How long after a pregnant woman receives a recommended vaccine in the 2nd or 3rd trimester does the fetus start benefiting from passive immunity?

From the Guidelines

After a pregnant woman receives a recommended vaccine in the second or third trimester, the fetus begins benefiting from passive immunity approximately 2 weeks following vaccination, as this is the time needed for the mother's immune system to produce protective antibodies in response to the vaccine. This passive immunity is crucial as it protects the newborn during the first few months of life when they are too young to receive certain vaccines and their own immune system is still developing 1. The most commonly recommended vaccines during pregnancy include the Tdap vaccine (tetanus, diphtheria, and pertussis) ideally given between 27-36 weeks gestation, and the seasonal influenza vaccine, which can be given at any point during pregnancy.

Key Considerations

• The timing of vaccination is important, with vaccinating earlier in the 27-36 week time period maximizing passive antibody transfer to the infant 1.
• Substantial active transport of maternal immunoglobulin G does not take place before 30 weeks of gestation, highlighting the importance of vaccination during the third trimester 1.
• The effectiveness of passive immunity varies depending on the specific vaccine, the timing of administration during pregnancy, and maternal antibody levels, but generally provides protection for the infant for about 2-6 months after birth until their own immune system can respond effectively to vaccines.

Recommendations

• Pregnant women should be vaccinated with Tdap during the third trimester, preferably from 27 through 36 weeks’ gestation, to optimize the concentration of vaccine-induced antipertussis antibodies transported from mother to infant 1.
• Tdap may be simultaneously administered with an inactivated influenza vaccine to pregnant women 1.
• If a woman did not receive Tdap during her current pregnancy and did not receive a prior dose of Tdap, then Tdap should be administered immediately postpartum 1.

From the FDA Drug Label

Antibodies to pertussis antigens from individuals vaccinated during the third trimester of pregnancy are transferred transplacentally to prevent pertussis in infants younger than 2 months of age.

The fetus starts benefiting from passive immunity after the antibodies to pertussis antigens are transferred transplacentally. However, the exact time frame for this transfer is not specified in the provided drug labels. Key points:

• The transfer of antibodies occurs after vaccination during the third trimester.
• The antibodies are transferred transplacentally to prevent pertussis in infants.
• The exact timing of when the fetus starts benefiting from passive immunity is not directly stated in the labels 2 2.

From the Research

Timing of Passive Immunity

• The timing of passive immunity transfer from mother to fetus after vaccination during pregnancy is not precisely defined in terms of exact days or weeks, but studies suggest that it can occur relatively quickly after vaccination.
• According to 3, high levels of anti-pertussis antibodies were detected in cord blood samples from women vaccinated with recombinant pertussis vaccines during pregnancy, indicating effective transfer of passive immunity to the fetus.

Factors Influencing Passive Immunity

• The level of passive immunity transferred to the fetus can depend on factors such as the type of vaccine used, the gestational age at vaccination, and the maternal immune response.
• A study published in 4 found that immunogenicity and vaccine effectiveness were impacted by the timing of pertussis vaccination in pregnancy, with optimal immune responses at birth suggested following early third trimester vaccination.
• Another study 5 reported that the optimal time for Tdap vaccination during pregnancy is at 27-36 weeks' gestation, as recommended by the Advisory Committee on Immunizations Practices (ACIP).

Vaccine-Specific Responses

• Different vaccines may have varying timelines for inducing passive immunity in the fetus.
• For example, a study on influenza vaccination during pregnancy found that maternal immunization can reduce the risk of vaccine-preventable diseases in the first months of life before the start or completion of the suggested vaccination schedule 6.
• The study in 3 specifically investigated the transplacental transfer of maternal antibodies following immunization with recombinant pertussis vaccines during pregnancy and found high levels of passive immunity in infants.

Gestational Age and Vaccination

• The gestational age at the time of vaccination can influence the level of passive immunity transferred to the fetus.
• Research suggests that vaccination in the second or third trimester can result in high levels of passive immunity in infants, as seen in the study on recombinant pertussis vaccination 3.
• However, the exact timing within these trimesters may impact the effectiveness of passive immunity transfer, as indicated by the study on pertussis vaccination timing 4.