Discuss the vaccine strategies that can be employed for the prevention of common childhood virus diseases
Successful immunization programs require both effective vaccines and effective systems to deliver them. Most countries have their vaccine policies set by central government, and the requirements and goals vary greatly between developed and developing countries. Numerous questions need to be addressed before a vaccination program is set up for a particular childhood virus disease:
1. Is the disease worth preventing by vaccination ?
The answer would be a straightforward yes in the case of potentially severe virus infections such as measles and poliovirus, but less straightforward in the case of usually mild virus infections such as mumps and varicella-zoster. The actual needs and priority would also differ between developed and developing countries. eg. rubella vaccination programs are the norm in developed countries whereas most developing countries do not have a rubella vaccination program.
2. What are the goals of the program?
Is the aim to control and eradicate the infection in the general population, or is it just to prevent susceptible individuals from contracting the disease?
For control and eradication to be successful, one must be able to implement an universal vaccination program effectively so that a good coverage rate is achieved. One does not need a 100% coverage rate in order to eliminate a particular virus disease from the community as herd immunity will impede the transmission of the virus. Good access to the target population is essential and this may be achieved by compulsory vaccination. eg. in the U.S. every child must produce a measles vaccine certificate before being admitted to school (schooling itself is compulsory).
The alternate aim would be to control the disease in those individuals who are particularly susceptible to severe infection eg. immunization against VZV in leukaemic children and the immunization against influenza in children with chronic diseases.
How good is the vaccine?
The safety, efficacy, volatility and the cost of the vaccine
should be assessed. Serious adverse effects, however rare, may
discourage parents from taking their child for immunization eg.
in the case of pertussis vaccine. Live vaccines should not be
given to immunocompromized individuals. For universal vaccination,
the vaccine should be as effective as possible as it would be
impractical to test each individual for seroconversion afterwards.
Some vaccines are less stable than others and require good cold
storage facilities eg. Inadequate refrigeration facilities are
thought to be one of the causes of vaccine failure in the case of
the Sabin polio vaccine in third world countries. Finally the
cost of the vaccine and the cost of administering the whole
program should be considered. Cost would be a far more important
consideration in developing countries than developed countries.
The following are common childhood viral diseases for which vaccines are available or being developed.
Measles is a potentially severe disease with severe complications. It is one of the diseases included in the Expanded program on immunization (EPI) of the World Health Organization the aim of which is to control the disease. Universal childhood vaccination is practiced in most countries and the first dose of the vaccine is given at 9 to 12 months. Measles is highly infectious and is proving hard to eradicate. It is increasingly seen in those babies less than 1 year of age who have not been vaccinated.
Rubella is usually a mild disease but can cause severe congenital abnormalities (congenital rubella syndrome) in those infants whose mothers are infected in the first trimester of pregnancy. The aim of any vaccination program is to prevent congenital rubella syndrome. Most developing countries do not have a rubella vaccination program. In developed countries, there had been 2 different strategies in the past. With universal vaccination, all preschool children of both sexes are to be vaccinated the aim of which is to control and eradicate the disease altogether. For this strategy to work, a good coverage is essential as mathematical models show that poor coverage may lead to an increase in the incidence of congenital rubella syndrome as more women of child-bearing become infected by the wild virus. The other strategy is to selectively vaccinate 10 - 14 year schoolgirls before they reach child-bearing age. Again this strategy requires good coverage. A consensus of opinion has emerged in recent years in favour of universal vaccination. The fact that rubella vaccine can be combined with mumps and measles (MMR) makes it attractive to parents to have their child vaccinated. At present, no vaccines are available for Fifth (erythema infectiousum) and Sixth (Roseala Infantum) diseases. Since both diseases are mild, there is no impetus to develop vaccines against these diseases
Two vaccines are available for polio: inactivated (Salk) vaccine and a live (Sable) attenuated vaccine. Polio is one of the infections included in the EPI schedule. Polio vaccine is given as universal vaccination in most countries, as early as just after birth. Because of the ease of administration and the fact that it can induce local immunity, the oral Sabin vaccine is used by most countries in preference to the inactivated Salk vaccine. The latter vaccine is reserved for immunocompromized children and is used exclusively for universal vaccination in some Scandinavian countries. The reason why the inactivated vaccine is used in some countries is that the Sabin vaccine is known on rare occasions to revert to virulence and cases of paralytic polio had been reported. However, the cost of IPV is currently 8 times that of OPV. The fact that inactivated polio vaccine can be combined with DPT vaccine makes this an increasingly attractive option in developed countries which can afford the increased cost of this vaccine. Polio is part of the EPI program of the WHO and is the first virus on the list targeted for eradication. For this program, OPV is chosen because unlike IPV, it will eliminate local virus infection as well as disease. In 2007, 1307 cases were reported worldwide with India and Nigeria being responsible for the majority of cases. Therefore, it will probably take at least several years for eradication to be achieved.
Although mumps is usually a mild disease, it is the leading cause of aseptic meningitis and was responsible for 1500 hospital admissions in the U.K. per year. Although the vaccine is expensive, cost benefit analysis in the U.S. had shown that the cost to the community in not giving the vaccine outweighs the cost of giving the vaccine. However, this would not apply to developing countries. Mumps vaccine usually form part of the MMR vaccine which is given to all children.
Vaccines are available yearly against the prevailing strains
of influenza A and B. Annual universal vaccination against
influenza would not be practical, especially in healthy children.
The vaccine should only be given to those children who are at
increased risk of developing complications eg. those with chronic
heart, respiratory and renal diseases, those with endocrine
disorders and under immunosuppression.
There are now vaccines available against rotaviruses. Rotaviruses cause substantial morbidity and mortality, especially in developing countries. Therefore there is a case for universal vaccination of babies and young children. However, the high cost of the vaccine will hamper its acceptance in poorer countries.
Vaccines are currently available against hepatitis A and B. Although hepatitis A is a common childhood disease, particularly in developing countries, it is highly doubtful whether universal vaccination is a good proposition since the disease is usually subclinical or very mild in children. Although hepatitis B is usually subclinical in newborn and young children, most become carriers who are prone to develop cirrhosis and hepatocellular carcinoma later in life. Therefore in areas of high prevalence, universal vaccination of all newborn is generally recommended, and in areas of low prevalence, vaccination of babies born to mothers who are hepatitis B carriers is recommended.
No vaccines are currently available for HSV, CMV and EBV. A vaccine against EBV would be particularly useful as it is associated with Burkitt's lymphomas in African children and nasopharyngeal carcinomas in Southern China. A live attenuated vaccine is available against VZV which has yet to come into general use. It had been used with considerable success in leukaemic children but since it is a live vaccine, it often causes a mild illness in the vaccine recipients. Universal vaccination against VZV is currently being promoted in the US on the basis of cost-benefit analysis studies. When indirect costs such as days parents have to take off work, and days off from school is taken into account, the cost benefit of vaccination is apparent. However, when only direct costs such as hospital admission and treatment is taken into account, then the cost benefit ratio drops to 1:1 or even less.
To conclude, there are a large number of factors involved in deciding an appropriate vaccination strategy for a particular virus in a particular country. These factors include financial cost of vaccine, incidence of infection, burden of disease, and ease of administering the vaccine. For eradication of a virus to be achieved, effective co-ordination by the WHO would be required between countries.