By Audrey Dorélien | November 12, 2019

Flu season is approaching, yet many people in the U.S. are not getting their flu shots. While all adults should get vaccinated, it is especially important for women who are planning on getting pregnant and those who are expecting. But while the Centers for Disease Control and Prevention (CDC) reports that pregnant women are twice as likely to be hospitalized if they get the flu, only 54 percent of pregnant women reported getting a flu shot during the past flu season.

Pregnant women are twice as likely to be hospitalized if they get the flu, but only 54 percent of pregnant women reported getting a flu shot during the past flu season.

While the CDC reports that maternal flu vaccination reduces hospitalizations for pregnant women and transfers protective antibodies to newborns, there may be even more benefits. My latest research shows that maternal flu vaccination also protects fetuses from neonatal mortality and preterm births while they are in the womb.

Do It for the Herd

The flu is a bigger deal than most people think. Each year 10 to 20 percent of the population gets the flu. The CDC estimates that during the last year’s flu season, there were at least:

• 36,400 deaths from the flu;
• 531,00 to 647,000 flu-related hospitalizations; and
• 4 to 42.9 million flu cases.

The easiest way to protect yourself against to flu is to get vaccinated. Flu vaccination rates among children ages six months to 17 years have increased over the past nine flu seasons; however, the adult vaccination rate has largely stagnated.

Not everyone can get vaccinated, including children younger than six months and people with compromised immune systems. Some groups, like the elderly, have a lower immune response to flu vaccination. But if enough healthy people get vaccinated, the flu virus will not be able to spread. This effect, called “herd immunity,” protects both those who get vaccinated and those around them. An important group that may be protected by herd immunity is developing fetuses.

Protecting the Unborn

My research suggests that the CDC death and hospitalization estimates substantially understate the health costs of the flu, because children who are exposed to the disease in utero suffer severe health consequences.

To study the risks of in utero exposure to the flu, I looked at data during a period when relatively few potential mothers received vaccinations (1988-2004). In order to control for seasonal variations and differences across counties, I compare the birth outcomes of women that conceived at the same time of the year in the same county but in years with stronger and weaker flu epidemics.

I find that timing of the flu exposure matters. In the first trimester, exposure to a moderate flu season leads to a 45 percent increase in risk of neonatal and infant mortality compared to a weak flu season.

In the first trimester, exposure to a moderate flu season leads to a 45 percent increase in risk of neonatal and infant mortality compared to a weak flu season.

The majority of these deaths are due to congenital abnormalities. Exposure during the first trimester also increases the risk of preterm births by about 10 percent.

In the third trimester, I find that exposure to a stronger flu season increases the risk of low birthweight by 15 percent. The majority of this effect is due to earlier births. Exposure during the third trimester also increases the risk of preterm births by 16 percent. This is in line with evidence showing that exposure during the third trimester can trigger premature labor.

These effects are large and meaningful. They are equivalent to one-third of the U.S. black-white racial gap in infant mortality and 12 percent of the black-white difference in preterm rates. The comparisons above are between a mild and moderate flu season. The worst flu seasons in my study lead to changes in infant health that are as large as the black-white gap.

The worst flu seasons in my study lead to changes in infant health that are as large as the U.S. black-white racial gap.

Put another way, moderate exposure changes could increase the infant mortality rate from six to eight per 1,000, and extreme exposure changes could increase the infant mortality rate from six to 19 per 1,000 if the vulnerable period occurs during a peak flu month.

Policy Implications

My findings indicate that factors that reduce in utero flu exposure, such as herd immunity and maternal vaccination, protect the fetus and improve infant health at birth.

The CDC estimates that 54 percent of pregnant women received a flu vaccine during the last flu season. The rate is even lower for black, non-Hispanic women, a population that experiences high maternal mortality and poorer birth outcomes than other groups. Given the benefits of maternal vaccination for mothers, fetuses, and infants, these rates are far too low.

The American College of Obstetricians and Gynecologists and the Advisory Committee on Immunization Practices (ACIP) recommend that all women who are or will be pregnant during the flu season, regardless of trimester, should be vaccinated.

However, my findings suggest that ACIP should make an even stronger recommendation: all women who may become pregnant should be up-to-date on their flu vaccination and should get their flu shot prior to getting pregnant, since even the earliest exposure can be harmful to the fetus.

My findings suggest that ACIP should make an even stronger recommendation: all women who may become pregnant should be up-to-date on their flu vaccination and should get their flu shot prior to getting pregnant, since even the earliest exposure can be harmful to the fetus.

For adults, the flu vaccine is important and necessary to both protect yourself and provide herd immunity to others. For pregnant women and women who may become pregnant, it’s even more important. Increasing the rate of maternal flu vaccination, particularly among those communities with the lowest rates, is a crucial way to protect the health of newborn children and developing fetuses.