There are two vaccines available for whooping cough. Tdap is for everyone aged 11 years and older, including pregnant women. A vaccine called DTaP is available for younger children.
For the study, published in JAMA, researchers assessed the health records of 123,494 women in order to evaluate adverse events that may have been associated with Tdap vaccination during the course of pregnancy.
"Given limited prior safety data, continued widespread pertussis transmission, and current recommendations to routinely vaccinate during pregnancy, our study provides important information on the safety of Tdap vaccination during pregnancy," state the authors.
The Tdap vaccine contains tetanus toxoid, reduced diphtheria toxoid and acellular pertussis. Following administration of the vaccine, protective antibodies are created - some of which will be passed on, giving the baby some level of protection from whooping cough in early life.
The Centers for Disease Control and Prevention (CDC) recommend that pregnant women receive the Tdap vaccine during the third trimester of each pregnancy - between the 27th and 36th weeks - in order to pass protection on to their baby.
This was not always the recommendation. When the Tdap vaccine was first licensed in 2005, it was advised to be given after birth due to fears that newborns could inadvertently receive whooping cough from it.
However, infant deaths and a widespread whooping cough outbreak led to California becoming the first state to recommend the routine administration of Tdap during pregnancy in 2010.
Whooping cough most commonly affects infants
Whooping cough is a contagious respiratory disease that is characterized by uncontrollable and violent coughing. These necessitate deep breaths, resulting in the "whooping" sound that gives the disease its name. It most commonly affects infants and can be fatal, particularly among those under 1 year of age.
For the study, Dr. Elyse Kharbanda, of the HealthPartners Institute for Education and Research in Minneapolis, MN, and colleagues analyzed data from two California Vaccine Safety Datalink databases.
The 123,494 women whose data was assessed all had pregnancies that ended with a live birth between January 1, 2010, and November 15, 2012. Of this total, 26,229 (21%) received the Tdap vaccine during their pregnancy.
The researchers found that Tdap vaccination was not associated with hypertensive disorders of pregnancy, small-for-gestational age (SGA) births or preterm births - defined as those with less than 37 weeks' gestation.
Of the women receiving Tdap during pregnancy, 8.4% had an SGA birth and 6.3% had a preterm birth. These findings were compared with those for women who were unexposed to the vaccine, among whom 8.3% had an SGA birth and 7.8% had a preterm birth.
'Continued monitoring will be important'
A small increase in the risk of chorioamnionitis - inflammation of the membranes surrounding the fetus - was observed in the women who received Tdap, with 6.1% diagnosed with the condition, compared with 5.5% of women unexposed to the vaccine. The increase in likelihood was lower in women vaccinated between 27 and 36 weeks' gestation.
The authors suggest that this particular finding should be treated with caution, however, as the magnitude of this risk was so small.
They also note that the study is limited, as the birth data used only represented women from one state who had continuous insurance coverage. This meant that higher-risk pregnancies, those occurring in women with intermittent insurance coverage, were underrepresented in their analysis.
"Because current recommendations from the Advisory Committee on Immunization Practices are to administer Tdap in every pregnancy, continued monitoring of the safety of repeated Tdap doses in a geographically diverse population will be important," acknowledge the authors.
The results of this study, funded by the CDC, should allay any fears surrounding the vaccination. As babies in the US do not receive their own whooping cough vaccines until they are 2 months old, the provision of early, short-term protection is vital.
Earlier in the year, a study in the UK reported that around 20% of children aged 5-15 visiting their doctor with a persistent cough have whooping cough, despite nearly all children being fully vaccinated for the disease.