Study Finds Flu Vaccine Reduces Risk of Flu Illness in Pregnant Women by Half

First study to use lab-confirmed influenza outcomes among pregnant women shows that flu vaccination significantly reduces flu-related illness

A study recently published in the journal Clinical Infectious Diseases (CID) provides new data on the benefits of flu vaccination for pregnant women showing that vaccination reduced their risk of flu illness by half from 2010 to 2012. This information comes as the nation is experiencing a rapid increase in seasonal flu activity, predominated by influenza A H1N1. The H1N1 virus emerged in 2009 to cause a pandemic and was particularly hard on pregnant women, who were four times more likely to be hospitalized from flu than non-pregnant women. The CDC has already received reports this season of pregnant women hospitalized from flu complications and, in some cases, dying from flu complications.

CDC recommends the flu shot as a priority for pregnant women because studies and history have shown that pregnant women are at an increased risk for flu-related hospitalizations and death when compared with non-pregnant women. During the 2009 H1N1 pandemic, expectant mothers seemed at an even greater risk than usual.

Getting a flu shot is the best way to prevent flu infection. The CID study supports this recommendation, indicating that flu vaccine works about as well in pregnant women as in other young, healthy adults and can reduce the risk of flu-associated respiratory illness among pregnant women by one-half. The recently published study showed influenza vaccine effectiveness estimates among pregnant women were similar to those found in other studies among young adults, which ranged from 44% to 51% from 2010 to 2012.

Influenza activity is widespread in most of the country at this time and is likely to continue for the next several weeks at least. It's not too late for pregnant women to get vaccinated and still benefit from the protection the flu vaccine can provide. Vaccination not only protects the pregnant mother from flu and its complications, but also has been shown to provide some immunity to the newborn child during the first six months of their lives.*

Influenza vaccination rates among pregnant women have remained fairly consistent at about 40% for the past few flu seasons, following an increase in uptake after the 2009 pandemic. As of early November 2013, flu vaccination coverage among pregnant women was 40.7%, which is similar to 2012-2013 early season vaccination coverage (42.9%).

CDC urges pregnant women to get vaccinated and also urges their caregivers to actively support influenza vaccination among their patients. Research has found that a recommendation for influenza vaccination by a health care provider is an important factor in a woman's decision about whether or not to get vaccinated. Health care providers should continue to encourage pregnant women to protect themselves and their babies by getting vaccinated.

It's also important for pregnant women and their providers to realize that flu vaccine is not perfect and some vaccinated people still become infected. Therefore, clinicians and the public should be aware that influenza antiviral drugs may be used as a second line of defense against flu; pregnant women should be treated based on the physician’s best judgment without waiting for confirmatory testing.

Study Methods

Though flu vaccine effectiveness (VE) has been examined in studies of pregnant women that looked at influenza-like illnesses, this is the first study to use laboratory confirmed influenza as an outcome. Respiratory specimens were collected from patients using nasal swabs and tested using a laboratory test called RT-PCR. Prior flu VE studies of pregnant women used non-specific respiratory illness outcomes to measure vaccine effectiveness.

Researchers estimated vaccine effectiveness in influenza illness by considering vaccinated and unvaccinated pregnant women with acute respiratory illness (ARI) who were laboratory-confirmed as having flu (i.e., influenza cases) and those with ARI who tested negative for influenza infection (influenza-negative controls). This study design looking at influenza-negative controls is believed to minimize the potential bias to VE estimates introduced by individuals’ health care seeking behavior. The women in the study had had at least one prenatal visit at the study sites. As part of a secondary analysis, researchers also considered pregnant women who had no respiratory illness (matched ARI-negative controls) as part of their VE estimates

Vaccine effectiveness models were adjusted for the patients’ age, race, ethnicity (Hispanic or non-Hispanic), high risk medical conditions, study site, influenza season, and days between illness onset and testing. Flu vaccine effectiveness estimates during the study period were 44% when compared to women with ARI who tested negative for flu infection, and 53% compared with pregnant women who had no respiratory illness. These results are similar to VE estimates from the same seasons looking at the general population, indicating that the flu vaccine was as effective for pregnant women as it was for other adults.

Additional analysis of the study’s VE data over the course of two flu seasons showed that the effect of vaccination during the influenza season when the woman was pregnant may not have been independent of prior season vaccination. Interestingly, women who received a flu vaccine during either flu season or in both flu seasons had similarly reduced likelihoods of having influenza infection.

For more information about pregnancy and flu, visit Pregnant Women & Influenza (Flu).

The study is available online in the Clinical Infectious Disease Advance Access.

*Babies younger than 6 months of age are too young to get a flu vaccine. To protect an infant who is younger than 6 months from getting the flu, the mother should get a flu shot during her pregnancy. An additional recommended way to protect the baby is for all of the baby’s caregivers and close contacts (including parents, brothers and sisters, grandparents and babysitters) to get vaccinated against the flu.