The optimal window for administering the Tdap vaccine during pregnancy is later rather than earlier in the third trimester. How much later?
Prussian nobleman Otto Von Bismarck is credited with the statement, "Laws are like sausage, it is better not to see them being made.” Those of us familiar with the workings of our Congress would agree that that statement still applies today. The deliberations of the Advisory Committee on Immunization Practices (ACIP) of the CDC may fall into this category as well.
At their last meeting in October, the CDC reviewed some new data1 before making a final recommendation for the use of Tdap in pregnant women. While for other adults Tdap is administered only once, women are supposed to get a dose with every pregnancy. So, four pregnancies in four years would mean 4 Tdap shots in four years. The current recommendation is to give the Tdap injection sometime between 27 and 36 weeks of gestation. The goal is to maximize the antibody levels against the vaccine pertussis antigens in the newborn baby. While we don’t have a precise immune correlate for pertussis (ie, an antibody titer level that we know confers immunity) we do know that the antibody levels obtained in the first year after vaccination with Tdap result in a vaccine efficacy of about 95%. Babies don’t get their first pertussis vaccine until 2 months of age and a single shot only provides partial protection, so starting life with good antibody levels transferred across the placenta will help prevent pertussis infection in the first few months of life.
Lay persons tend to assume we physicians know a lot more outside our specialty than we often do, so we all tend to get asked for our “opinion” about a subject we often don’t deal with. Here’s a scenario: your pregnant next door neighbor catches you at a neighborhood party and, of course, the conversation gravitates to her pregnancy and she asks you, “My OB told me at my last visit that I need a Tdap shot during my last trimester. When is the best time to get it?”
You tell her (choose the best answer):
A. Get it at 36 weeks. Antibody transport to the baby peaks in the last two weeks prior to delivery.
B. Get it at 28-32 weeks. Studies show that giving it in this time frame results in the highest antibody levels in the baby after birth.
C. Get it anytime between 27-36 weeks, which is what the CDC recommends.
D. Get it at 32-34 weeks since waiting until 36 weeks won’t offer much protection if the baby comes 2 weeks early.
Answer: B. Get the Tdap vaccination at 28-32 weeks. Studies show that giving it in this time frame results in the highest antibody levels in the baby after birth.
C is a true statement, but not the best answer. B is the best answer from an “evidence-based medicine” perspective.1 To be clear, the Tdap vaccine can be given any time during pregnancy, but the optimum time is between 27-30 weeks.
So, why didn't the ACIP change the recommendation to reflect this new data which was presented at the October meeting?* The committee’s rationale is consistent with its attempts to factor in the “real world” when making their vaccine recommendations. Members were concerned that changing the recommendation would lead to “recommendation fatigue” (someone’s actual words) resulting in confusion that could cause the provider to make no recommendation and provide no Tdap shot for the pregnant mom. Personally, I give my OB colleagues a little more credit and suspect the ACIP’s concern is in error.
I do appreciate, however, the ACIP’s efforts to simplify our lives in regard to vaccine recommendations. A good example is when two competing vaccines that have slightly different recommended schedules from the manufacturer (based on the initial clinical studies leading to FDA approval) are “harmonized,” with a recommendation that applies to both issued by the ACIP. For example, Rotateq and Rotarix, the two rotavirus vaccines for infants made by Merck and GSK, respectively, have a different maximum age for administration of the first and last dose of the vaccine. The ACIP used expert opinion (since no data exist) to come up with a single recommendation for both vaccines. In the ideal world we would only practice evidence based medicine. In the real world we do the best we can with what we know.
Balancing the need to provide optimum recommendations with the reality of how things happen in the real world can be like making sausage. We love the end result, but may not like how we got there.
Article showing peak antibody levels from vaccine given at different gestational ages:
1. Naidu MA, Muljadi R, Davies-Tuck ML, Wallace EM, Giles ML. The optimal gestation for pertussis vaccination during pregnancy: a prospective cohort study. AJOG. 2016;215:237.e1–237.e6
*If you’d like, you can watch theactual ACIP deliberationson YouTube starting at the 1:15 time frame. If you do watch it you will qualify for the “Vaccine Nerd” club of which I am a charter member.