The safety of the two vaccines routinely recommended for pregnant women were reported in two separate studies recently, providing additional reassurance that the shots designed to protect against the flu and whooping cough also pose little risk to a woman or her fetus.
The influenza vaccine paper, published Nov. 18 in Clinical Infectious Diseases, pooled the findings of seven previous studies to find a slightly lower risk of stillbirth and miscarriage in women who received H1N1 flu vaccines during pregnancy compared to unvaccinated women.
"This is a somewhat comprehensive meta-analysis," said senior author Saad Omer, an associate professor of global health at Emory University. "The findings were not really surprising but they were reassuring in that all the bigger and higher quality studies, with a lower chance of randomness in the results, pointed in the same direction."
Even though the study found an apparent protective effect from the vaccine – lower risk of miscarriage or stillbirth in vaccinated mothers – it's not yet clear if that's a real effect or one influenced by other factors. Omer and his colleagues are looking into possible biological explanations, such as a reduction in inflammation in those who were vaccinated. These studies were all focused on H1N1 strains (including one 1976 strain), but Omer's team is also presently analyzing data from three randomized controlled trials of flu vaccination during pregnancy, all funded by the Gates Foundation and conducted in South Africa, Nepal and Mali. Only the South African trial's results have been published, but as the others wrap up, the data from all three can be combined for a larger sample to look for more rare outcomes. "So far the data looks very reassuring," he said.
The second study, published in JAMA Nov. 12, analyzed more than 120,000 live births from 2010 through 2012 at two California sites participating in the Vaccine Safety Datalink program. About one in five of these women (21%) received the Tdap during pregnancy. The researchers found no difference in the rates of preterm birth or underweight babies among mothers who did or did not receive the vaccine, and those who got the vaccine before 20 weeks of pregnancy were at no higher risk for high blood pressure during pregnancy than those who didn't.
The only risk observed was a 19 percent higher risk in Tdap-vaccinated mothers for the bacterial infection chorioamnionitis. This risk, however, was potentially due to unrelated factors not accounted for in the overall analysis, such as receipt of an epidural since nearly all the women with this infection received an epidural. (The authors didn't have enough data about epidurals for the full sample to account for this factor.)
The CDC Advisory Committee on Immunization Practices (ACIP) began recommending in October 2012 that all pregnant women receive Tdap vaccine between 27 and 36 weeks of every pregnancy. Tdap protects against tetanus, diphtheria and, most importantly, pertussis, also known as whooping cough. Pertussis rates have soared in recent years, primarily due to waning in the current vaccine but also partly due to low vaccine uptake in some areas.
Infants under 2 to 3 months old are the most vulnerable, but babies don't get their first pertussis vaccine until 2 months old. Their best chance of protection against whooping cough, then, is getting antibodies from their mothers during pregnancy. A small study in JAMA earlier this year showed that newborns whose mothers received the Tdap during pregnancy had nearly five times the concentration of pertussis antibodies as babies whose mothers did not get the booster.
Meanwhile, the CDC has recommended for more than a decade that women receive the influenza vaccine before each flu season since pregnant women are at increased risk for flu-related complications, including premature labor, preterm birth and miscarriage. Yet barriers to vaccination have remained despite plenty of evidence regarding the safety of flu vaccines during pregnancy, and rates of Tdap uptake are even lower. A study last month found fewer than a third of pregnant women received the Tdap.
Yet these recommendations are so important that a letter (PDF) from the Department of Health and Human Services went out to providers in October to emphasize the importance of both immunizations for women during pregnancy. The letter was signed by the heads of 14 professional organizations, including the American Academy of Pediatricians, the American College of Nurse-Midwives and the American College of Obstetricians and Gynecologists.
"We – young parents – have been programmed, rightfully so, to be cautious in terms of what they are exposed to during pregnancy," Omer said. "That overall cautiousness is good, but sometimes that spills over into something that is very useful."
He said women will ask "Why risk it?" about getting a vaccine, just in case there is an adverse effect. "But what people don't realize is the other side," he said. "There is substantial evidence that flu itself has impact on the mother, the fetus and the baby. By not getting vaccinated, they are taking more of a chance. That message hasn't been more effectively communicated so far."
Similarly, not getting the Tdap during pregnancy means less opportunity for babies to ward off whooping cough in those first few months before they get their first shots. The previously recommended strategy of postpartum Tdap vaccination and "cocooning" – where other household members get the vaccine to create a protective "cocoon" around a newborn – have not borne out as effective enough. But vaccination during pregnancy appears to offer more protection. And rates of pertussis show no sign of slowing down.
