Ways to beat rubella bogey
While concerns arise whether there will be a rubella outbreak in Hong Kong, one may suspect whether our vaccination policy can ever eradicate rubella. It is known that rubella is worldwide in distribution.
Even though symptoms of rubella are few in children (as many as 20 to 50 per cent of the cases may occur without an evident rash), it does, however, bring significant congenital defects in offspring of women who are infected by rubella during pregnancy.
Since 1977, R M Anderson and R M May have done extensive collaboration on studies of population dynamics of infectious disease.
In their comprehensive work of 1991, they used mathematical models to predict the impact of vaccination. Their predictions are found to be in excellent agreement with observed data. Of course mathematical models would not be useful if they were not grounded on a clear understanding of the epidemiological data and public health problems.
However, if the basic dynamics of the system are not fully understood, then even years of practical experience and loads of data can never guarantee a sensible answer.
Anderson and May compared the consequences of different vaccination policies. Their numerical studies suggest that levels of herd immunity of around 85-88 per cent should be sufficient to eradicate rubella, and such levels of herd immunity can be attained by 90 per cent coverage in vaccination of children between the ages of 15 months and two years.
This critical level has been exceeded in the United States, where vaccination is essential for all children before they enter pre-school kindergarten but after the age of 15 months, to guard against maternal-antibody protection. Hence rubella seems well on the way to eradication.
In late 1988, a two-stage policy combining vaccination to girls, and only girls, between the ages of 10 and 15 years with immunisation of pre-school children with a combined measles-mumps-rubella (MMR) vaccine was begun in Britain. Calculations by Anderson and May led us to conclude that rubella could be eradicated within 40 years or so if 80-85 per cent of children were to be vaccinated at age two years in addition to maintaining 85 per cent coverage in vaccination of girls between the ages of 10 and 15.
In 1990 Hong Kong adopted a programme of immunisation similar to Britain's, and since last year, not only girls, but all Primary Six students have been given a second rubella vaccine. The average coverage rate for most vaccines has been high (above 90 per cent as reported in 1988 and 1989). However, the social and environmental factors within our community may affect the impact of vaccination. In view of our crowded environment, the force of infection would be higher when compared to Britain.
Moreover, with the large number of young immigrants in recent years, it is difficult to be certain whether the critical levels can be maintained. Vaccination of new immigrants has never been required here.
Until there are policies which demand all young immigrants who have no record of rubella immunisation to be vaccinated, and encourage the vaccination of susceptible female immigrants of reproductive age, it will still be a long way before we can eradicate rubella in Hong Kong.
DR JUDY TAM Assistant Professor Department of Applied Mathematics The Hong Kong Polytechnic University