Pregnant women face high risk of pandemic flu

H1N1 vaccine trials under way

Pregnant women appear to be particularly vulnerable to H1N1 influenza A virus and will be the top priority to receive a vaccine expected to be available this fall against the pandemic strain, the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices (ACIP) recently recommended.

Recently published Centers for Disease Control and Prevention data underscore the risk, finding more complications and hospitalizations after H1N1 infection during pregnancy.1 "These data lend support to the present recommendation to promptly treat pregnant women with H1N1 influenza virus infection with anti-influenza drugs," the authors note. Between April 15 and June 16, 2009, six deaths in pregnant women were reported to the CDC; all were in women who had developed pneumonia and subsequent acute respiratory distress syndrome requiring mechanical ventilation, the researchers found.

The H1N1 vaccine is still being tested and there was some question whether pregnant women would be asked to roll up their sleeves to receive an experimental immunization during the trials. Somewhat surprisingly, the answer is yes.

"We are running a pregnant women's study with the H1N1," says Donald Kennedy, MD, a professor of infectious diseases at Saint Louis University. "Don't forget the people that have done the absolute worst with this H1N1 strain are pregnant women."

No data were available yet because the women still were being recruited for the study, which will take place at the Center for Vaccine Development at the university.

"It's not going to be an enormous number because obviously you've got to find them and it's a very small population," he says. "The H1N1 vaccine that we are testing has been made in the same way seasonal flu vaccine has been made for years. There's obviously debate about safety and it's a real issue. But in reality, with some of the devastating consequences this virus has had in selected populations, it's either do something or accept the consequences. We already recommend seasonal vaccine for pregnant women for the very reason that they do very poorly if they get the flu during their pregnancy."

In all likelihood, the arrival of H1N1 will precede any statistically valid results, but infection preventionists should emphasize to their colleagues that pregnant women are the primary risk group.

"The highest priority message is to treat pregnant women with influenza-like illness as soon as possible; treatment should not be withheld pending results of testing for influenza, if testing is done," the CDC reports on its H1N1 web site. "Early treatment with influenza antiviral medications is recommended for pregnant women with suspected influenza illness. Clinicians should not wait for test results to initiate treatment since these medications work best if started as early as possible after illness onset."

An excess of influenza-associated deaths among pregnant women were reported during the pandemics of 1918-1919 and 1957-1958, the CDC reports. Adverse pregnancy outcomes have likewise been reported following previous influenza pandemics, with increased rates of spontaneous abortion and preterm birth reported, especially among women with pneumonia. "Many pregnant women will go on to have a typical course of uncomplicated influenza," the CDC notes. "However, for some pregnant women, illness might progress rapidly, and might be complicated by secondary bacterial infections including pneumonia."

Other H1N1 risk groups

In addition to pregnant women, ACIP recommended that H1N1 vaccination efforts focus on four other populations:

  • people who live with or care for children younger than 6 months of age;
  • health care and emergency services personnel;
  • people between the ages of 6 months through 24 years of age;
  • people from ages 25 through 64 years who are at higher risk for novel H1N1 because of chronic health disorders or compromised immune systems.

The groups listed above total approximately 159 million people in the United States. If there is a shortage of vaccine, the priority list changes somewhat:

  • pregnant women;
  • people who live with or care for children younger than 6 months of age;
  • health care and emergency services personnel with direct patient contact;
  • children 6 months through 4 years of age;
  • children 5 through 18 years of age who have chronic medical conditions.


  1. Jamiesan DG et al. H1N1 2009 influenza virus infection during pregnancy in the USA. Lancet 2009; published online July 29, 2009.