It has struck students, clerks and doctors by the hundreds in a matter of months. They are the latest batch of Hong Kong patients to have been infected with the epidemic viral disease, rubella. The latest outbreak of rubella, or German measles - usually a mild disease characterised by a red flush on the skin, a rash and a slight fever - has highlighted the need for an effective vaccination programme to control infectious diseases. When at least 18 public health workers - some of them doctors working in emergency rooms - were confirmed in the past few weeks as rubella sufferers, questions were raised as to whether doctors in public hospitals should be vaccinated against the disease themselves so as to protect their patients, and more specifically, pregnant women, from infection. Rubella would be of little concern were it not for the effects it can have on the unborn child, especially during the first three months of pregnancy when the vital organs are forming and developing. Overseas studies carried out on aborted foetuses show that up to the beginning of the second trimester of pregnancy the virus may infect the placenta in 80 to 90 per cent of cases, and the foetus in 60 to 70 per cent of cases, although not all infections result in malformations. The overall malformation rate in the first trimester is 25 to 35 per cent, or close to 50 per cent if infection occurs in the first month. Professor John Tam, of the Chinese University's department of microbiology, says the risk of foetal abnormality drops to 10 per cent if infection occurs in the second month. Between the 13th and 16th week, the rate is six to 10 per cent. Prenatal infection after the 17th week may still occur but, apart from transient growth retardation, no abnormalities have been observed. Professor Lau Yu-lung, a leading paediatrician with the University of Hong Kong, says the effects of infection, which can include heart defects, cataracts, glaucoma, deafness, retardation and diabetes, vary considerably in children. Some infants with congenital rubella may be growth-retarded and have radiolucent bone disease, jaundice, and purple skin lesions (the 'blueberry muffin' effect). According to Professor Lau, some present few or no obvious clinical manifestations at birth. 'I have seen these babies. Some might have disabilities that could not be surgically corrected, others might have very mild symptoms. As is the case with deafness, you might not know a child had a hearing problem until he was two or three years old,' he says. Department of Health figures record a fitful rise in rubella incidence since 1984, when nine babies had congenital rubella syndrome. Eight cases were reported in 1989. The average between 1984 and 1992 was 4.7 cases a year. It is often not the number of cases, but the potential severity of disability, that prompts health administrators to urge comprehensive vaccination. The overriding objective is always to protect pregnant women and their unborn children. According to Professor Lau, after an epidemic in the United States affected many pregnant women and caused a host of congenital abnormalities in 1964, it was recommended as public policy that rubella vaccine be administered in combination with measles and mumps vaccine to all children at 12 months of age or over, with special emphasis being placed on immunising post-pubertal males and females, such as college students and military recruits. The US vaccination programme has been hailed as effective and the targeting of both sexes stands in stark contrast to Britain and Hong Kong, where girls only have been immunised. 'The current outbreak shows if we had taken up an approach of universal immunisation, say, 20 years ago there would have been no problem. But for a long time we followed the UK practice,' says Professor Lau. The Department of Health, which used to arrange vaccinations for girls only, started injecting all one-year-olds in 1990. Since last year all Primary Six pupils have been given a second inoculation, Professor Lau says. 'This means there are still boys in different age groups we haven't targeted - a group of males aged between eight and 11 and another group of over-13.' Another group of children who may lack immunity are new immigrants. Latest Department of Health figures show that, among the 1,545 patients who showed up as infected between January and April this year, more than 85 per cent were male and many were teenagers: 1,040 were between 10 and 19 years of age. The youngest patients were 125 aged nine or under, while there were 31 aged 50 or over. When asked whether he believes medical staff should be inoculated, Professor Lau says: 'Personally, I feel those doctors working with pregnant women should consider immunisation, which has few side-effects.' Professor Tam, of Chinese University, observes: 'The United Kingdom has a policy to vaccinate male and female medical staff who are in contact with pregnant women.' American health policy is for a special effort to be made to ensure that all individuals who plan to work in educational institutions or child-care centres should be vaccinated. Male and female health-care workers who may be exposed to patients with rubella should also be immunised, under US policy. But the Hospital Authority is leaving it up to its staff to decide. Postnatal rubella is transmitted chiefly through direct or droplet contact from nose and throat secretions. Between a quarter and a half of all infections are asymptomatic, and the greatest risk of transmitting the disease appears to occur in the few days before, and five to seven days after, the onset of the rash. A small number of infants born with rubella continue to spread the virus to others for a year or more. Women are advised to undergo premarital screening for rubella immunity in case of unplanned pregnancy. After vaccination, they are advised to wait three months before trying to conceive. The vaccine, though it carries few side-effects, should not be given to pregnant women. If it is inadvertently given, or if pregnancy occurs within three months of immunisation, the patient should be conselled on the risk, however hypothetical, to the foetus.