Pertussis, also known as 'whooping cough', is a highly infectious bacterial disease of the respiratory tract and is spread by droplet infection.
Pertussis infection is caused by a gram-negative bacterium called Bordetella pertussis which is found in the back of the throat of an infected person. The incubation period is on average 7-10 days (range from 5-21 days) and the infectious period is for 21 days after the onset of symptoms.
Pertussis has the highest incidence in infants. Young infants are at highest risk of severe complications, hospitalisation and death. Adults and older children are often the source of infection for younger siblings at home.
Protection against whooping cough infection is not life-long and even after natural disease, individuals can get re-infected and spread infection to others. The same is true after whooping cough vaccination, although infection in fully vaccinated individuals is normally very mild. Because of this, the disease still continues to occur in cycles despite the high coverage of the vaccine. Protection from vaccination against serious whooping cough is very high for the first few years of life, when the risk of complications is greatest. Protection is extended further by the booster dose given shortly before children go to school. However, vaccinated people can get a mild infection, particularly as immunity wanes in late adolescence and adulthood, and these people may spread infection to those children who are too young to be vaccinated. When pertussis is circulating, this boosts people’s protection and eventually helps to stop transmission. However, a few years later immunity wanes again and infection can spread more easily – leading to these intermittent epidemics.
An irritating cough gradually becomes outbursts of coughing (paroxysms), usually within one to two weeks, and often lasts for two to three months ("one-hundred-day cough"). Symptoms may start similar to a common cold, progressing to coughing and choking spells which may be followed by vomiting and/or a characteristic ‘whoop’. In young infants, the typical ‘whoop’ may never develop and coughing spasms can be followed by difficulty in breathing (apnoea). Severe complications and deaths occur mostly in infants under six months of age. Serious illness is less common in older children and adults (1 in 15,000 - 21,000), however, they have the potential to transmit infection to vulnerable babies.
Before the introduction of pertussis immunisation in the 1950s, the average annual number of notifications in England and Wales exceeded 100,000. In 1972, when vaccine acceptance was over 80%, there were only 2069 notifications of pertussis. Public anxiety about the safety and efficacy of the vaccine, following a report published which suggested a possible link between the vaccine and a group of children with brain damage, saw immunisation coverage drop to 30% in 1975 resulting in major epidemics in 1977/79 and 1981/83. As a result, there were more than 200,000 extra notifications and 100 deaths in 1970s and 1980s. Vaccine coverage steadily increased over the next decade as public and professional confidence in the vaccine was restored, reaching 95% by second birthday in 1995 and remaining at between 93% and 95% until 2010 when it increased to 96%. Correspondingly, overall notifications decreased dramatically during this period. The Health Protection Agency (HPA) initiated a programme of enhanced surveillance to monitor the number of cases of whooping cough and vaccine efficacy in 1994.
However, despite a high vaccination uptake, the burden of pertussis in England and Wales remains highest in infants too young to be fully protected through vaccination. Following a detailed study of pertussis in infants on paediatric intensive care units (PICUs) and analysis conducted by the HPA, in November 2001, a pre-school booster dose of pertussis vaccine (given between 3-5 years of age) was added to the routine immunisation schedule with the aim of reducing illness in older age groups thereby reducing transmission of pertussis to babies too young to be fully protected. Continued improvements in pertussis control in children <10 years of age, including disease reduction in young infants, have been observed in England/Wales since the preschool booster was introduced.
Pertussis epidemiology can be affected by a number of factors, including changes in laboratory techniques and heightened awareness of the disease. Overall increases in laboratory confirmed cases have been observed in older age groups and subsequent to the introduction of new laboratory methods from 2001. Increased pertussis activity was identified from the third quarter of 2011, however, with cases in those aged 15 years and older exceeding expected levels. This may be in part due to increased awareness amongst health professionals improving case ascertainment in older age groups. This is reflected by the increased demand for serology testing which is the predominant method of confirmation in adolescents and adults who typically present with milder features late in the course of the illness. However, waning immunity following vaccination and /or natural infection is also likely to be an important contributory factor. This increased activity has continued into the first quarter of 2012 and has extended into <3 month infants. Although a greater number of cases are being confirmed in older age groups, the incidence in these age groups remains relatively low.
In England and Wales, whooping cough is statutorily notifiable whereby a diagnosis is usually made on clinical grounds without the requirement for laboratory confirmation. Notifications provide timely data relating to trends over time and by age.
Diagnosis can be confirmed through isolation of the B. pertussis organism through culture. However culture lacks sensitivity and since 2001, the enhanced diagnostic methods based on PCR and serological testing, have been made available by the HPA. These methods have increased case ascertainment, particularly the use of serology in adults.
Classic (severe) pertussis, as defined by the World Health Organization (WHO), consists of at least 21 days of cough illness with paroxysms, associated whoops or post-tussis vomiting, and culture confirmation. Mild pertussis is any laboratory-confirmed pertussis disease that does not meet the criteria for classic disease.
Acellular pertussis vaccine is given in the primary course with diphtheria, tetanus, polio and Hib, as DTaP/IPV/Hib, given at aged 2, 3, & 4 months of age. A further booster dose with acellular pertussis, given as dTaP-IPV, is given with the preschool boosters 3 years after the completion of the primary course and before school entry.
Children and adults can catch pertussis even if they were vaccinated in the past because both natural and vaccine immunity wane over time. Pertussis commonly lasts 6-8 weeks even when treated with antibiotics, with severity of symptoms related to age. The most severe infections are usually in infants, and over 90% of laboratory confirmed cases in infants are hospitalised. Morbidity and mortality is greatest in those aged less than 6 months of age. Close contacts of pertussis cases and those who are particularly vulnerable, unvaccinated, partially vaccinated or less than five years of age are given erythromycin treatment or prophylaxis. Although the evidence base for using newer macrolides antibiotics such as azithromycin and clarythromicin is less extensive than erythromycin these are considered to be suitable alternatives for treatment and prophylaxis and are better tolerated.
In 1974 a paper was published suggesting a link between the whooping cough vaccine and brain damage. Because of this concern, the number of children receiving the vaccine fell. As a consequence, the number of cases and deaths from the disease rose. Subsequent research shows that, if there is a long-term risk of brain damage from the whole cell vaccine, it is rare, whereas the disease itself is known to cause brain damage and death. The vaccine that caused these unfounded concerns is no longer used in the UK and has been replaced with acellular vaccine (see below).
Until 2004 a whole-cell whooping cough vaccine was used routinely in the UK. This vaccine was made from non-live whole bacteria that cause whooping cough. The whole cell vaccines provided excellent protection against whooping cough but unfortunately tended to cause side effects such as fever, irritability and redness and swelling at the injection site. Acellular vaccines, which contain only the parts of the whooping cough bacteria that are important for an immune response, have been carefully developed over the years so that they now offer similar protection to the whole cell vaccines. As they cause fewer side effects than the whole cells vaccines, it was possible to introduce a booster acellular pertussis vaccine dose in 2001. In 2004, the UK changed its primary vaccination programme so that we now only offer the acellular vaccines to protect children in the UK against whooping cough.