Acellular pertussis is a type of vaccine against whooping cough that contains only a few purified proteins from the bacterium Bordetella pertussis, rather than the whole killed bacterium. It replaced the older whole-cell pertussis vaccine in the 1990s because it causes significantly fewer side effects while still providing protective immunity. In the United States, acellular pertussis is never given on its own. It’s always combined with tetanus and diphtheria vaccines in the formulations known as DTaP (for young children) and Tdap (for adolescents and adults).
How Acellular Differs From Whole-Cell Vaccines
The original pertussis vaccine, developed decades ago, used entire killed Bordetella pertussis bacteria. That whole-cell approach was effective, but it exposed the immune system to thousands of bacterial components at once, many of which weren’t necessary for building immunity. The result was a higher rate of side effects: fever, fussiness, and swelling at the injection site were common, and rare but more serious reactions drew public concern.
Acellular pertussis strips the vaccine down to just the components that matter. Instead of a whole bacterium, it contains between one and five purified proteins that the bacterium naturally produces. The five proteins tested and used in various combinations are pertussis toxin, filamentous hemagglutinin, pertactin (an outer membrane protein), and two fimbrial proteins. Different manufacturers use different combinations. One widely used formulation contains three of these proteins, while another contains all five. The word “acellular” simply means “without cells,” reflecting the absence of whole bacterial cells.
What’s in DTaP vs. Tdap
DTaP is the version given to infants and children under seven. It contains higher amounts of the pertussis and diphtheria components, calibrated for young immune systems encountering these antigens for the first time. Tdap is the adolescent and adult formulation, which contains reduced quantities of both the pertussis and diphtheria proteins. The lowercase letters in “Tdap” signal those lower doses. Tdap is less likely to cause injection-site swelling and pain, which tends to be more of an issue in older children and adults receiving repeat doses.
Both formulations contain the same types of pertussis proteins. The difference is purely in the amount.
How It Trains the Immune System
Because acellular pertussis contains only a handful of purified proteins paired with a single immune-boosting ingredient, it triggers a narrower immune response than either natural infection or the old whole-cell vaccine. Specifically, it tends to activate a branch of the immune system that produces antibodies (a Th2-dominant response) while generating weaker cellular immunity. Whole-cell vaccines and natural infection, by contrast, activate a broader combination of immune pathways, including the types of immune cells that directly attack infected cells and help clear bacteria from the airways.
This difference has practical consequences. Studies comparing blood samples from children vaccinated with acellular versus whole-cell vaccines found distinctly different immune profiles years later. Children primed with the acellular vaccine produced high levels of antibody-signaling molecules but very little of the signaling molecules associated with bacterial clearance. Children primed with the whole-cell vaccine showed the opposite pattern. Research in animal models confirmed the same split: acellular vaccines produced strong antibody responses but minimal cellular immunity, while whole-cell vaccines generated robust cellular responses.
How Long Protection Lasts
Protection from the acellular pertussis vaccine wanes faster than researchers initially hoped. After the fifth childhood dose (given around age four to six), immunity drops substantially over the following five years. A large study conducted during a California pertussis outbreak, published in the New England Journal of Medicine, found that children aged eight to eleven who had received the full five-dose DTaP series had the highest pertussis rates, suggesting the fifth dose’s protection was relatively short-lived. Earlier clinical trials had estimated protection lasting five to six years after three or four doses, but real-world data showed immunity declining steadily each year after that final childhood shot.
This waning is a key reason adolescents need a Tdap booster and why pertussis outbreaks still occur even in well-vaccinated populations. The vaccine prevents severe disease and complications effectively, but its ability to block infection altogether fades with time.
The Recommended Schedule
Children receive five doses of DTaP: at 2, 4, and 6 months, again between 15 and 18 months, and a final dose between ages 4 and 6. At age 11 or 12, a single Tdap booster is recommended. Adults who never received Tdap should get one dose at any time. After that, no additional pertussis-specific boosters are currently recommended, though adults need a tetanus-diphtheria booster every 10 years, which can be given as either Td or Tdap.
Pregnant women are a special case. The CDC recommends a dose of Tdap during every pregnancy, ideally between 27 and 36 weeks of gestation, with earlier in that window being better. This timing allows the mother to build antibodies that cross the placenta and give the newborn passive protection during the vulnerable first weeks of life, before the baby is old enough for their own vaccinations.
Side Effects
Acellular pertussis vaccines are considerably milder than the old whole-cell versions. The most common reactions are local: redness at the injection site (about 25% of reports), fever (roughly 20%), and swelling at the injection site (about 15%). Warmth at the injection site, mild rash, and hives occur less frequently. These reactions are typically mild, resolve on their own, and are consistent with what was observed in the clinical trials that led to the vaccine’s approval.
Serious adverse events are rare. The safety profile was the entire reason acellular pertussis was developed. The trade-off, as researchers have come to understand it, is a vaccine that causes far fewer reactions but also produces a somewhat less durable and less broadly protective immune response than its whole-cell predecessor.