Pregnant individuals are strongly advised by major health organizations, including the Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG), to get a flu shot during any trimester. Influenza infection poses a significant health risk to both the expectant mother and the developing baby, making vaccination a simple, yet highly effective, preventative measure. This guidance is rooted in the extensive safety data and the unique protective benefits the shot offers to the newborn.
Understanding the Safety Profile
The influenza vaccine used during pregnancy has an established record of safety for both the mother and the fetus. The shot contains an inactivated, or killed, virus, meaning it cannot cause influenza illness. The vaccine works by presenting harmless viral components to the immune system so that the body can build a protective response.
Contracting influenza while pregnant can lead to severe complications, including a higher risk of pneumonia, hospitalization, and even intensive care unit admission. The physical changes of pregnancy, particularly in the respiratory and immune systems, increase a person’s susceptibility to severe illness from the virus. Vaccination is a direct way to mitigate these risks, with studies showing a reduction in the risk of flu-related hospitalization for pregnant individuals by an average of 40%.
Large-scale data from the Vaccine Safety Datalink (VSD) have consistently found no increased risk of major birth defects, miscarriage, stillbirth, or premature delivery following vaccination during any trimester. Common side effects are mild, such as soreness at the injection site, a low-grade fever, or muscle aches, which are temporary and resolve quickly.
Protection for the Baby After Birth
One of the most significant reasons for vaccination during pregnancy is the unique protection it confers to the baby after delivery, a process known as passive immunity. When the mother receives the flu shot, her immune system produces antibodies specifically targeted against the circulating influenza strains. These protective antibodies are then transferred across the placenta to the developing fetus.
The antibodies received in utero remain in the newborn’s system for the first several months of life, providing a temporary shield against the virus. This passive immunity is important because infants under six months of age are too young to receive their own influenza vaccine and are highly vulnerable to severe, life-threatening complications from the flu. Maternal vaccination has been shown to reduce the risk of laboratory-confirmed influenza in infants during their first six months by up to 44%.
This transferred protection helps bridge the gap until the baby is old enough for their own immunization at six months of age. The level of antibody transfer is often highest when vaccination occurs during the second or third trimester, though vaccination at any time during pregnancy provides some benefit.
When and Which Vaccine to Receive
The influenza vaccine can be safely administered during any trimester. The ideal time for vaccination is as soon as the current season’s vaccine becomes available, which is usually in September or October. If the flu season extends, it is beneficial to receive the shot later in the winter or even into the spring if one has not yet been vaccinated.
The type of vaccine is a specific consideration for pregnant individuals. Only the inactivated influenza vaccine (IIV), which is the standard injectable shot, or the recombinant influenza vaccine (RIV) is recommended. Conversely, the live attenuated influenza vaccine (LAIV), which is the nasal spray, contains a live, albeit weakened, virus and is not recommended for use during pregnancy.
Standard-dose, preservative-free, or thimerosal-containing inactivated vaccines are all acceptable options for pregnant people. High-dose or adjuvanted vaccines, which are typically reserved for older adults, are generally not necessary unless specifically advised by a healthcare provider. It is always best to consult with an obstetrician or midwife to confirm the correct vaccine type and address any specific allergy concerns before immunization.