The pertussis vaccine protects against whooping cough, a highly contagious bacterial infection that can be life-threatening for infants. It’s not given as a standalone shot. Instead, it’s combined with vaccines for diphtheria and tetanus in two formulations: DTaP for children under 7, and Tdap for older kids, teens, and adults. Together, these vaccines helped reduce whooping cough cases in the U.S. from roughly 200,000 per year in the early 20th century to about 5,000 in 2023.
DTaP vs. Tdap: Two Versions of the Same Vaccine
DTaP is the version given to babies and young children. The capital letters indicate it contains full-strength doses of all three components: diphtheria, tetanus, and pertussis. Tdap, used for preteens through adults, contains reduced amounts of the diphtheria and pertussis components. The lowercase letters signal those smaller doses, which are enough to boost immunity that was first built in childhood but aren’t sufficient to start protection from scratch.
How the Vaccine Works
Modern pertussis vaccines are “acellular,” meaning they contain only purified protein fragments from the whooping cough bacterium rather than the whole organism. When injected, these fragments prompt your immune system to produce antibodies that recognize and neutralize the real bacterium if you’re exposed later. The vaccine also trains a type of white blood cell called CD4 T cells, which help fight infection in two ways: they assist antibody-producing cells and they directly activate other immune cells that can kill bacteria.
Older whole-cell vaccines, used widely before the late 1990s, triggered a broader and more aggressive immune response. They were effective, but they also caused more side effects, including high fevers, prolonged crying, and in rare cases, seizures or episodes where children became limp and unresponsive (roughly one case per 1,750 doses). Those safety concerns drove the switch to acellular vaccines, which cause significantly fewer reactions. The tradeoff, as researchers later discovered, is that immunity from the acellular version doesn’t last as long.
The Recommended Schedule
Children receive five doses of DTaP to build and maintain protection through early childhood:
- 2 months
- 4 months
- 6 months
- 15 through 18 months
- 4 through 6 years
The first three doses build a high level of protection. The final two are boosters that extend it through the early school years. Preteens then get a single Tdap shot between ages 11 and 12 to restore waning immunity. After that, adults need a booster of Tdap or Td (tetanus-diphtheria only) every 10 years.
Protection During Pregnancy
Pregnant women are recommended to receive a Tdap shot during every pregnancy, ideally between 27 and 36 weeks of gestation. Getting vaccinated earlier in that window is better. The goal is to give the mother’s body enough time to produce antibodies that cross the placenta and reach the baby before birth, providing passive protection during the first weeks of life when the infant is too young to be vaccinated. At least two weeks between vaccination and delivery appears necessary for meaningful antibody transfer. This strategy is one of the most effective tools for preventing whooping cough in newborns, who face the highest risk of severe complications and death from the disease.
How Effective the Vaccine Is
Right after completing the childhood series, the acellular pertussis vaccine is about 91% effective at preventing whooping cough. That’s strong initial protection, but it doesn’t hold steady. Effectiveness drops by roughly 10% each year. One large study tracking children after their fifth DTaP dose found that protection was 98% in the first year, fell to about 87% by year three, and was down to around 71% by year five.
The pattern is similar after the preteen Tdap booster, which starts at about 85% effectiveness and also declines by roughly 12% per year. A study of adolescents in Wisconsin found that protection dropped from 75% in the first year after Tdap to about 35% by year two, and was essentially gone by years three and four. This waning immunity is a well-recognized limitation of the current acellular vaccines and is a major reason booster doses are needed at multiple points throughout life. It also helps explain why outbreaks still occur, particularly among teens and young adults who are several years past their last dose.
Common Side Effects
Most side effects are mild and resolve within one to three days. For DTaP in children, the most common reactions include soreness or swelling at the injection site, low-grade fever, fussiness, fatigue, decreased appetite, and occasional vomiting. Severe injection site reactions are rare and may be less likely when the shot is given in the thigh rather than the arm.
Tdap side effects in older children and adults follow a similar pattern: pain, redness, or swelling at the injection site, mild fever, headache, fatigue, and sometimes stomach symptoms like nausea or diarrhea. Studies have consistently found that serious reactions to either version are uncommon.
Who Should Not Get the Pertussis Vaccine
There is only one contraindication specific to the pertussis component: encephalopathy (a serious brain condition) occurring within seven days of a previous dose that can’t be explained by another cause. People with this history should not receive the pertussis-containing vaccine again, though they can still receive the tetanus and diphtheria components separately. For the vast majority of people, including those with mild illnesses, egg allergies, or a family history of seizures, the vaccine is considered safe.
Why Immunity Fades Faster Than Expected
The shift from whole-cell to acellular vaccines improved safety but changed how the immune system responds. Whole-cell vaccines stimulate a type of immune response (called Th1) that is particularly effective at activating cells that kill bacteria directly. Acellular vaccines lean more heavily on a different pathway (Th2) that focuses on antibody production. Both approaches generate strong antibody levels initially, but the Th2-skewed response from acellular vaccines appears to fade more quickly and may be less effective at preventing people from carrying and spreading the bacterium even when they don’t get sick themselves. This difference in immune programming, established in infancy, seems to persist even after later booster doses.