious adverse events were not considered related to pregnancy.

Overall, pregnancy loss occurred more frequently in the immediate vaccination group than in the unvaccinated group, though the difference was not statistically significant. In the vaccinated group, 14 of 31 pregnancies ended in pregnancy loss, compared with 11 of 33 in the unvaccinated group.

The authors of the article considered the information valuable, but not conclusive, regarding pregnancy outcomes in women vaccinated with rVSV-ZEBOV. While the difference in pregnancy losses was not statistically significant, the authors cautioned reading too much into a limited sample size.

“Only a small number of pregnancies occurred among participants in STRIVE, and data from larger study samples would be needed to rule out a meaningful difference in the percentage of pregnancy losses,” the authors wrote.

In light of the increase in pregnancy losses and many ambiguities the authors recommended caution.

“These data highlight the need for Ebola vaccination decisions to balance the possible risk for an adverse pregnancy outcome with the risk for Ebola exposure.”