Comparison of Tdap vaccine immunogenicity between pregnant and non-pregnant active duty service members.

BACKGROUND: Tetanus, diphtheria, and acellular pertussis (Tdap) vaccination in pregnancy is recommended to provide passive immunity to newborns. Few studies have assessed how pregnancy modifies vaccinated individuals' immune response to Tdap vaccination, with most focused on short-term responses (i.e., within 1 month). Here, we assessed anti-pertussis toxin (PT) immunoglobulin G (IgG) levels 9-15 months following Tdap vaccination in pregnant and non-pregnant active duty service members (ADSMs). METHODS: This observational cohort study included 240 ADSMs pregnant at Tdap vaccination, 1:1 matched with 240 non-pregnant ADSMs on age, time between vaccine and serum collection, and receipt of other same-day vaccines (2011-2016). Serum samples (0-6 weeks before and 9-15 months after vaccination) were obtained from the Department of Defense Serum Repository. Anti-PT IgG levels were assessed using geometric mean concentrations (GMCs), and associations between pregnancy status at vaccination and anti-PT IgG levels were estimated through multivariable linear regression models. RESULTS: Pregnant vs. non-pregnant ADSMs had higher anti-PT IgG GMCs in both pre- (9.01 vs. 5.81, P < .01) and post-vaccine (27.71 vs. 22.33, P = .02) serums. Post-vaccine multivariable models adjusting for pre-vaccine levels were not significantly different between pregnant vs. non-pregnant ADSMs (b = -0.05, P = .42). Associations were modified by time since prior Tdap booster, and statistically different for pregnant individuals who received a booster <2 years prior (b = -0.22, P = .02). CONCLUSION: Tdap vaccination in pregnancy yielded a similar long-term anti-PT IgG response to vaccination outside of pregnancy; findings substantiate the current recommended vaccine schedule.