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Measles could take hold as UK vaccination levels drop

BMJ 2003; 327 doi: (Published 14 August 2003) Cite this as: BMJ 2003;327:360
  1. David Spurgeon
  1. Quebec

    Measles could again become endemic in the United Kingdom as a result of a continuing decline in the number of infants being vaccinated, concludes a study of recent outbreaks in England and Wales (Science 2003;301:804).

    There has been a drop in uptake of the combined measles, mumps, and rubella vaccine from its national average of 91% of children in 1998. Only 79% of children reaching the age of 2 years between January and March 2003 had had the combined vaccine.

    The fall off began after widespread publicity concerning the alleged side effects of the vaccine, although all claims of serious side effects have been refuted, say the authors. The drop has led to a growing pool of susceptible children and has occurred at the same time as a number of large outbreaks of measles.

    “My hope is that this is a warning signal for the parents,” said Vincent Jansen, first author of the paper and a Wellcome Trust researcher from Royal Holloway University of London. “We are approaching the danger zone where measles could again become an endemic disease in the United Kingdom. We are not there yet, but it may be going that way.”

    Dr Jansen said that although the drop in vaccinations coincides with an increase in the size of measles outbreaks, the conclusion that there is a causal connection cannot be drawn from their data. However, the study does conclude that the “reproductive number”—the mean number of secondary infections per infection, which can be used to predict the course of an epidemic—has increased in recent measles outbreaks in England and Wales. The study's authors say that the reproductive number is “approximately proportional to the fraction of the population that is not immunised.”

    The study says, “If the reproductive number is smaller than one, the disease will not persist but will manifest itself in outbreaks of varying size triggered by importations of the disease. If the reproductive number approaches one, large outbreaks become increasingly likely, and, if it exceeds one, the disease can become endemic. If the reproductive number equals one, the situation is said to be at criticality. A decline in vaccine uptake will lead to increasingly large outbreaks of measles and, finally, the reappearance of measles as an epidemic disease.”

    The researchers estimated the reproductive number for 1995-8 as 0.47 (90% bootstrap confidence interval 0.36 to 0.55) and for 1999-2002 as 0.82 (0.71 to 0.87) (P<0.00011). And by comparing the distribution of outbreak sizes before 1999 with the distribution for the years 1999-2002 they show that “the shape of the distribution is close to the distribution at criticality.”

    Mary Ramsay, an author from the Health Protection Agency, London, notes that while it is impossible to say when a resurgence of measles might occur, some areas where vaccination rates have been low for relatively long periods, such as London, risk seeing larger outbreaks in future.

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