Incubation period days

Period of communicability From 1 day before the first signs of infection until 4 days after the rash starts (or 4 days before to 4 days after the rash begins).

Occurrence and distribution Measles has been a severe infection in Western countries for a considerable period of time, producing mortality in poor and slum populations similar to what is seen in developing countries. Introduced with European exploration, it caused devastating epidemics, particularly in island communities, some of which never recovered their former population numbers. However, in many developing countries in which it is a major problem, there is evidence that measles has been present for several hundred years, with the pattern having changed from sporadic epidemics with all ages involved to one of endemicity in which the under-5-year-olds are predominantly affected.

Control and prevention of measles is by vaccination. As measles is such an infectious disease, it can be reckoned that every child will develop it. Some 10% will either have such a mild infection or be partially protected by maternal antibodies as to appear not to have been infected. A further 10-20% will not have measles until the following year due to the epidemic effect; therefore, the expected number of cases of measles can be calculated from the birth rate minus 25%. If the birth rate in a developing country is 50,000, then 75% of this means that 37.5 cases of measles per 1000 can be expected each year, which represents 37,500 cases in an administrative unit of a million people. Calculations like these can be used to estimate the number of children to be vaccinated and hence the vaccine requirements.

Eighty per cent of susceptibles will need to be vaccinated to produce control of the disease, but a lower target may be acceptable in more isolated communities. This target will need to be achieved every year in rural areas, but as much as every 6 months in urban areas. Measles vaccine is 90% effective if the cold chain is not broken.

Maternal antibodies protect the newborn infant for the first 6 months of life, but thereafter the child becomes readily susceptible to infection with a peak around 1 year. The seroconversion rate is some 76% at the age of 6 months, 88% at 9 months and 100% at 12 months. Giving measles vaccination at 1 year would produce the best conversion, but, by this time in developing countries some 50% of the population would have already had the disease. Giving it at 6 months will be before all but a few have had the disease, however the ser-oconversion rate is so poor at this time that not many will be protected. The best compromise is a first vaccination at 9 months, with the possibility of a second opportunity through periodic mass campaigns. In condi tions of high infectivity, such as during an epidemic, admission to hospital or refugee camp or if the infant has HIV infection, then reducing the age of vaccination to 6 months is justified. In this case, another vaccination should be given at 12-15 months.

In developed countries, vaccination is given at 12-15 months so that the time taken to reduce the incidence in the population will be less, as shown in Fig. 12.1. The greater the coverage the more rapidly this is achieved. For example, 60% coverage will theoretically take 12 years to reduce the incidence to zero if vaccination is given at 12 months, but never be achieved at 9 months. However, 70% coverage will achieve zero incidence with vaccination given at 9 months, which is being achieved by an increasing number of developing countries.

Effective measles vaccination coverage will not only reduce the number of children developing the disease in an epidemic, but will have the secondary benefit of raising the age of developing the disease, as can be seen in Fig. 12.2. Epidemics had occurred in Namanyere, Tanzania, regularly every second year until 1978 when there was only a minor increase, the main epidemic being delayed until 1979. This meant that children born in 1977, who could have expected to become infected in their second year of life (1978), had their measles put off until 1979 when they were beyond the age of maximum mortality.

The chances of a susceptible child developing measles when admitted to hospital is very high as it is already sick with another complaint. It is fortunate that measles vaccine can produce protective immunity quicker than the wild virus (about 8 days for the vaccine and 10 days for the disease), so as long as the child is vaccinated within 48 h of admission, it will be protected. Because of the severity of disease in the debilitated child, there are very few contraindications and the malnourished and those with minor infection should all be vaccinated. HIV infection is not a contraindication

Years

Years

Fig. 12.1. The relative impact of immunization programmes on measles incidence in the age group 0-19 years, according to age at vaccination and population coverage. (Reproduced by permission from Cvetanovic, B., Grab, B. and Dixon, H. (1982) Bulletin of the World Health Organization, 60(3), 405-422.)

Years

Fig. 12.1. The relative impact of immunization programmes on measles incidence in the age group 0-19 years, according to age at vaccination and population coverage. (Reproduced by permission from Cvetanovic, B., Grab, B. and Dixon, H. (1982) Bulletin of the World Health Organization, 60(3), 405-422.)

as the child is more likely to die from measles than from complications of receiving a live vaccine.

The policy in many countries is to give a second measles vaccination at 4-5 years or on school entry. Other countries, especially in South and Central America, give a second measles vaccination using vaccination days or special campaigns. Countries of the western hemisphere have set a target for the cessation of measles transmission by 2007.

Fig. 12.2. Prolongation of the time interval between measles epidemics due to vaccination in Namanyere, Tanzania.

Year

Fig. 12.2. Prolongation of the time interval between measles epidemics due to vaccination in Namanyere, Tanzania.

As vaccination coverage is increasing, a potential problem could arise because less maternal antibody is produced by mothers who have acquired their immunity from vaccination rather than by having measles. This means that infants of a younger age will become susceptible, so vaccination may need to be given earlier if coverage is not complete.

Measles vaccine is conveniently combined with rubella (MR) or both with rubella and mumps (MMR). Despite adverse publicity given to the MMR vaccine, no complications have been confirmed and the vaccine should continue to be used.

Treatment There is no specific treatment, but supportive therapy with fluids and easily digested foods needs to be given. Vitamin A supplementation should be given to all children with measles. Complications may require additional treatment, such as antibiotics for bacterial chest infections.

Surveillance All cases of measles should be recorded and monthly totals charted to indicate epidemics and estimate when new epidemics will occur (Fig. 12.2). Measles has a seasonal pattern, which can vary markedly from country to country (Fig. 1.7); so this needs to be determined to work out the best time to supplement routine vaccination programmes. If measles cases are focal, then this may indicate gaps in vaccination coverage (Fig. 3.2). Since all children should be vaccinated, an estimate of vaccine coverage can be made by comparing the number of new vaccinations with the number of children born.

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