The DTaP vaccine has a strong safety record backed by decades of use and continuous monitoring. It protects children against diphtheria, tetanus, and pertussis (whooping cough), and the version used today causes significantly fewer side effects than the older whole-cell vaccine it replaced. Most children experience nothing more than mild soreness or a low-grade fever that resolves within a day or two.
Common Side Effects and How Often They Happen
The most frequent reactions to DTaP are mild and show up within three days of the shot. About 31% of children develop some redness at the injection site (a small patch under 20 mm) across the full vaccine series, and roughly 20% get minor swelling. These are signs of a normal immune response, not a problem.
Low-grade fever (around 100°F to 101°F) occurs in about 21% of children over the course of the series, though it’s less common after the first dose (about 4%) and slightly more likely after the second and third. Moderate fussiness affects about 12% of children across all doses. Higher fevers, above 104°F, are rare, occurring in fewer than 1% of children.
Side effects tend to increase slightly with each successive dose, which is why you might notice more redness or irritability after the second or third shot than after the first. This is normal and reflects the immune system building a stronger response each time.
How Today’s DTaP Compares to the Older Vaccine
The United States switched from the whole-cell pertussis vaccine (DTP) to the acellular version (DTaP) in the 1990s, and the difference in side effects was dramatic. The old vaccine caused severe reactions like inconsolable crying, very high fever, or a limp, unresponsive episode in about 1 out of every 100 doses. Fever-related seizures occurred in roughly 1 out of every 1,750 doses.
With DTaP, those same severe reactions happen in approximately 1 out of every 10,000 doses, a hundredfold reduction. When Canada made the switch, hospital admissions for vaccine-related febrile seizures dropped by 79%, and episodes of limpness and unresponsiveness fell by 60 to 67%. The “acellular” label means the vaccine uses only purified pieces of the pertussis bacteria rather than the whole organism, which is why it triggers far fewer reactions.
Rare Serious Reactions
Severe allergic reactions (anaphylaxis) to DTaP are possible but extremely uncommon with any vaccine. Febrile seizures, which are brief convulsions triggered by a rapid rise in temperature, can occur but are not associated with long-term harm. Children who have a febrile seizure after vaccination recover fully and do not develop epilepsy as a result.
The only medical condition that specifically rules out the pertussis component is encephalopathy (a serious brain disorder) occurring within seven days of a previous dose and not explained by another cause. A family history of vaccine reactions is not a reason to skip DTaP.
Children with a history of Guillain-Barré syndrome within six weeks of a previous tetanus-containing vaccine, or those who had a severe local reaction after a prior dose, are typically evaluated individually before receiving another dose.
What About Ingredients?
One of the most common concerns parents have is about vaccine ingredients. DTaP contains aluminum salts, which serve as adjuvants, substances that help the immune system mount a stronger response so less of the active ingredient is needed. Aluminum-containing vaccines have been used for many decades, and the FDA notes they have “only uncommonly been associated with severe local reactions.” The amount of aluminum in a vaccine dose is far less than what children encounter through food, breast milk, and formula in the course of normal life.
Thimerosal, the mercury-containing preservative that generated concern in the late 1990s, was removed from childhood vaccines in the United States in 2001. Current DTaP vaccines do not contain it. Small amounts of formaldehyde are used during manufacturing to inactivate bacterial toxins, but the quantity remaining in the final product is tiny compared to what the body produces naturally through normal metabolism.
The Five-Dose Schedule
Children receive DTaP as a five-dose series: at 2 months, 4 months, 6 months, 15 to 18 months, and 4 to 6 years. The first three doses build the foundation of protection, while the fourth and fifth doses serve as boosters to extend immunity. DTaP is sometimes given as part of a combination vaccine that also covers other diseases like polio and hepatitis B. Studies have found that combination vaccines do not cause adverse events at a higher rate than giving each vaccine separately, with the exception of a very small additional risk of febrile seizures (less than 4 per 100,000 vaccinations in one large study).
How Vaccine Safety Is Tracked
DTaP doesn’t just get tested before approval and then forgotten. The CDC operates multiple overlapping surveillance systems that continuously monitor vaccines after they reach the public. The Vaccine Adverse Event Reporting System (VAERS) functions as an early warning system, collecting reports from healthcare providers and patients. When VAERS detects an unusual pattern of reports for a particular vaccine, scientists investigate further using more rigorous systems like the Vaccine Safety Datalink (VSD), which draws on medical records from millions of patients.
This layered approach means that even very rare problems, ones that might not surface in clinical trials involving tens of thousands of participants, can be identified and acted on once millions of doses have been administered. It’s the same system that caught the rare blood clotting issue with one of the COVID-19 vaccines within weeks of its rollout.
Safety During Pregnancy
The adult version of this vaccine, called Tdap, is routinely recommended during pregnancy to pass protective antibodies to the baby before birth. A large body of evidence shows it does not increase the risk of high blood pressure during pregnancy, preterm birth, or low birth weight. Getting a flu shot at the same time as Tdap during pregnancy is also safe and does not lead to higher rates of adverse outcomes compared to spacing the two shots apart.