The flu shot is consistently recommended as a safe and effective way to protect both the mother and the developing baby during any stage of pregnancy. Major health organizations agree that the influenza vaccine is a routine and necessary part of prenatal care. The vaccine given to pregnant individuals is the inactivated influenza vaccine, which contains non-live, or “killed,” virus particles that cannot cause influenza illness. This consensus is based on decades of research and millions of vaccinated women, establishing an excellent safety profile for this preventive measure.
The Critical Need for Vaccination During Pregnancy
Pregnancy causes physiological changes that place women at an increased risk for severe illness if they contract influenza. The immune system, heart, and lungs all undergo modifications, making it harder to fight off respiratory infections. Increased heart rate and stroke volume, combined with decreased lung capacity as the uterus grows, can lead to serious complications like pneumonia. Pregnant women are at a higher risk of hospitalization compared to non-pregnant women of the same age.
A flu infection during pregnancy also carries risks for the developing baby. Exposure to the influenza virus has been linked to adverse outcomes like preterm birth, low birth weight, and stillbirth. The severe systemic inflammation triggered by the virus in the mother’s body can restrict blood flow and oxygen supply. This restriction ultimately affects the placenta and the fetus.
Safety Profile for Mother and Fetus
The injectable flu shot is made with inactivated virus particles, meaning the virus is dead and cannot replicate or cause the flu. This distinction alleviates concerns about the vaccine causing illness in the mother or infection in the fetus. Extensive surveillance and studies across all trimesters have consistently confirmed the vaccine’s safety for both the pregnant woman and the baby.
Data from large-scale projects have found no association between receiving the inactivated influenza vaccine during pregnancy and an increased risk of adverse outcomes. Studies have shown no increased risk of spontaneous abortion, stillbirth, preterm birth, or congenital malformations. This robust body of evidence supports the recommendation to vaccinate at any point during gestation.
A common concern involves thimerosal, a mercury-containing preservative used in some multi-dose vials of the flu vaccine. The mercury in thimerosal is ethylmercury, which is cleared from the body quickly and is different from the more toxic methylmercury. Scientific evidence confirms that the amount of thimerosal in the vaccine is not harmful to the mother or the developing fetus. Thimerosal-free options are also widely available, and the benefits of flu prevention far outweigh any theoretical risk.
Protection Extended to the Newborn
One benefit of maternal vaccination is the passive immunity it provides to the newborn. After the mother receives the flu shot, her immune system creates antibodies to fight the virus. These protective immunoglobulin G (IgG) antibodies are actively transported across the placenta to the fetus, primarily during the third trimester.
This transfer provides a temporary shield of immunity that protects the infant during the first six months of life. Infants under six months old are too young to receive their own flu vaccine, making them vulnerable to severe influenza complications. Research indicates that maternal vaccination can reduce the risk of influenza hospitalization in the baby by a significant margin. The protection provided by these maternal antibodies wanes over time, emphasizing the importance of this initial passive defense.
Recommended Vaccine Types and Timing
All women who are or will be pregnant during the influenza season should receive the inactivated influenza vaccine. This is the standard “flu shot” given by injection. The vaccine can be safely administered at any time during pregnancy, regardless of the trimester.
The only type of flu vaccine that must be avoided during pregnancy is the live attenuated influenza vaccine (LAIV), which is the nasal spray. Because this vaccine contains a weakened, live virus, it is contraindicated due to a theoretical risk of crossing the placenta. The ideal time for vaccination is as soon as the seasonal vaccine becomes available, typically in the fall, to ensure protection throughout the flu season.