What Are the Side Effects of the Tdap Shot?

The most common side effects of the Tdap shot are soreness, redness, and swelling at the injection site, with arm pain reported by roughly two-thirds of recipients in clinical trials and typically fading within a few days. Beyond the sore arm, a smaller share of people experience headache, fatigue, or mild muscle aches. Serious reactions are rare. But the picture gets more interesting when you look at how the vaccine behaves in different groups, including pregnant women, older adults, and adolescents, and at the handful of uncommon reactions that are worth knowing about.

The Typical Sore Arm and What Comes With It

Injection-site pain is far and away the most frequently reported reaction. In a trial comparing Tdap given at the same time as an influenza vaccine versus given separately, about two-thirds of participants reported pain at the Tdap injection site, and most of it was mild and gone by day three.¹ A large review of passive surveillance data from the U.S. Vaccine Adverse Event Reporting System found that among reports filed for adults 65 and older who received Tdap, about a quarter involved injection-site redness and roughly one in five mentioned pain or swelling at the site.²

Beyond the arm, whole-body symptoms are less frequent but not unusual. In a randomized trial comparing a Tdap booster with a plain tetanus-diphtheria (Td) booster, about one in five Tdap recipients reported headache, and slightly fewer reported fatigue or muscle aches.³ Fever, when it shows up at all, tends to be low-grade. A systematic review of Tdap in pregnancy pegged the rate at about 3% or below.⁴ Most people who experience these symptoms find them manageable, and they rarely last more than a couple of days.

How Tdap Compares to a Plain Td Booster

One worry people sometimes have is that the added pertussis component makes Tdap rougher than the basic Td shot. The data suggest otherwise. An active surveillance study using the Vaccine Safety Datalink found that the risk of adverse events after Tdap was not significantly higher than after Td.⁵ In a randomized head-to-head trial, rates of local reactions like pain, swelling, and redness were comparable between Tdap and Td groups, and the systemic symptoms (headache, fatigue, muscle aches) ran within a few percentage points of each other for both vaccines.³

A pregnancy trial in Mali reinforced this: mild pain was the most common symptom in both Tdap and Td groups, and no participant in the Tdap arm reported a systemic symptom in the week after vaccination, while 6% in the Td arm did.⁶ That said, reactions to any DTP-class vaccine are not driven solely by the pertussis antigens. Aluminum adjuvants and impurities in crude toxoids can also provoke local inflammation, so even Td boosters produce sore arms.⁷

Tdap During Pregnancy

The CDC recommends Tdap during every pregnancy, ideally early in the third trimester, to pass along pertussis antibodies to the newborn. This means millions of doses are given to pregnant people each year, and safety in that group has been closely studied. A systematic review of the evidence found that headache, general tiredness, and muscle aches were the most commonly reported systemic reactions, with fever rates at 3% or lower. Rates of medically attended events were similar between vaccinated and unvaccinated pregnant women.⁴

A randomized clinical trial found that about four in five pregnant women who received Tdap reported an injection-site reaction, which sounds high until you note that the same proportion reported reactions when Tdap was given postpartum instead. The trial found no serious adverse events linked to the vaccine in mothers or infants, and growth and development were similar in both infant groups.⁸ A VAERS review of pregnancy reports over a five-year span found 132 confirmed reports after Tdap in pregnancy, of which six were classified as serious. Those serious cases involved conditions like ectopic pregnancy rupture and stillbirth from placental abruption, and none were attributed to the vaccine itself. No maternal or infant deaths were reported.⁹

Getting Tdap in Back-to-Back Pregnancies

People who have pregnancies close together sometimes wonder whether repeated Tdap doses cause worse side effects or pose any new risks. The safety profile does not appear to worsen with successive doses. An immunogenicity study of Tdap in consecutive pregnancies found that maternal antibody levels at delivery were comparable between the first and second pregnancies. Cord blood antibody levels were lower after the second pregnancy, which reflects a known blunting of transplacental transfer rather than any safety concern in the mother.¹⁰ The practical upshot is that each pregnancy still warrants a Tdap dose for the newborn’s protection, and there is no evidence that repeated shots create escalating side effects for the parent.

Rare but Serious Reactions

Serious adverse events after Tdap are uncommon. In the VAERS review of adults 65 and older, only 6% of the roughly 1,800 reports were classified as serious. Among those, nervous system disorders were the most frequently coded category, followed by infections. Seven deaths were reported, but none were attributed to the vaccine.² A broader disproportionality analysis covering a decade of VAERS data (over 20,000 reports) found that about 96% of all adverse-event reports were classified as non-serious, and the majority of reactions occurred within a single day of vaccination.¹¹

Postlicensure surveillance has flagged a handful of very rare conditions that have been reported after Tdap in adults and adolescents, including Guillain-Barré syndrome, encephalitis, seizures, Bell’s palsy, anaphylaxis, and low platelet counts.¹² Of these, anaphylaxis is the most acutely dangerous and is the reason you are asked to wait 15 minutes in the clinic after any vaccination. It is extremely rare with Tdap and treatable with epinephrine when it does occur.

Guillain-Barré Syndrome and the Limits of Causality

Guillain-Barré syndrome, a condition where the immune system attacks the nerves causing muscle weakness and sometimes paralysis, has drawn particular attention because it has a known (though very small) association with some vaccines. Case reports exist after DTaP and Tdap, but establishing a definitive causal link has proven elusive. A pediatric case report and literature review concluded that while there is a temporal correlation between vaccination and GBS in some patients, “the exact causal relationship between the two remains uncertain.”¹³ The risk, if it exists at all, is on the order of one or two extra cases per million doses, which means it does not change the calculus for the vast majority of people. If you have a history of GBS within six weeks of a previous tetanus-containing vaccine, your doctor may weigh the decision more carefully.

Fainting After the Shot

Syncope, or fainting, is not a side effect of the vaccine itself but rather a vasovagal response to the experience of getting an injection. It is most common in adolescents and young adults. A study of adolescent syncope after vaccination found a rate of about 3 per 10,000 vaccination events. The rate climbed when more vaccines were given at the same visit: roughly 1.5 per 10,000 after a single shot, rising to about 10 per 10,000 after three or more simultaneous vaccines. Around 15% of fainting episodes resulted in an injury, usually from falling.¹⁴ For context, that same study found fainting was actually more common after a routine blood draw than after a vaccination. The practical takeaway is simple: sit down for 15 minutes after your shot, especially if you are a teenager or have fainted before.

Shoulder Injury from Improper Injection Technique

Shoulder Injury Related to Vaccine Administration, or SIRVA, is a reaction that has nothing to do with what is in the vaccine and everything to do with where it goes. If the needle enters too high on the shoulder and injects vaccine material into the bursa or rotator cuff tendons rather than the deltoid muscle, it can trigger significant inflammation, pain, and reduced range of motion. A case report documented a healthy adult woman who developed acute shoulder pain and fatigue within hours of a tetanus toxoid vaccine. She had no prior shoulder problems, and conservative treatment with anti-inflammatory medication and physical therapy provided only partial relief after a month.¹⁵

The good news is that SIRVA is preventable. Proper technique means injecting about two to three finger-widths below the bony point at the top of the shoulder (the acromion), using a needle length appropriate for the person’s body size, and making sure the arm is properly positioned. The existing literature suggests that SIRVA results from an inflammatory response that occurs when vaccine material reaches tissues containing a preexisting antibody.¹⁶ If you develop shoulder pain that does not improve within a day or two and is clearly different from the normal injection-site soreness, it is worth getting evaluated.

Arthus Reactions and Timing Between Doses

An Arthus reaction is an intense local immune response, think dramatic swelling, pain, and sometimes tissue damage at the injection site, that can occur when someone with high circulating antibodies receives another dose of the same antigen. This has historically been a concern with tetanus-containing vaccines if doses are given too close together. In practice, the fear is overblown. A review of eight published studies where Tdap was given within five years of a prior tetanus-containing vaccine found zero Arthus cases.¹⁷

That said, there is a modest increase in ordinary injection-site reactions when doses are spaced more closely together. A study looking specifically at how soon after a Td vaccine you could safely give Tdap found no whole-limb swelling or Arthus-like reactions even at shorter intervals, but noted a slight uptick in injection-site discomfort. The conclusion: Tdap can be safely given as little as 18 months after a prior tetanus-diphtheria vaccine.¹⁸ If you cut your hand on rusty metal and need a tetanus shot even though you recently had one, your doctor does not need to worry much about this.

Getting Tdap and a Flu Shot at the Same Time

A common logistical question is whether combining Tdap with other vaccines at the same visit makes side effects worse. When Tdap and influenza vaccine were given together versus sequentially, the rates and severity of injection-site reactions at the Tdap site were not meaningfully different between groups.¹ A separate study on co-administration of Tdap and influenza vaccine confirmed that adverse event reporting was similar whether the vaccines were given together or apart, and concluded that giving them simultaneously does not compromise the tolerability of either vaccine.¹⁹ You will end up with two sore arms instead of one, but neither arm is likely to hurt more than it would have on its own.

How Age Affects the Experience

Reactogenicity can vary by age in ways that sometimes surprise people. A VAERS review focused specifically on adults 65 and older found that the most common reports involved local reactions, but also flagged that nearly a third of reports in that age group involved coding terms suggesting the vaccine was administered incorrectly, such as wrong route or wrong site.²⁰ This is worth knowing because some of what gets reported as a “vaccine side effect” in older adults is really the consequence of an administration error. The same review concluded there were no safety concerns that warranted further study in that age group.

At the other end of the spectrum, a disproportionality analysis of VAERS data found a notable safety signal in adolescents for opisthotonus, a type of involuntary muscle spasm where the body arches backward. Among non-pregnant women specifically, signals for bursal fluid accumulation and brachial radiculitis (nerve inflammation in the arm) were disproportionately elevated, though no such signals appeared in males.¹¹ These are statistical signals worth monitoring, not established risks, and disproportionality in a passive reporting system does not prove causation. But they do point to the value of the ongoing surveillance systems that track these vaccines in real time.

How Childhood Vaccination History Shapes Your Booster Response

Something you might not expect: the type of pertussis vaccine you received as a child can influence how your body responds to a Tdap booster as a teenager or adult. A randomized study of adolescents compared immune responses to two different Tdap vaccines, stratifying by whether participants had been primed in childhood with the older whole-cell pertussis vaccine (DTwP) or the newer acellular version (DTaP). The DTaP-primed group produced pertussis antibody responses that were as much as 71% lower than those seen in DTwP-primed adolescents in a prior study of the same vaccines.²¹ This is an immunogenicity finding rather than a side-effect finding, but it matters because most people vaccinated in the U.S. since the late 1990s received DTaP as children. Their immune systems respond differently to pertussis boosters, which is part of the reason pertussis outbreaks can still occur even in highly vaccinated populations. The side-effect profile does not appear to differ based on childhood priming, but the effectiveness conversation is ongoing.