Providing free flu vaccinations for all children under 12, at an annual cost of HK$100 million, would be a radical measure. But it is one the government is right to consider at a time of heightened concern over the spread of flu. Such a move would, however, raise questions about the cost effectiveness of the vaccination programme and its benefits in terms of community health. The international medical community has long debated these issues without reaching a consensus. In Hong Kong, our experience of Sars would be a factor in the debate. Official consideration of the idea follows concerns raised by the deaths of four children with flu-like symptoms. It comes after the drastic step taken on Thursday of closing all primary schools and kindergartens for two weeks as a precaution against the spread of infection. Both moves were driven by memories of the Sars outbreak five years ago that remain fresh in the public mind. The government is understandably keen to find a way to become better prepared for an annual flu outbreak, which has the potential to become a health crisis. The school closure has a precedent during the Sars crisis. It has a clear benefit: breaking the cycle of infection between children and, through their families and friends, into the wider community. Flu vaccinations have proved an effective protective measure and are recommended for groups most at risk from serious complications - children up to two years and the elderly. They are not to be compared, however, with child vaccinations that confer lifetime immunity from a range of illnesses. The flu vaccines have to be given annually and re-engineered every flu season to match mutated strains of the virus. New strains can emerge too late to be matched, although a mismatched vaccine can still be more than 50 per cent effective, according to a study by US health authorities. The rationale for vaccination of schoolchildren can be compared with that for closing schools - denying the virus the chance to spread among and beyond a vulnerable population. If children are vaccinated it will result in less hospital and doctor visits and fewer deaths. And the general population will be less exposed to the virus. This argument has helped persuade an advisory panel on immunisation to the United States Centres for Disease Control and Prevention to recommend that all children from six months to 18 years receive annual flu shots, compared with six months to five years now. A reminder that flu experts are not unanimous about the benefits is to be found in disagreement with the findings of the panel's study, based on the 2003-04 flu season and 153 child deaths. In Hong Kong, given that vaccination of most of the primary school and kindergarten population could have reassured the public and averted the need for closures, there is certainly a case for the government to consider it. The issues include effectiveness, cost and how to implement it. The effectiveness of such a measure here may be enhanced by the density of the population and the favourable conditions for transmitting communicable diseases. Implementation should be on a voluntary basis, supported by an education campaign aimed at convincing most parents of the advantages of vaccination, and reassuring them that it is safe. If cost-effectiveness is measured by any estimate of the savings in the cost of treating those who fall ill and lost productivity through absenteeism, it is arguable whether such a policy should be introduced. If it is measured by lives saved from fatal complications of flu, or by peace of mind in a city still living in the shadow of Sars, many will say there is no question that this should be done.