The Tdap vaccine contains three sets of active ingredients designed to protect against tetanus, diphtheria, and pertussis (whooping cough). Each 0.5-mL dose delivers inactivated toxins from the tetanus and diphtheria bacteria, plus purified protein fragments from the pertussis bacterium. Beyond those active components, the vaccine includes a small number of inactive ingredients that help it work effectively and stay stable.
The Three Active Components
The “T,” “d,” and “ap” in Tdap each refer to a different part of the vaccine. The lowercase letters for diphtheria and pertussis indicate that these components are present in smaller amounts compared to the childhood version (DTaP), since adolescent and adult immune systems need less material for a strong booster response.
Tetanus toxoid is a chemically inactivated form of the toxin produced by the tetanus bacterium. It can no longer cause harm, but your immune system still recognizes it and builds antibodies against it. Both U.S. Tdap brands contain 5 Lf (a standard unit of measure for toxoid potency) of tetanus toxoid per dose.
Diphtheria toxoid works the same way. The diphtheria bacterium produces a toxin that damages the heart, kidneys, and nervous system, and the inactivated version in the vaccine trains your body to neutralize it. The two brands differ slightly here: Adacel (made by Sanofi Pasteur) contains 2 Lf of diphtheria toxoid, while Boostrix (made by GlaxoSmithKline) contains 2.5 Lf.
Acellular pertussis antigens are purified proteins taken from the surface of the whooping cough bacterium. “Acellular” means the vaccine does not contain whole bacterial cells, only selected proteins. This is what distinguishes modern pertussis vaccines from older whole-cell versions and is the reason side effects are milder. The specific proteins vary by brand, and they target different ways the pertussis bacterium attaches to and damages your airways.
How the Two Brands Differ
The United States uses two Tdap vaccines, and while they serve the same purpose, their formulations aren’t identical.
Adacel contains four pertussis proteins: detoxified pertussis toxin (2.5 micrograms), a protein that helps the bacterium stick to airway cells called FHA (5 micrograms), pertactin (3 micrograms), and fimbriae or FIM (5 micrograms). Boostrix contains three pertussis proteins: inactivated pertussis toxin (8 micrograms), FHA (8 micrograms), and pertactin (2.5 micrograms), but no fimbriae.
Boostrix is licensed for anyone 10 years and older, while Adacel is licensed for ages 10 through 64. In practice, both are used interchangeably for routine boosters, and either can be given during pregnancy.
Inactive Ingredients
A handful of non-active ingredients round out each dose. These fall into a few categories.
- Aluminum salts: These act as an adjuvant, a substance that strengthens your immune response to the vaccine’s active components. Aluminum salts (typically aluminum hydroxide or aluminum phosphate) have been used in vaccines for decades. The amount in a single dose is far less than what you’d absorb from a typical day of eating and drinking.
- Residual formaldehyde: Formaldehyde is used during manufacturing to inactivate the tetanus and diphtheria toxins, turning them into harmless toxoids. Trace amounts may remain in the final product. Your body naturally produces and breaks down formaldehyde as part of normal metabolism, and the residual quantity in a vaccine dose is a tiny fraction of what’s already circulating in your bloodstream.
- Residual glutaraldehyde: Another chemical used to detoxify bacterial toxins during production. Like formaldehyde, only trace amounts carry over into the finished vaccine.
Neither Adacel nor Boostrix contains a preservative. Thimerosal, the mercury-based preservative sometimes asked about, is not in either formulation.
How These Ingredients Trigger Immunity
None of the components in Tdap can cause tetanus, diphtheria, or whooping cough. The toxoids are chemically neutralized, and the pertussis proteins are isolated fragments, not living bacteria. What they can do is look enough like the real threats that your immune system mounts a defense against them.
When the vaccine is injected, your immune cells encounter the toxoids and pertussis proteins and treat them as foreign invaders. Over the following days and weeks, your body produces antibodies specifically shaped to lock onto these proteins. It also creates memory cells that stick around long-term. If you’re later exposed to the actual bacteria, those memory cells ramp up antibody production quickly, neutralizing the toxins or the bacterium before you get seriously ill.
The aluminum adjuvant plays a supporting role here. It holds the vaccine components near the injection site longer, giving immune cells more time to interact with them. This produces a stronger, longer-lasting response than the antigens alone would generate.
How Long Protection Lasts
Tdap protection is not permanent. Immunity to pertussis in particular wanes after a few years, which is one reason whooping cough outbreaks still occur even in well-vaccinated communities. Despite this decline, there is currently no recommendation for repeated pertussis boosters. The standard schedule calls for one dose of Tdap (typically given around age 11 or 12), followed by a tetanus and diphtheria booster every 10 years. That booster can be either Td or Tdap.
Pregnant women are recommended to receive one dose of Tdap during each pregnancy, ideally during the early part of the third trimester. This timing allows the mother’s freshly produced antibodies to cross the placenta and protect the newborn during the first few months of life, before the baby is old enough for their own vaccinations. The vaccine is also given for wound management when a tetanus booster is needed.