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Summary of Adverse Events in Pregnant Women Following Administration of TIV and LAIV in VAERS, 1990-2009

The Centers for Disease Control and Prevention (CDC) recently conducted a review of reports to the Vaccine Adverse Event Reporting System (VAERS) of pregnant women who received seasonal flu vaccines. The review was done to assess potential vaccine safety concerns. The results of this study were published in the article, “Adverse Events in Pregnant Women Following Administration of TIV and LAIV in VAERS, 1990-2009” and are available in the American Journal of Obstetrics and Gynecology.

Overall, the study concluded that no unusual patterns of pregnancy complications or fetal outcomes were observed in the VAERS reports of pregnant women after the administration of trivalent inactivated influenza vaccine (TIV) or the flu shot, or the live attenuated influenza vaccine (LAIV) or the flu nasal spray. Data from CDC’s 19-year review of VAERS add to an existing body of evidence supporting the safety of the flu shot in pregnant women; the nasal spray vaccine is not recommended for use in pregnant women.

CDC and the Food and Drug Administration (FDA) will continue to monitor vaccine safety during the 2010-11 flu season.

Below are additional results of this study.

Study Results:

  • CDC searched for VAERS reports of adverse events in pregnant women who were administered the flu shot from July 1, 1990 through June 30, 2009, or nasal spray vaccine from July 1, 2003 through June 30, 2009.
  • Two medical officers conducted an independent clinical review of all VAERS reports retrieved to distinguish between pregnancy-related and non-pregnancy-related reports.
  • CDC identified a total of 148 reports of AEs in pregnant women following administration of the flu shot and 27 reports after the inadvertent administration of nasal spray vaccine.
  • No maternal deaths were reported.
  • No specific AEs were recorded in 20.3% of flu shot and 59% of nasal spray vaccine reports.
  • Twenty flu shot (13.5%) and 1 nasal spray vaccine (4%) related reports were classified as serious.
  • According to the Code of Federal Regulations, a report is considered serious if an adverse event results in death, life-threatening illness, hospitalization or prolongation of hospitalization, permanent disability or a birth defect to the vaccine recipient.
  • The most common pregnancy-related adverse event was spontaneous abortion - 17 reports (11.5%) after the flu shot and 3 reports (11%) following administration of the nasal spray vaccine.*
  • The reporting rate of spontaneous abortion was 1.9 per million pregnant women vaccinated with both the flu shot and flu nasal spray vaccine.*
  • The second most common pregnancy-specific adverse event following administration of the flu shot was stillbirth—6 reports (4%).*
  • A total of 7 (4.7%) congenital malformations were reported as the main diagnosis.
  • Three reports described women with Guillian-Barre syndrome (GBS); medical records allowed verification of this diagnosis in 2 cases.
  • Three reports of Bell’s palsy were identified, but information was not sufficient to verify this diagnosis.

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Limitations of VAERS

  • When reviewing data from VAERS, please keep in mind what the system is designed to do and what it is unable to do:
    • VAERS is a national reporting system, in which reports are submitted voluntarily by people who think an adverse event occurred after vaccination. VAERS does not solicit reports in any systematic way from all people who have been vaccinated. Reports can be submitted by anyone, including health care providers, patients, or family members. Because of this feature, VAERS reports may and often do include incorrect and incomplete information. VAERS reports often lead to more complete follow-up and review of medical records.
    • VAERS staff follow-up on all serious and other selected adverse event reports and obtain additional medical, laboratory, and/or autopsy records when available. As a result of the follow-up/review process, coding terms for individual VAERS reports may change based on the information received. VAERS data are publicly available through the U.S. Vaccine Adverse Event Reporting System (VAERS; http://vaers.hhs.gov) and CDC’s website, WONDER (http://wonder.cdc.gov/vaers.html); the public VAERS data should be used with caution because the numbers and conditions do not reflect data collected during follow-up. Events reported in VAERS should not be viewed as evidence that the vaccine directly caused the event. Reports do not infer causality; further investigation is required.
    • Underreporting, or failure to report events, is also one of the main limitations of VAERS. Serious medical events are more likely to be reported than minor events.
    • Most importantly, VAERS cannot usually determine cause-and-effect. VAERS accepts all reports without regard to whether or not the event was caused by the vaccine. The report of an adverse event to VAERS does not mean that a vaccine caused the event. It only indicates that the event occurred sometime after administration of the vaccine. Proof that the event was caused by the vaccine is not required in order for VAERS to accept the report.
    • No reports are deleted from VAERS. Therefore, it is possible to have more than one VAERS report on an individual case (e.g., a physician and a patient may have filed separate reports for the same case).
    • For all serious reports, VAERS staff seeks follow-up medical records on each case, and medical officers review them closely to determine if any additional action or studies may be needed.
    • The most reliable information about vaccine side effects can be found in the manufacturers’ vaccine package insert, vaccine information statements (VISs), or the Advisory Committee on Immunizations Practice’s (ACIP's) statements on vaccines.

* Previous studies have shown that approximately 10.4% - 22.4% of all pregnancies result in spontaneous abortion and almost 0.5% of pregnancies result in stillbirth regardless of vaccination.

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