Acellular pertussis vaccines

Pertussis (whooping cough) is a bacterial respiratory infection caused by the gram-negative bacillus Bardetella pertussis. Its hallmark is a cough illness that lasts many weeks. A related organism, Bordetella parapertussis, can produce a less severe pertussis-like syndrome. Pertussis may occur at any age (infection of adolescents and adults is increasingly recognized) but is mast severe in infants and young children. Infants are susceptible to pertussis within the first few weeks or months of life, when the risk for mortality from whooping cough is highest. Pertussis is acquired through direct transmission from close respiratory contact and is highly contagious; as many as 90% of susceptible household contacts will acquire the disease. Before the routine use of whole-cell pertussis vaccine, virtually every child experienced pertussis infection; as many as 270,000 cases of pertussis were reported each year in the United States, with 10,000 deaths.(15) With vaccination, the prevalence of pertussis declined to a nadir of 1010 reported cases in 1976(12) but has progressively increased since. The 7796 cases reported for 1996 was the highest total in more than 30 years. Currently, in countries such as the United States, in which pertussis vaccination is common, the age distribution of pertussis has changed markedly. The increase in pertussis has been greatest among older children and adults, probably reflecting waning vaccine-induced immunity. Because pertussis often is undiagnosed in adolescents and adults, the actual number of cases likely greatly exceeds the number reported. Worldwide, an estimated 40 million pertussis cases occurred in 1994, resulting in 5 million episodes of pneumonia, 360,000 deaths, and 50,000 neurologic complications (including permanent brain damage).(25) The incubation period averages 9 or 10 days. Onset is gradual; an intermittent cough progresses to paroxysms and the characteristic whoop begins. The whoop is caused by forced inspiration through a narrowed glottis immediately following a paroxysm of a dozen or more rapid, short coughs without intervening inspiration. Cyanosis and vomiting may ensue, and these children often are left exhausted; unfortunately, several paroxysms may occur within a few minutes. Between paroxysms, these children may appear normal. Recovery is gradual. Minor complications include subconjunctival hemorrhages, epistaxis, edema of the face, and ulcer of the frenulum. Major complications include pneumonia, encephalopathy, and wasting.