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September 9, 2009

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NIAID Trial of 2009 H1N1 Influenza Vaccine in Pregnant Women at the Vaccine and Treatment Evaluation Units

1.   Why is it important to test vaccines for 2009 H1N1 influenza in pregnant women?

Pregnant women are at increased risk of complications following infection by seasonal influenza viruses and, similarly, following infection with the 2009 H1N1 influenza virus. Since the beginning of the 2009 H1N1 influenza outbreak, 28 pregnant women have died after becoming infected with the virus, according to the Centers for Disease Control and Prevention (CDC).

Pregnant women infected with the 2009 H1N1 influenza virus also have a higher rate of hospitalization than the general population. At the last official count, 100 pregnant women with 2009 H1N1 infection have been hospitalized in intensive care units in the United States.
U.S. public health officials, following the recommendation of the CDC’s Advisory Committee on Immunization Practices, have designated pregnant women among the top priority groups to receive the 2009 H1N1 influenza vaccine.

Clinical trials of 2009 H1N1 vaccines in pregnant women will supply needed information to public health officials to help answer such questions as how many doses of vaccine are needed to elicit immune responses indicative of protection against the 2009 H1N1 virus. The clinical trials in pregnant women, like other trials of 2009 H1N1 influenza vaccines now being conducted by the National Institute of Allergy and Infectious Diseases (NIAID), are collecting critically needed data. The trials are not intended to serve as a mechanism for mass immunization of any of the populations designated as high-priority groups to receive 2009 H1N1 influenza vaccine.

2. When did the NIAID-supported trial of 2009 H1N1 influenza vaccines in pregnant women begin?

The NIAID-sponsored trial testing 2009 H1N1 influenza vaccines in pregnant women began enrolling volunteers on September 9. The trial is being conducted by the NIAID Vaccine and Treatment Evaluation Units, a network of clinical trials sites at medical research institutions nationwide.

3. What is the design of the 2009 H1N1 influenza vaccine trial in pregnant women?

The trial is testing a 2009 H1N1 flu vaccine produced by Sanofi Pasteur in the U.S. vaccine facility, Swiftwater, PA. The study has enrolled 120 women 18 to 39 years old in their second or third trimester of pregnancy (14 to 34 weeks). All volunteers will receive two injections of the vaccine spaced three weeks apart. Half of the volunteers will receive 15-microgram doses of vaccine, and the other half will receive 30-microgram doses of vaccine.

In addition to gathering information about the vaccine’s safety, study investigators will look for indications of immune system responses (such as the production of antibodies against 2009 H1N1 influenza virus). Blood samples will be drawn at set time points following the first and second injections to provide information about the immune response to the vaccine. In addition, at the time of delivery, blood samples from both the mother and the umbilical cord will be collected to measure maternal antibodies transferred to the infants through the placenta.

4. What are the preliminary results of this trial?

A preliminary analysis of 50 samples showed that the women had a robust immune response to the vaccine. Blood samples taken 21 days after the first injection showed levels of antibodies directed against the 2009 H1N1 virus that are comparable to levels seen in healthy non-pregnant adults ages 18 to 64. Specifically, 92 percent of women who received a 15-microgram dose of vaccine showed antibody levels likely to be predictive of protection against infection. Among women who received a 30-microgram dose of vaccine, 96 percent had antibody levels likely to be predictive of protection against infection.

5. Why weren’t women in their first trimester of pregnancy enrolled in the trial?

Miscarriage (loss of pregnancy) occurs more frequently during the first trimester than during later trimesters. Vaccination against seasonal influenza is not associated with miscarriage but the expected higher rate of miscarriage in the first trimester—from causes unrelated to vaccine or any other intervention—could make interpreting results of the trial more difficult, and so women in their first trimester of pregnancy were not included in this trial.

[See PD Tamma et al. Safety of influenza vaccination during pregnancy. American Journal of Obstetrics & Gynecology. DOI: 10.1016.j.ajog.2009.09.034. 2009.]

6. Is there a detailed description of the study available?

A detailed description of this study can be found at NCT 00963430 on the Web site.

7. What sites enrolled volunteers?

The study is being conducted through the NIAID Vaccine and Treatment Evaluation Units. The following sites enrolled trial volunteers: Baylor College of Medicine VTEU in Houston; Group Health Cooperative Center for Health Studies VTEU in Seattle; Saint Louis University VTEU; Vanderbilt University VTEU in Nashville; and Duke University in Durham, N.C.

8. Is it possible to become infected with 2009 H1N1 influenza virus by receiving this vaccine?

No. The vaccine is made from an inactivated form of 2009 H1N1 influenza virus. It is impossible to become infected with the virus by receiving this vaccine.

9. Does the vaccine contain either the preservative thimerosal or an adjuvant?

No. The vaccine does not contain thimerosal, a preservative. It also does not contain an adjuvant. An adjuvant is a substance that is added to vaccine to improve the body’s immune response to the vaccine.

10. How does the 2009 H1N1 influenza vaccine compare with the seasonal influenza vaccine?

The 2009 H1N1 flu vaccine being tested in this clinical trial is very similar to the annual influenza vaccine recommended by CDC for all pregnant women. The vaccine is produced in the same way as most seasonal flu vaccines—virus is grown in fertilized chicken eggs, then the virus is inactivated and used to make the vaccine. Seasonal inactivated virus influenza vaccines have been used in pregnant women in the United States since the 1960s and have an excellent safety profile.

The recommendation that all pregnant women receive seasonal influenza vaccine is based in part on results from multiple studies of inactivated influenza vaccine that collectively assessed the safety of vaccine in more than 10,000 pregnant women and found no difference in adverse events reported in vaccinated and unvaccinated women. [See: NE MacDonald et al. Influenza immunization in pregnancy. Obstetrics & Gynecology. 114: 365-368. Aug. 2009.]

11. How is safety being monitored in the trial?

Safety is being monitored continuously throughout the trial, both by the study investigators and by an independent panel of experts known as a Safety Monitoring Committee. This committee was convened by NIAID to review the data from this and other NIAID-sponsored 2009 H1N1 influenza vaccine trials. The committee has regularly scheduled meetings during the course of the trial and also is available for ad hoc meetings as needed.

12. How long will the trial last?

Study investigators will monitor the health of the participants for 7 months following the first injection.

13. Why is this trial enrolling only 120 women?

The trial size is based on vaccine availability and the number of individuals needed to yield sufficient information to determine the safety of the H1N1 vaccine in pregnant women, and to measure their immune response to the vaccine. The information gathered from this study will be compared with information from similar studies comparing one and two doses of 2009 H1N1 influenza vaccines. Those trials have enrolled larger numbers of healthy adults and elderly individuals, but do not include pregnant women.

14. Will the vaccine provide protection to an infant whose mother was vaccinated as well as to the vaccinated woman?

It is expected that the vaccine will provide protection for infants against infection with 2009 H1N1 influenza virus. This protection is likely a result of transfer of antibodies across the placenta from the mother to her infant. In a study of maternal immunization published in 2008, there was a 63 percent reduction of influenza illness in infants up to 6 months of age whose mothers had received inactivated seasonal influenza vaccine while pregnant.

[See K Zaman et al. Effectiveness of maternal influenza immunization in mothers and infants. New England Journal of Medicine DOI:10.1056/NEJMoa0708630 (2008).]

15. Will protection against 2009 H1N1 influenza be transferred to a newborn from a vaccinated mother who breastfeeds her infant?

It is unknown whether protection against 2009 H1N1 will be transferred from a vaccinated mother to her breastfeeding infant. However, measurable antibodies have been detected in breast milk following vaccination of pregnant women with pneumococcal, meningococcal and group B streptococcal vaccines, indicating a potential for these antibodies in preventing disease.

16. Where can I get additional information on other NIAID-sponsored trials of 2009 H1N1 influenza vaccines?

For more information about other NIAID-sponsored clinical trials of 2009 H1N1 flu vaccines, see the following Questions and Answers documents: Clinical Trials of 2009 H1N1 Influenza Vaccines Conducted by the NIAID-Supported Vaccine and Treatment Evaluation Units and Pediatric Trials of Candidate 2009 H1N1 Vaccine at NIAID Vaccine and Treatment Evaluation Units (VTEUs).

17. Where can I get additional information on influenza and pregnancy?

Information for the general public and for healthcare providers about 2009 H1N1 influenza vaccines in pregnant women is available from the CDC.

Additional information on seasonal influenza disease and influenza vaccination for populations including pregnant women is available in the CDC report, Prevention and control of seasonal influenza with vaccines. MMWR 58 (RR08) (July 31, 2009).

A Webchat on 2009 H1N1 influenza and pregnant women featuring a panel of experts, including NIAID Director Dr. Anthony S. Fauci, is available.

Media inquiries can be directed to the NIAID Office of Communications at 301-402-1663,

NIAID conducts and supports research—at NIH, throughout the United States, and worldwide—to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID website.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit

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Last Updated November 02, 2009

Last Reviewed November 02, 2009