Safety and immunogenicity of a single dose of Tdap compared to Td in pregnant women in Mali and 3 its effect on infant immune responses: a single-centre, randomised, double-blind, active-controlled phase 2 study.

Background: While maternal pertussis vaccination is a strategy to reduce infant morbidity, safety and immunogenicity data are limited in sub-Saharan Africa. We aimed to evaluate the safety of a single dose of tetanus, diphtheria and acellular pertussis vaccine (Tdap) vaccine compared to tetanus and diphtheria vaccine (Td) vaccine in pregnant women in Bamako, Mali and to assess the pertussis toxin (PT) antibody response at birth.

Methods: In this phase 2, single-centre, randomised, double-blind, active-controlled study, from 23 January 2019 to 10 July 2019, healthy 18-39 year old women in the second trimester of a singleton pregnancy were randomised 2:1 to receive Tdap or Td. Blood was tested for serum immunoglobulin G (IgG) against PT and other vaccine antigens using a qualified Meso Scale Discovery multiplex immunoassay. The co-primary objectives evaluated safety and birth anti-PT levels. Infant immune responses to whole-cell pertussis vaccine (DTwP) were assessed. Statistical analysis was descriptive. This trial is registered with clinicaltrials.gov, NCT03589768.

Findings: 133 women received Tdap and 67 received Td, with 126 and 66 livebirths, respectively. In the Tdap group, 22 serious adverse events (SAEs) including one maternal death occurred in 20 participants (15·0%), with 10 SAEs in 10 participants (14·9%) in the Td group. Among infants, 18 events occurred among 13 participants (10.3%) and 8 SAEs in 6 participants (9.1%), including three and two infant deaths, occurred in Tdap and Td groups, respectively. None were related to study vaccines. Anti-PT geometric mean concentration (GMC) at birth in the Tdap group was higher than in the Td group (55.4 [46.2-66.6] IU/ml vs 7.9 [5.4-11.5] IU/ml). One month after the third dose of DTwP, the GMC in infants born to mothers in the Tdap group were lower compared to the Td group (20.2 [13.7-29.9] IU/ml vs 77.2 [32.2-184.8] IU/ml). By 6 months of age, the anti- PT GMCs were 17.3 [12.8-23.4] IU/ml and 67.1 [35.5-126.7] IU/ml in Tdap and Td groups, respectively. At birth, anti-tetanus toxin (TT) GMCs were higher in infants in the Td vs Tdap group (5.9 [5.0-7.0] IU/ml vs 4.1 [3.5-4.8] IU/ml). Anti-diphtheria toxin GMCs were similar in both groups.

Interpretation: Tdap administered to pregnant women in Mali is safe and well-tolerated. Infants of mothers who received Tdap were born with high PT and protective anti-TT antibody levels. By six months of age, after primary vaccination, the PT levels were lower in the Tdap group compared to the Td group. The blunted immune responses to primary DTwP vaccination in the Tdap infant group warrant further study.

Funding: This project was funded by National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH), under contract numbers 75N93021C00012 (The Emmes Company), and HHSN27220130000221 (University of Maryland, Baltimore). Dr. Susana Portillo was supported by NIH award no. T32AI007524. NIAID, NIH provided Tdap vaccine (BOOSTRIX).