Discuss the eradication of viruses that cause disease in man
Viruses account for the bulk of infectious diseases. Effective vaccines are now available against many viruses making eradication a viable proposition. Smallpox is the first and only virus disease to be completely eradicated. It was eradicated through mass vaccination, and more importantly, the tracing and isolating of known cases and contacts. There were certain features of smallpox which made it a relatively easy target for eradication.
- Smallpox was a severe disease with significant morbidity and mortality. It had already been eradicated from many developed countries before the WHO campaign began, thus demonstrating the feasibility of global eradication. Eradication would result in significant savings in terms of the cost of vaccination to non-endemic countries. Therefore, the will power was there for eradication.
- The disease was characteristic and thus easily diagnosed, therefore, there were no cultural or social barriers to case tracing and control.
- There were no animal reservoir for smallpox
- The incubation period for smallpox was long and infected individuals were infectious after the incubation period. The communicability of smallpox was low. Therefore, people living in the area surrounding a known case of smallpox can be readily protected by vaccination.
- There was no carrier state
- There was only one serotype of virus and an effective vaccine was available which conferred lifelong immunity.
Any program for the eradication of a particular virus would involve universal vaccination of children, preferably at as young an age as possible. As in the case of smallpox, the following questions should be addressed when the feasibility of eradication of a particular human virus disease is considered.
1. Is the disease worth eradicating?
In the case of severe viral diseases like smallpox and poliomyelitis, the case for eradication is straightforward. However, for milder virus diseases such as chickenpox and mumps, the case for eradication is less straightforward. The priorities would differ between developed and developing countries. The cost-benefit ratio of such an eradication program must be taken into account
2. Is there any animal reservoir?
Viruses with a known animal reservoir such as rabies will be very difficult to eradicate. Eradication of the virus would involve eradication of the virus from the animal reservoir as well. This would involve the vaccination of the reservoir. In the case of rabies, eradication from some countries have been achieved by strict quarantine procedures for imported animals and vaccination of pets. However, eradication of rabies from the wildlife would prove to be extremely difficult.
3. Is there a carrier state?
Viruses which could produce a carrier state or persistent infection in humans such as hepatitis B and the herpes viruses would be very difficult to eradicate unless the carriers could be treated. The possibility that some of these viruses may be vertically transmitted from mother to child by transplacental transmission would confer extra difficulty in terms of eradication.
4. Is effective vaccination available
Effective vaccines are now available against some viruses such as rubella, measles and poliomyelitis. These viruses are antigenically stable and are restricted to one or a few serotypes. Other viruses such as influenza A and B are antigenically unstable and the formulation of the vaccine has to be modified annually. These viruses would be almost impossible to eradicate by universal vaccination. Vaccine are still not available against the majority of viruses such as HIV and hepatitis C. In some of the cases, the development of a vaccine is very difficult not because of the multiplicity of serotypes as in the case of rhinoviruses and HIV. In other cases such as in the case of RSV, the development of a vaccine had proved to be very difficult because of other practical difficulties. In the case of RSV, the vaccine would have to be more immunogenic than the natural virus because natural infection does not appear to confer long term immunity.
5. How communicable is the disease? What level of coverage is required for eradication?
A 100% coverage rate for vaccine uptake is unlikely to be attainable. However, eradication does not require 100% coverage as herd immunity will impede the transmission of the virus. The coverage rate required for eradication depends mainly on the transmissibility of the virus. eg. Smallpox had a low rate of transmission whereas measles had a high rate of transmission. Thus measles is proving difficult to eradicate in countries such as the US even though the vaccine coverage is very good.
Whether a virus disease can be eradicated or not depends onmany factors, not least on the will power to implement such a policy. The eradication of smallpox serves as a model for global cooperation. It is unlikely that every virus disease is eradicable.