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Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

  • The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
  • The English language content on this website is being archived for historic and reference purposes only.
  • For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

Questions & Answers

2009 H1N1 Flu In The News

April 21, 2010 10:30 AM ET

2009 H1N1 and Pregnancy

This document provides updated information on pregnancy as a risk factor for serious 2009 H1N1-related complications based on findings from a recent study. 

Has pregnancy been considered a risk factor for serious influenza-related complications in the past?

Yes. Pregnant women were shown to be at increased risk for severe complications from influenza in previous pandemics, during seasonal influenza outbreaks, and from early data on 2009 H1N1. A study conducted during the first month of the outbreak found that the rate of hospitalizations for 2009 H1N1 was four times higher in pregnant women than the rest of the population. Compared with people in the general population, pregnant women with 2009 H1N1 flu are more likely to be hospitalized or die.

Why are pregnant women at increased risk from 2009 H1N1?

In order for a pregnant woman to adapt to carrying a fetus that is genetically different from her, her immune system must undergo changes.  Those changes make women more likely to be severely affected with some infections, including influenza.  Changes in women’s cardiac and respiratory systems also play a role.

What has been learned from the 2009 H1N1 pandemic about pregnancy and risk of death?

Researchers analyzed surveillance data of 2009 H1N1 in pregnant women reported to CDC with symptom onset from April to August 2009, including data on maternal characteristics, underlying illness, severity of illness, and maternal outcomes related to timing of antiviral treatment, to further describe the effects of 2009 H1N1 on pregnant women in the United States.

Data from this study are consistent with previous studies from the United States  and other countries  that show pregnant women are at increased risk for serious illness and death associated with 2009 H1N1 infection.

Although pregnant women account for approximately 1% of the U.S. population, this group accounted for 5% of U.S. deaths from 2009 H1N1 reported to CDC from April 14 – August 21, 2009.

Data were updated with information on women admitted to an intensive care unit (ICU) or women who died with symptom onset through December 31, 2009.  During the entire time period (from April through December 2009), CDC received reports of 280 pregnant women with 2009 H1N1 who were admitted to ICUs, 56 of whom died.

What has been learned from the 2009 H1N1 pandemic about pregnancy and risk of hospitalizations and other serious complications?

A high rate of preterm birth (30%) was reported among women who had been infected with 2009 H1N1 influenza, although data on infant outcomes were incomplete. These data are consistent with data suggesting a high rate of preterm delivery during previous pandemics.  The usual rate of preterm birth in the United States is ~ 13%.

A higher proportion of ICU admissions and deaths occur in the second and especially third trimester, although pregnant women in all three trimesters are at increased risk of influenza-associated complications. 

How many pregnant women had severe complications from 2009 H1N1 influenza?

Based on reports to CDC from state and local health departments, 280 pregnant women with 2009 H1N1 influenza with symptom onset between April and December 2009 were admitted to an intensive care unit – of these, 56 women died. 

What has been learned about the use of antiviral treatment among pregnant women?

Delayed treatment with antiviral medications was associated with more severe illness and death, consistent with previous data on seasonal influenza and 2009 H1N1, where early treatment has been associated with reduced illness duration, symptom severity, death, secondary complications, hospitalizations, and need for antibiotics.

Receipt of early (within 2 days from symptom onset) and intermediate (3-4 days from symptom onset) treatment with antiviral medications among pregnant women was associated with less severe disease, fewer admissions to an ICU and fewer deaths. Pregnant women with intermediate treatment were nearly 10 times more likely to die than those treated early (5% compared to 0.5%) and those treated late were much more likely to die (27% compared to 0.5%) than those treated early.  Data on timing of antiviral treatment were available for pregnant women with symptom onset before August 21, 2009 – during that time, 30 pregnant women died and only one of those had been treated with an antiviral medication within 2 days of symptom onset. 

Are there other underlying conditions associated with hospitalizations and deaths among pregnant women?

Underlying conditions were common among hospitalized pregnant women (55%), women admitted to ICU (63%), and deaths (78%). The most common underlying condition was asthma, seen in 23% of hospitalized patients, 26% of ICU admissions and 44% of deaths.  Obesity was the second most common underlying condition (13%), followed by pregestational or gestational diabetes (7%), anemia (3%), and hypertension (3%).

Are there specific demographics associated with serious complications from 2009 H1N1 in pregnant women?

Demographic and clinical characteristics of pregnant women with severe outcomes (hospitalization, ICU admission and maternal deaths) were assessed, but associations were unable to be determined due to reporting bias.

Hispanic was the most commonly reported racial-ethnic group, (33%), followed by non-Hispanic white (23%), non-Hispanic black (19%), and Asian/Pacific Islander (6%).  However, this racial-ethnic breakdown was similar to the racial/ethnic distribution of live births forthe states that reported the most cases.

Maternal age ranged from 14–43 years, with a median of 25 years among all reported women, and was similar among deaths.

What should pregnant women do based on these findings?

Getting a flu vaccine against 2009 H1N1 is the most important action people can take to prevent 2009 H1N1 influenza and its complications.  Because pregnant women are at a higher risk of serious 2009 H1N1-related complications, it’s especially important that they get vaccinated against 2009 H1N1.

With the Advisory Committee on Immunization Practices recommendation on February 24, 2010 to adopt universal influenza vaccination for the 2010-2011 season, everyone 6 months and older is now recommended to get an annual flu vaccination.  Next year’s seasonal influenza vaccine will contain the 2009 H1N1 virus, in addition to two other viruses that research indicates are most likely to circulate during the upcoming season.

For many years pregnant women have been advised to receive the seasonal influenza vaccine – this vaccine has been shown to reduce the risk of influenza in the woman and in her baby up to 6 months of age (at an age when the baby is too young to get the flu vaccine because they don’t have an adequate immune response at this age).  Studies have shown no evidence of increased maternal or fetal risks from influenza vaccines when used during pregnancy.

Pregnant women should get the flu shot (the killed vaccine), not the nasal spray (the live attenuated vaccine).  The live attenuated vaccine is not approved for use in pregnant women.

Pregnant women who think they have influenza should contact their doctor promptly.  If their doctor thinks a pregnant patient might have 2009 H1N1 influenza, she should be treated as soon as possible.  Treatment should not be delayed while awaiting test results because rapid testing for 2009 H1N1 has been shown to have low sensitivity.

Why didn’t all states provide data to CDC on pregnant women?

State and local health departments report data to CDC on a voluntary basis. 

Can you provide data by state?  How many pregnant women in my state had 2009 H1N1?

Information on a specific state’s experience should be obtained from the individual state.  We are unable to provide data on a state-by-state basis.

Are these all the cases of 2009 H1N1 influenza in the United States for this time period?

No – some states did not report data to CDC, and mild cases were likely missed in all states.  However, the states that reported data represented more than 97% of all births in the United States.  In addition, states had different requirements for case reporting and these changed as the outbreak continued.  In addition, testing for 2009 H1N1 changed as the outbreak continued.  Especially during later months, some states focused on cases that were more severely ill (hospitalized, admitted to an intensive care unit or died).  Therefore, we believe that severely affected cases were more likely to be reported to CDC and included in this report.  

Is the flu vaccine safe in pregnancy? 

The seasonal flu shot has been given to millions of pregnant women over many years. Flu shots have not been shown to cause harm to pregnant women or their babies. The 2009 H1N1 flu vaccine was made in the same way and at the same places where the seasonal flu vaccine is made.

What proportion of women were vaccinated against 2009 H1N1 and seasonal influenza this year? 

Preliminary data from a time of limited vaccine availability suggest that the uptake of 2009 H1N1 influenza vaccine was higher than is usually seen for seasonal influenza vaccine – based on a survey of only 150 women – uptake was 38% (95% CI 24-52%).  We are not aware of data on percent of women who received the seasonal influenza vaccine during the 2009-2010 influenza season.

What are the fetal effects of influenza infection? 

We don’t fully understand the effects of influenza infection on the fetus.  Some studies have shown that pregnant women who develop a fever in the first trimester have an increased risk of having a baby with a certain group of birth defects of the brain and spine, called neural tube defects (these include spina bifida and anencephaly).  For that reason, pregnant women with a fever need to be treated with acetaminophen (Tylenol).  Whether influenza causes other problems for a fetus is not known.  In most cases, it appears that the influenza virus does not cross the placenta to infect the baby, although this has occurred in some cases.

Can influenza be passed to the baby during pregnancy? 

In most cases, it appear that the influenza virus does not cross the placenta to infect the baby, although this has occurred in some cases, including one pregnant woman who had 2009 H1N1 near the end of her pregnancy whose newborn was infected with 2009 H1N1.  However, this appears to be rare.

Why wasn’t the 2009 H1N1 vaccine available sooner and more readily (i.e. shortages)?  

The way vaccines are made takes time – for a vaccine to be produced, the virus needs to grow for a time in eggs.  The 2009 H1N1 virus grew more slowly than was initially anticipated, resulting in a delay in availability.

Was 2009 H1N1 really any worse than seasonal influenza? 

We don’t know if 2009 H1N1 was worse than seasonal influenza for pregnant women.  Pregnant women are more severely affected with both seasonal influenza and 2009 H1N1 influenza (pregnant women are more likely to be hospitalized for both seasonal and 2009 H1N1 influenza and to die when they have 2009 H1N1 influenza)

Should pregnant women get thimerosal free vaccines? 

There is no evidence that thimerosal (a mercury preservative in vaccine that comes in multi-dose vials) is harmful to a pregnant woman or a fetus. However, because some women are concerned about thimerosal during pregnancy, vaccine companies made preservative-free seasonal flu vaccine and 2009 H1N1 flu vaccine in single-dose syringes for pregnant women and small children. CDC advises pregnant women to get flu shots either with or without thimerosal.

Is there an adjuvant in the flu vaccine? 

There is no adjuvant in the flu vaccines that are available in the United States.

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