DPT vaccine protects against diphtheria, pertussis (whooping cough) and tetanus. Whooping cough or pertussis is a strictly respiratory human disease. It is one of the most common vaccine-preventable childhood disease. Infants under 6 months of age, who are unimmunized or incompletely immunized, continue to suffer the greatest disease burden and mortality. The causal agent is Bordetella pertussis, a Gram negative bacterium. Pertussis causes nearly 300 000 deaths in children every year. Most deaths take place in developing countries. Pertussis is very infectious with high secondary attack rates in households. Complications of pertussis include pneumonia, failure to thrive, post-tussive vomiting, seizures, encephalopathy, cerebral hypoxia, secondary bacterial infection, pulmonary hypertension, sub-conjunctival hemorrhage, and rectal prolapse.

Types of DPT vaccine :
DPwT: Popularly known as triple antigen, DTwP is composed of tetanus and diphtheria toxoids as well as killed whole cell pertussis bacilli adsorbed on insoluble aluminium salts which act as adjuvants. The content of diphtheria toxoid varies from 20 to 30 Lf and that of tetanus toxoid varies from 5 to 25 Lf per dose. Whole-cell pertussis vaccine is a suspension of killed B pertussis organisms. For more than 40 years, the immunization of infants with whole cell pertussis vaccine (Pw) vaccines has been very successful in reducing severe infections and deaths by more than 95%.

DPaT: It is the same diphtheria and tetanus toxoid along with acellular pertussis vaccine. The high rates of side effects to these Pw vaccines directed scientists to develop a second type of vaccine, called the acellular pertussis (Pa) vaccine, composed of inactivated bacterial proteins. The major toxins of pertussis are tracheal cytotoxin (TCT), responsible for the destruction of tracheal ciliated cells, pertussis toxin (PT), and adenylate cyclase-hemolysin (AC-Hly), which synergistically inhibit immune host defences. The major adhesins are filamentous hemagglutinin (FHA), pertactin (PRN), and fimbriae (FIM) which allow the bacteria to adhere to epithelial and immune host cells. All Pa vaccines contain chemically inactivated PT. To this protein, depending on the manufacturer, is added FHA or FHA and PRN or FHA, PRN and FIM. Pa vaccines are easier to produce and better tolerated by infants. However, they are much more expensive. DTaP vaccines are not more efficacious than DTwP vaccines, they have fewer adverse effects. The DTaP vaccines may be preferred to DTwP vaccines in those children with history of severe adverse effects following DTwP vaccines or children with neurologic disorders, if resources permit.

dT vaccine: It has 1/10th dose of diphtheria toxoid than present in DPT. If full dose of diphtheria toxoid is given as present in DPT in children above 10 years of age, it can lead to serious side effects like cardiac toxicity, serum sickness like reactions etc. With lack of natural boosting due to mass vaccination, diphtheria can occur beyond 10-15 years of age in vaccinees. One needs to give booster of diphtheria at 10 years and maybe every 10 years thereafter to maintain protective titres. dT is useful in such cases as a booster at 10 years (instead of TT).

TdaP vaccine: Recently, a low dose, adult-formulated diphtheria, tetanus, acellular pertussis (Tdap) vaccine has been licensed and recommended in several countries for use in adults and adolescents. Efficacy data from a large trial in the USA (APERT), estimates vaccine protection from culture or PCR proven symptomatic pertussis disease is 92%, though protection against less severe coughing illness is likely to be only 50-60%. In individuals who have completed primary and booster vaccination with DTwP/ DTaP, Tdap boosters every 10 years provide sufficient protection.

Dosing and storage
Dose is 0.5ml intramuscularly. It should be stored at 2-8 degree Celsius. The standard schedule is three primary doses at 6, 10 and 14 weeks and two boosters at 15-18 months and 5 years.

Adverse Effects
Pain, swelling, redness, difficulty in walking, fever, lassitude, anorexia, vomiting, irritability and excessive crying is seen in 30-40% of vaccinees. It persists for 24-72 hours & responds to paracetamol. Rarely if the injection has gone into deep fascia there could be swelling & redness of lateral aspect of thigh. Sometimes a nodule forms at the injection site which may persist for several days to weeks. It may soften and form a sterile abscess. It does not merit any treatment except analgesics. If it shows fluctuation, it can be drained.

Whole cell pertussis component may lead to severe adverse effect. It includes hyperpyrexia i.e. fever more than 1050 F, excessive crying & screaming spells lasting for more than 4 hours, convulsion occurring within next 7 days, encephalitis in next 14 days, unconsciousness or altered sensorium, hypotonic, hyporeflexic episode (HHE), shock etc. If a patient develops any of those adverse reactions it is a contraindication to further use of DPwT & instead only DPaT should be used.

Contraindications to pertussis vaccine
Absolute contraindications to any pertussis vaccination (including DTwP vaccine) is history of anaphylaxis or development of encephalopathy within 7 days following previous DTwP vaccination. For patients with history of encephalopathy following vaccination, any pertussis vaccine is contraindicated and only diphtheria & tetanus vaccines may be used. Progressive/evolving neurological illnesses, is a relative contraindication to first dose of DTwP immunization.