Preliminary Findings of Covid-19 Vaccine Safety in Pregnant People

Many pregnant persons in the United States are receiving coronavirus disease 2019 (Covid-19) vaccines, but data are limited on their safety in pregnancy. From December 14, 2020, to February 28, 2021, we used data from the “v-safe after vaccination health checker” surveillance system, the v-safe pregnancy registry, and the Vaccine Adverse Event Reporting System (VAERS) to characterize the initial safety of mRNA Covid-19 vaccines in pregnant persons.

RESULTS

A total of 35,691 v-safe participants 16 to 54 years of age identified as pregnant. Injection-site pain was reported more frequently among pregnant persons than among nonpregnant women, whereas headache, myalgia, chills, and fever were reported less frequently. Among 3958 participants enrolled in the v-safe pregnancy registry, 827 had a completed pregnancy, of which 115 (13.9%) resulted in a pregnancy loss and 712 (86.1%) resulted in a live birth (mostly among participants with vaccination in the third trimester). Adverse neonatal outcomes included preterm birth (in 9.4%) and small size for gestational age (in 3.2%); no neonatal deaths were reported. Although not directly comparable, calculated proportions of adverse pregnancy and neonatal outcomes in persons vaccinated against Covid-19 who had a completed pregnancy were similar to incidences reported in studies involving pregnant women that were conducted before the Covid-19 pandemic. Among 221 pregnancy-related adverse events reported to the VAERS, the most frequently reported event was spontaneous abortion (46 cases).

CONCLUSIONS

Preliminary findings did not show obvious safety signals among pregnant persons who received mRNA Covid-19 vaccines. However, more longitudinal follow-up, including follow-up of large numbers of women vaccinated earlier in pregnancy, is necessary to inform maternal, pregnancy, and infant outcomes.

The first coronavirus disease 2019 (Covid-19) vaccines available in the United States were messenger RNA (mRNA) vaccines: BNT162b2 (Pfizer–BioNTech) and mRNA-1273 (Moderna). In December 2020, the vaccines were granted Emergency Use Authorization (EUA) by the Food and Drug Administration (FDA) as a two-dose series, 3 weeks apart for Pfizer–BioNTech and 1 month apart for Moderna, and were recommended for use by the Advisory Committee on Immunization Practices (ACIP).^1-4 Pregnant persons were excluded from preauthorization clinical trials, and only limited human data on safety during pregnancy were available at the time of authorization. However, pregnant persons with Covid-19 are at increased risk for severe illness (e.g., resulting in admission to an intensive care unit, extracorporeal membrane oxygenation, or mechanical ventilation) and death, as compared with nonpregnant persons of reproductive age.⁵ Furthermore, pregnant persons with Covid-19 might be at increased risk for adverse pregnancy outcomes, such as preterm birth, as compared with pregnant persons without Covid-19.⁶ The Centers for Disease Control and Prevention (CDC) and ACIP, in collaboration with the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics, have issued guidance indicating that Covid-19 vaccines should not be withheld from pregnant persons.^7-9

Postauthorization monitoring in pregnant persons is necessary to characterize the safety of these new Covid-19 vaccines, which use mRNA, lipid nanoparticles, and state-of-the-art manufacturing processes. Furthermore, establishing their safety profiles is critical to inform recommendations on maternal vaccination against Covid-19. We report preliminary findings of mRNA Covid-19 vaccine safety in pregnant persons from three U.S. vaccine safety monitoring systems: the “v-safe after vaccination health checker” surveillance system,¹⁰ the v-safe pregnancy registry,¹¹ and the Vaccine Adverse Event Reporting System (VAERS).¹²

MONITORING SYSTEMS AND COVERED POPULATIONS

V-safe Surveillance System and Pregnancy Registry

V-safe is a new CDC smartphone-based active-surveillance system developed for the Covid-19 vaccination program; enrollment is voluntary. V-safe sends text messages to participants with weblinks to online surveys that assess for adverse reactions and health status during a postvaccination follow-up period. Follow-up continues 12 months after the final dose of a Covid-19 vaccine. During the first week after vaccination with any dose of a Covid-19 vaccine, participants are prompted to report local and systemic signs and symptoms during daily surveys and rank them as mild, moderate, or severe; surveys at all time points assess for events of adverse health effects. If participants indicate that they required medical care at any time point, they are asked to complete a report to the VAERS through active telephone outreach.

To identify persons who received one or both Covid-19 vaccine doses while pregnant or who became pregnant after Covid-19 vaccination, v-safe surveys include pregnancy questions for persons who do not report their sex as male. Persons who identify as pregnant are then contacted by telephone and, if they meet inclusion criteria, are offered enrollment in the v-safe pregnancy registry. Eligible persons are those who received vaccination during pregnancy or in the periconception period (30 days before the last menstrual period through 14 days after) and are 18 years of age or older. For persons who choose to enroll, the pregnancy registry telephone-based survey collects detailed information about the participant, including medical and obstetric history, pregnancy complications, birth outcomes, and contact information for obstetric and pediatric health care providers to obtain medical records; infants are followed through the first 3 months of life. Details about v-safe and v-safe pregnancy registry methods have been published previously.^10,11

OUTCOMES

V-safe outcomes included participant-reported local and systemic reactogenicity to the BNT162b2 (Pfizer–BioNTech) vaccine and the mRNA-1273 (Moderna) vaccine on the day after vaccination among all pregnant persons 16 to 54 years of age and among nonpregnant women 16 to 54 years of age as a comparator. For analysis of pregnancy outcomes in the v-safe pregnancy registry, data were restricted to completed pregnancies (i.e., live-born infant, spontaneous abortion, induced abortion, or stillbirth). Participant-reported pregnancy outcomes included pregnancy loss (spontaneous abortion and stillbirth) and neonatal outcomes (preterm birth, congenital anomalies, small size for gestational age, and neonatal death) (Table S1 in the Supplementary Appendix, available with the full text of this article at NEJM.org). In the VAERS, outcomes included non–pregnancy-specific adverse events and pregnancy- and neonatal-specific adverse events.

STATISTICAL ANALYSIS

Demographic information and pregnancy characteristics are described for both v-safe and VAERS participants. Descriptive analyses were performed with the use of v-safe survey data for persons who identified as pregnant through February 28, 2021 (35,691 persons); persons enrolled in the v-safe pregnancy registry who were vaccinated through February 28, 2021 (3958 persons); and VAERS reports involving pregnant women received through February 28, 2021 (221 persons). Local and systemic reactogenicity was compared between persons who identified as pregnant and nonpregnant women. Descriptive analyses were conducted with the use of SAS software, version 9.4 (SAS Institute). All activities were reviewed by the CDC and were conducted in accordance with applicable federal law and CDC policy.

V-SAFE SURVEILLANCE: LOCAL AND SYSTEMIC REACTOGENICITY IN PREGNANT PERSONS

From December 14, 2020, to February 28, 2021, a total of 35,691 v-safe participants identified as pregnant. Age distributions were similar among the participants who received the Pfizer–BioNTech vaccine and those who received the Moderna vaccine, with the majority of the participants being 25 to 34 years of age (61.9% and 60.6% for each vaccine, respectively) and non-Hispanic White (76.2% and 75.4%, respectively); most participants (85.8% and 87.4%, respectively) reported being pregnant at the time of vaccination (Table 1). Solicited reports of injection-site pain, fatigue, headache, and myalgia were the most frequent local and systemic reactions after either dose for both vaccines (Table 2) and were reported more frequently after dose 2 for both vaccines. Participant-measured temperature at or above 38°C was reported by less than 1% of the participants on day 1 after dose 1 and by 8.0% after dose 2 for both vaccines.

Most Frequent Local and Systemic Reactions Reported in the V-safe Surveillance System on the Day after mRNA Covid-19 Vaccination.

These patterns of reporting, with respect to both most frequently reported solicited reactions and the higher reporting of reactogenicity after dose 2, were similar to patterns observed among nonpregnant women (Figure 1). Small differences in reporting frequency between pregnant persons and nonpregnant women were observed for specific reactions (injection-site pain was reported more frequently among pregnant persons, and other systemic reactions were reported more frequently among nonpregnant women), but the overall reactogenicity profile was similar. Pregnant persons did not report having severe reactions more frequently than nonpregnant women, except for nausea and vomiting, which were reported slightly more frequently only after dose 2 (Table S3).