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(1) Background: The adverse-effect profile and short-term obstetric and neonatal outcomes among pregnant women who were vaccinated with the BNT162b2 vaccine at any stage of pregnancy do not indicate any safety concerns. The vaccine is effective in generating a humoral immune response in pregnant women. (2) Objective: To determine the vaccine-induced immunity and adverse events associated with the third (booster) dose of the BNT162b2 vaccine compared to the first and second dose of the vaccine among pregnant women. (3) Study design: A prospective cohort study in a tertiary referral center comparing pregnant women who were vaccinated by the first and second dose of the BNT162b2 (Pfizer/BioNTech) vaccine to pregnant women vaccinated by a third (booster) dose, between January and November 2021. A digital questionnaire regarding adverse events was filled by both groups 2–4 weeks after vaccination. Blood samples were collected and tested for SARS-COV-2 IgG antibodies 28–32 days after the administration of the second or third BNT162b2 dose. (4) Results: Seventy-eight pregnant women who received the first and second doses of the vaccine were compared to eighty-four pregnant women who received the third dose of the vaccine. In terms of adverse events following vaccination, local rash/pain/swelling (93.6% vs. 72.6%, p < 0.001) was significantly less common after the third vaccination compared to after the second vaccination. Other adverse events, including early obstetric complications, did not differ between the two groups. SARS-CoV-2 IgG serum levels 28–32 days after the vaccination were significantly higher after the third vaccination compared to the second vaccination (1333.75 vs. 2177.93, respectively, p < 0.001). (5) Conclusion: This study confirms the safety regarding early adverse events and immunogenicity, and the lack of early obstetric complications of the BNT162b2 second- and third-dose vaccine in pregnant women. The third (booster) dose is effective in generating a stronger humoral immune response in pregnant women compared with the second dose.

Pregnant women with coronavirus disease 2019 have been shown to be at increased risk of severe maternal outcome 1 and are recommended to get vaccinated with the first and second doses of the BNT162b2 vaccine, which has shown encouraging results in terms of its safety and effectiveness in pregnancy 2,3 . The administration of a third (booster) dose of the BNT162b2 vaccine following the emergence of the B.1.617.2 (Delta) variant has proved to be effective in lowering rates of confirmed infection, severe illness and COVID-19-related death 4 and is, therefore, also recommended for pregnant women. We report on the adverse events and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunoglobulin G (IgG) serum levels in pregnant women who received the third dose of the BNT162b2 vaccine.This was a prospective cohort study between August and November 2021 of pregnant women who received the third dose of the BNT162b2 vaccine and were matched by age in a 1:1 ratio to a control group of non-pregnant women who received the third dose of the vaccine during the same time period. All women had received two doses of BNT162b2 vaccine at least 5 months earlier. Blood samples were collected and tested for SARS-CoV-2 IgG antibodies (in binding antibody unit (BAU)/mL) before and 33 ± 3 days after the administration of the third BNT162b2 dose. Women with medical conditions,