The Tdap vaccine has a strong safety record. Both FDA-approved versions, Boostrix and Adacel, have been in use since 2005, and ongoing surveillance across multiple monitoring systems has found no unexpected safety concerns in the general population, pregnant women, or adults over 65. The most common side effects are mild and resolve on their own within a few days.
Common Side Effects
The side effects most people experience after a Tdap shot are localized and short-lived. Pain, redness, or swelling at the injection site is the most frequent reaction. Some people also develop a mild fever, headache, fatigue, or stomach symptoms like nausea or diarrhea. These typically fade within one to three days without treatment.
While injection site reactions are common, studies have found that severe injection site reactions (significant swelling or pain that limits arm movement) occur at a low rate. If your arm is sore, gentle movement and a cool compress can help.
Serious Reactions Are Extremely Rare
Severe allergic reactions to vaccines in general occur at a rate of roughly 0.65 to 1.53 cases per million doses, based on a large analysis of over 7.6 million vaccine doses published by the American Academy of Pediatrics. For context, that’s less than a one-in-a-million chance.
Studies using the Vaccine Safety Datalink, a network that tracks health outcomes in millions of vaccinated people, have found no association between Tdap vaccination and Guillain-Barré syndrome or other neurological disorders. Broader analyses have also found no increased risk of allergies, blood disorders, or chronic illnesses following Tdap.
Who Should Not Get the Vaccine
A small number of people have genuine reasons to avoid or delay Tdap. You should not receive the vaccine if you have had a severe allergic reaction (anaphylaxis) to a previous dose or to any ingredient in the vaccine. The same applies if you experienced encephalopathy, a serious brain condition involving prolonged seizures or altered consciousness, within seven days of a prior dose of any diphtheria, tetanus, or pertussis vaccine.
Other reasons to hold off include:
- Guillain-Barré syndrome within six weeks of a previous tetanus-containing vaccine
- Unstable neurological conditions such as uncontrolled seizures, until treatment has been established and the condition is stable
- Arthus-type hypersensitivity (a severe local reaction) after a prior tetanus or diphtheria vaccine, in which case vaccination should be deferred until at least 10 years have passed
- Moderate or severe acute illness, which is a reason to wait until you’ve recovered, not a permanent contraindication
Safety During Pregnancy
The CDC recommends Tdap during the 27th through 36th week of every pregnancy, preferably in the earlier part of that window. After vaccination, a pregnant woman’s body produces protective antibodies and transfers high levels of them to the baby before birth. This gives the newborn short-term protection against whooping cough during the vulnerable first weeks of life, before the baby is old enough for their own vaccinations.
The safety data for Tdap in pregnancy is reassuring. The American College of Obstetricians and Gynecologists states there is no evidence of adverse fetal effects from vaccinating pregnant women with inactivated vaccines like Tdap. Safety data remains reassuring even for women who receive Tdap in back-to-back pregnancies spaced closely together. The FDA formally approved both Boostrix (in 2022) and Adacel (in 2023) specifically for use during pregnancy to prevent infant pertussis.
Beyond safety, the benefits extend to the baby’s outcomes. Infants whose mothers received Tdap during pregnancy had significantly lower rates of hospitalization and intensive care admission from whooping cough, even in cases where maternal vaccination didn’t completely prevent infection.
What’s in the Vaccine
Tdap contains small, inactivated pieces of three bacteria rather than live organisms. It uses lower doses of diphtheria and pertussis components than the DTaP version given to young children, which is one reason it tends to cause fewer reactions in older recipients.
The vaccine contains aluminum phosphate, which helps strengthen the immune response. A single dose of Adacel contains 0.33 mg of aluminum, a fraction of what most people consume daily through food and water. Trace amounts of formaldehyde, used during manufacturing to inactivate the bacterial toxins, remain in the final product at levels well below what the body naturally produces. Tdap does not contain thimerosal (a mercury-based preservative).
How Safety Is Monitored
Tdap safety doesn’t rest on pre-approval clinical trials alone. The CDC monitors vaccine safety through four complementary systems that continue tracking outcomes long after a vaccine reaches the market. The Vaccine Adverse Event Reporting System (VAERS) serves as a national early warning system, collecting reports of any health problem that occurs after vaccination. Scientists look for unusual patterns or clusters in these reports, called “signals.”
When a signal appears, researchers investigate further using the Vaccine Safety Datalink, which links vaccination records with medical outcomes for millions of people. This system can determine whether a health problem is actually occurring more often in vaccinated people or just coincidentally. For Tdap, these deeper investigations have consistently confirmed the vaccine’s safety profile.
Booster Schedule for Adults
Adults need a tetanus and diphtheria booster every 10 years to maintain protection. One of those boosters should be a Tdap dose (which adds pertussis protection); subsequent boosters can be either Tdap or Td. Pregnant women are the exception to the 10-year interval. They are recommended to get Tdap during every pregnancy regardless of when they last received a dose, because the goal is to pass fresh antibodies to each baby.