The Tdap vaccine is a combination immunization designed to provide protection against three bacterial diseases: tetanus, diphtheria, and pertussis (whooping cough). This single vaccine is an important public health tool for adolescents and adults, serving as a booster shot to maintain protection established during childhood.
The Diseases TDAP Prevents
Tetanus is caused by the bacterium Clostridium tetani, which is found in soil and enters the body through cuts or puncture wounds. The infection produces a toxin that affects the nervous system, leading to painful muscle stiffness and spasms throughout the body, often referred to as “lockjaw.”
Diphtheria is caused by Corynebacterium diphtheriae, which typically infects the nose and throat. The infection can lead to a thick coating, causing breathing difficulties, and the bacterial toxin can also damage the heart and nerves.
Pertussis (whooping cough) is a respiratory illness caused by Bordetella pertussis bacteria. While uncomfortable for older children and adults, it is dangerous for infants, causing severe, uncontrollable coughing fits that make it difficult to breathe, feed, or drink.
Measuring Initial Protection Levels
The Tdap vaccine provides high initial protection against the tetanus and diphtheria components. Following the full primary series and subsequent booster doses, effectiveness against tetanus and diphtheria is estimated to be nearly 100%. This protection is inferred from the generation of specific antitoxin levels in the blood.
Protection against the pertussis component is generally lower and more variable than the other two. Immediately after receiving a Tdap booster, effectiveness against pertussis ranges from approximately 68% to over 75% in adolescents and adults. This lower rate explains why pertussis is the most common breakthrough infection seen in vaccinated individuals. The acellular nature of the current pertussis vaccine component, which uses only parts of the bacteria, is thought to contribute to this difference in protection levels.
Duration of Immunity and Booster Needs
Waning immunity is most noticeable with the pertussis component. Protection against whooping cough does not last as long as protection against tetanus and diphtheria, and effectiveness can decline rapidly after the first year following vaccination.
Studies show that pertussis protection drops from its initial peak to as low as 8.9% to 34.2% within two to four years after vaccination. This rapid reduction in defense allows pertussis outbreaks to occur even in highly vaccinated populations. Routine booster doses of Tdap or Td are recommended every 10 years to maintain high levels of protection against tetanus and diphtheria.
A booster for the tetanus component may be recommended sooner than the standard 10-year interval if an individual sustains a contaminated wound or burn. A booster dose may be given if it has been more than five years since the last Tdap or Td vaccination. This schedule reflects the differing durability of the immune response to each of the three components.
Protecting Newborns Through Maternal Vaccination
The Tdap vaccine is administered during pregnancy to protect newborns. Pregnant individuals are recommended to receive a dose of Tdap during every pregnancy, ideally between 27 and 36 weeks of gestation. This timing maximizes the transfer of protective antibodies across the placenta to the developing fetus.
This strategy provides the infant with passive immunity, offering protection against pertussis during the first two months of life before they begin their own primary vaccination series. Research indicates that maternal Tdap vaccination is approximately 78% effective at preventing pertussis in infants younger than two months old. Maternal immunization is estimated to be about 90% effective at preventing hospitalization and severe disease in infants who contract pertussis.