Of Gestation and Germs: Influenza Vaccination in Pregnancy
Inna Berin, MD
Of Gestation and Germs: Influenza Vaccination in Pregnancy Every year, the influenza virus infects anywhere from 5% to 20% of all people in the United States. Infection with this virus leads to the disease commonly known as “the flu,” which is a respiratory illness affecting the nose, throat, and lungs. But unlike many other respiratory viruses such as the common cold, the flu causes severe illness and life-threatening complications in many of its victims; on average, in the U.S. alone, more than 200,000 people are hospitalized annually from flu-related problems. Influenza is a seasonal virus, generally infecting people only during certain times of the year. The exact timing of the flu season varies from year to year and is unpredictable, but most cases of the flu occur each year between October 1 and May 31; the 2011-2012 flu season, for example, is considered to begin on October 1, 2011, and continue through May 31, 2012. The peak of the flu season usually occurs anywhere between late November and March. Pregnant women (as well as some other groups of people such as older people, young children, and people with certain health conditions) are at higher risk for experiencing serious complications from the flu. Since the 2009-2010 pandemic caused by the H1N1 influenza virus commonly referred to as the “swine flu,” medical science has learned a lot about influenza in pregnancy. Before then, much of what we knew was based on indirect evidence. But since May 4, 2009, when the first known healthy pregnant woman died due to the flu, our knowledge of influenza in pregnancy has unfortunately expanded tremendously, and we now know with certainty, through careful and consistent direct evidence, that the flu affects pregnant women more severely than it affects the general population. Pregnancy is an immunocompromised state; during pregnancy, the immune system does not work at full capacity. Because of this, the body’s immune system in pregnancy has a harder time fighting off the influenza virus, and the flu therefore tends to be more severe. In fact, pregnant women have been disproportionately affected by severe disease in all influenza pandemics over the past century. In the 1918 flu pandemic, for example, half of all pregnant women with the flu experienced pneumonia. Of these, half died -- resulting in an astounding and tragic death rate of 25% among pregnant women who got the flu. In the 1957 pandemic, among women of reproductive age, half of all reported deaths occurred in pregnant women. In the recent 2009-2010 H1N1 influenza pandemic, a study from New York showed that pregnant women, when compared with non-pregnant women of reproductive age, were at increased risk for hospitalization, life-threatening breathing problems requiring admission to an intensive care unit, and death, and a recent national study by the U.S. Centers for Disease Control and Prevention (CDC) showed that pregnant women represented 23.5% to 31% of reproductive age women who needed hospitalization for influenza in 2009-2010. Influenza is spread mainly from one person to another through coughing or sneezing. A cough or sneeze by an infected person will release droplets containing many tiny influenza viruses, and a nearby person will breathe in these virus particles and become infected. Similarly, an infected person can spread the virus by coughing or sneezing into his/her hand, and then shaking hands with an uninfected person who then touches his or her own eyes, nose, or mouth. During the flu season, therefore, everyone, and especially pregnant women, should take routine everyday precautions to stay healthy, including washing your hands often with soap and water, or alternatively with an alcohol-based hand rub, to kill whatever live virus may be lying there in wait, and avoiding touching your eyes, nose, or mouth. Pregnant women should carefully follow public health advice such as regarding school closures, and should avoid crowds and increase physical distances among people in social and professional situations. However, the first and probably most important step in protecting against the flu is the annual influenza vaccine. The flu vaccine consists of inactivated influenza virus which, when injected into a person, acts as a simulated war game to train the body’s immune system to effectively fight against the influenza virus. After being vaccinated, if and when the body is infected with actual live influenza virus, the well-trained immune system will know how to immediately destroy the virus and prevent the person from getting sick with the flu. Although the influenza virus changes its own form slightly each year to try to outwit the body’s defenses, CDC scientists monitor the shifting virus, and develop a new influenza vaccine each year usually targeted against the three most likely types of influenza virus to strike the U.S. during the upcoming flu season. This annual triple influenza vaccine is technically called a “trivalent inactivated influenza vaccine,” and is very effective at decreasing a person’s chances of getting the flu. The CDC now recommends a yearly flu vaccination for everyone in the U.S. who is 6 months or older. Immunity sets in about two weeks after vaccination when the immune system is fully trained, and getting the flu vaccine provides protection that lasts throughout the flu season. Flu vaccines are most effective at prevention the earlier they are given; vaccines this year for the 2011-2012 flu season may become available as early as August 1. For pregnant women, the annual flu vaccine is also effective; studies have shown that influenza vaccination of pregnant women trains their immune systems to protect against the disease just as the vaccine does with everyone else. The flu vaccine has also been found to be safe throughout pregnancy; safety research and monitoring have not uncovered any increased risk of complications to either the mother, the fetus, or the newborn infant after influenza vaccination during pregnancy. Influenza vaccination for pregnant women was first recommended in the late 1950s, and has been recommended since 2005 for all pregnant women, regardless of pregnancy trimester, by the Advisory Committee on Immunization Practices of the CDC. Currently, because pregnant women are at such high risk for severe complications and even death from the flu, and because the vaccine is effective and safe in pregnancy, universal vaccination of pregnant women at any gestational age -- at any point in the pregnancy -- is recommended by both the CDC and the American College of Obstetricians and Gynecologists. In fact, in times of an influenza pandemic, or when there is a vaccine shortage and the vaccine is being rationed, pregnant women are given highest priority for vaccination because of their increased risk for flu-associated severe illness and death. Despite these recommendations, vaccination among pregnant women was low prior to the 2009-2010 H1N1 influenza pandemic, generally less than 15%; vaccination rates as low as 13% were reported during the 2006-2007 vaccination season. These low rates are largely due to nationally inadequate efforts in educating both healthcare providers and patients about the safety and efficacy of vaccination during pregnancy. Fortunately, however, vaccination rates have been improving dramatically since the 2009-2010 pandemic. Recent data from the National 2009 H1N1 Flu Survey show that 46% of all pregnant women received the H1N1 flu vaccine, and these higher levels seem to have been maintained for the 2010-2011 seasonal flu vaccine as well. In December 2010, the U.S. Department of Health and Human Services unveiled a “new health promotion and disease prevention agenda” called Healthy People 2020, which sets “new ten-year goals and objectives for health promotion and disease prevention.” Healthy People 2020 sets as one of its objectives the goal of vaccinating 80% of pregnant women each year with the annual seasonal influenza vaccine. One further issue which is currently being studied may encourage more pregnant women to undergo vaccination: the benefit to her newborn child. Vaccination in pregnancy seems to prevent not only the pregnant woman from getting the flu, but also seems to protect her newborn child. Infants up to 6 months old have the highest rate of influenza-related hospitalization among children, but are not eligible for vaccination because their immune systems are too immature to mount an adequate immune response after influenza vaccination. A recent randomized controlled research trial showed a 41-63% reduction in laboratory-confirmed influenza disease in infants of mothers who had been vaccinated during the third trimester of pregnancy. This reduction makes sense, probably occurring because studies consistently show that a pregnant woman who receives the influenza vaccine shares her immunity with her developing fetus by transferring antibodies to the fetus through the placenta (antibodies are the “ammunition” used by the immune system to battle an infection). Similarly, a recent CDC-funded multicenter study by the New Vaccine Surveillance Network showed that infants of mothers vaccinated during pregnancy were 45-48% less likely to have influenza hospitalizations than infants born to unvaccinated mothers. A recent model-based analysis of the cost effectiveness of vaccination in pregnancy concluded that universal vaccination of pregnant women substantially reduces severe illness and death from the flu in these women during pregnancy, in these same women after delivery, AND in the less-than-six-month-old infants of these women. Moreover, the vaccine accomplished this significant reduction at a cost that is comparable with other recommended vaccines used in adolescents and adults. Overall, all available evidence shows that a woman who chooses to receive the flu vaccine in pregnancy is making a decision that is safe, and that will be effective in protecting both herself and her baby, both of whom are at high risk for severe flu-related complications. The greatest benefit from the annual flu vaccine is when the vaccination is given as early as possible in the season -- so we strongly recommend that you discuss influenza vaccination with your primary doctor or obstetrician in the near future!