A pregnant woman's belly

Children whose mothers received the Tdap vaccine during pregnancy are at no higher risk of autism than others, a large new study published in the journal Pediatrics has found.

Pregnant women in the U.S. are advised to receive two vaccines. One is the flu shot, mainly because pregnant women are more likely to experience complications and even be hospitalized if they get the virus. The other is Tdap, a vaccine for adolescents and adults that protects against tetanus, diphtheria, and pertussis, also known as whooping cough.

more on whooping cough

Tdap is important because pertussis can be fatal to infants, and when a mother receives the shot, she passes on her immunity to her baby. 

But some people are concerned about getting vaccines during pregnancy, thanks in part to a thoroughly debunked 1990s study that linked the MMR (measles, mumps, and rubella) vaccine to autism. And an internet rumor that started a few years ago, claiming that Dtap, the childhood version of the vaccine against tetanus, diphtheria, and pertussis, was associated with autism, might have made some parents-to-be even more wary. (That rumor was also false.)

This new research should allay parents’ fears about the vaccine, say the researchers. “The results of this study can assure them that it’s not harmful to their unborn baby,” says Tracy Becerra-Culqui, Ph.D., M.P.H, a postdoctoral research fellow with Kaiser Permanente’s Department of Research & Evaluation in Pasadena, Calif., an author of the study.

What the Study Found

The research team reviewed the medical records of 81,993 babies born in California between 2011 and 2014. The mothers were all patients in the Kaiser Permanente system from the beginning of their pregnancies, and after birth, the babies remained in the health system for at least 15 months.

The researchers noted whether the mothers received the Tdap vaccine while pregnant, and looked for diagnoses of autism spectrum disorder (ASD) as the children grew up, either until they left the Kaiser Permanente system or through June 2017. Children whose moms received Tdap were no more likely to be diagnosed with ASD than children whose moms skipped it.

Although this study looked only at people in southern California, the region is so diverse that she expects that the results would apply to the U.S. population, Becerra-Culqui says.

The researchers did not take into consideration instances when a youngster in the study had an older sibling with ASD—which could signal a significantly higher risk of autism for the younger child. But when they looked only at mothers who had given birth to their first child, they still found that the Tdap vaccination had no effect on autism risk.

“I think we can have great confidence in these results,” says William Schaffner, M.D., professor of preventive medicine and infectious diseases at the Vanderbilt University School of Medicine in Nashville, who wasn’t involved in the study. “The results are completely reassuring and reinforce the recommendation that all women receive a Tdap vaccination during pregnancy.”

As for the overall safety of Tdap, previous studies have demonstrated no increase in the risk of other kinds of pregnancy complications, such as preterm birth or low birth weight, Becerra-Culqui points out.

In addition, most people experience no side effects, and those that occur tend to be minor, such as pain at the injection site, headache, or mild fever.

Why Tdap During Pregnancy Is So Important

Tens of thousands of cases of whooping cough occur in the U.S. every year. The numbers go up and down, but the most recent peak was in 2012, which saw more than 48,000 reported cases of the respiratory infection. And according to the Centers for Disease Control and Prevention, many more cases probably go unreported.

Whooping cough can cause violent fits of coughing and difficulty breathing, and it’s most dangerous for babies in the first few months of life: About half who contract it need to be hospitalized.

Usually, children start receiving vaccinations against it at 2 months of age, when they get their first Dtap shot. Before then, the immunity they inherit from their mothers can safeguard them from whooping cough.

Hence, the Tdap vaccine, which is meant to boost immunity that may have waned since the early childhood Dtap shots. By getting Tdap while pregnant, “The mother creates protection, antibodies, that cross the placenta and go into the baby,” Schaffner says. “So when the baby is born, the baby is protected by those antibodies.”

The CDC recommends that women receive a dose of Tdap during every single pregnancy, even if they’ve had the shot before. (Everyone should receive Tdap at age 11, and adults who’ve never received Tdap should have the shot.)

The optimal time to receive the shot, according to the CDC, is between the 27th and 36th week of pregnancy. (It's safe to get the flu shot at any point during pregnancy.)

You should also make sure that any family, friends, and caregivers who will have contact with your new baby are up-to-date on all of their vaccinations.

Clarification: This article has been updated to indicate that  babies, not mothers, remained in the Kaiser Permanente health system for at least 15 months after being born.