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An outbreak of measles in Japan has led to the closure of more than a hundred schools and universities in the past month and to calls for a new push to eradicate the virus completely. Japan is one of the few industrialised countries yet to eliminate the disease.
In 1978 Japan introduced a mandatory measles vaccination programme for preschool children. But mandatory vaccination stopped when the law was revised in 1994. Although vaccination rates remained at about 90%, according to the National Institute of Infectious Diseases, this was short of the 95% coverage needed to eradicate the disease from the general population.
Nobuhiko Okabe, director of the institute's infectious disease surveillance centre, warned last week that the gaps in immunisation coverage had led to the virus affecting older age groups than usual, causing greater risk of this extremely infectious disease spreading in the general population.
“The outbreak is not as widespread as in 2001, when between 200000 and 300000 patients, most of whom children, were estimated [to have been infected. But] an epidemic among teenagers and young adults, who can wander across a much wider range than infants, could extend infections to large numbers of people of all ages,” he warned.
Although there is no comprehensive count of patients with measles, a nationwide survey by the institute of about 450 medical institutions found 286 people aged 15 years and older had contracted the disease by 20 May, and there had been about 907 cases in children.
Every year about 20 million people worldwide, mainly children, contract measles. In 2005 there were 345000 measles related deaths, but in developed countries that have taken eradication measures the disease is now rare. In 2004 the World Health Organization received reports of only 37 cases of measles in the United States.
Peter Strebel, of WHO's expanded programme on immunisation, told the BMJ, “In general, outbreaks of measles that affect teenagers and young adults are usually the result of the accumulation of susceptible persons who either have never been vaccinated—for example, as a result of earlier years in which routine vaccination coverage was less than 95%—or who were vaccinated but did not respond—so called vaccine failures because the vaccine is approximately 85-95% effective depending on the age at which it is given.
“In Japan, I believe the main challenge has been getting on-time vaccination coverage with the first dose above 95%, and, until recently, the lack of a second dose of measles vaccine in their routine childhood immunisation schedule.”
Dr Okabe pointed out, “Japan did not revise the preventive vaccination law to make the second vaccination a requirement until . . . 2006.” He says that the problem has been compounded because “only children of certain ages are eligible to receive routine vaccinations free or partially free of charge. All other ages must pay the full expenses of vaccination.”
He recommends that when outbreaks occur “the government should find ways to inject public funds as a temporary measure to prevent further spread of the disease.”
Dr Strebel cautioned that measles is one of the most infectious agents known, and “thus it behoves all countries to maintain a high level of immunisation to prevent its spread.”