The authors used data on recent outbreaks in England and Wales to demonstrate a recent trend toward larger sized measles outbreaks in the United Kingdom. They report that the measure of the spreading ability of measles, its "reproductive number," is increasing. As the rates of early childhood vaccination have dipped, the reproductive number is rising to approach the point where outbreaks in the U.K. population may not simply fizzle out.
"My hope is that this is a warning signal for the parents," said Vincent Jansen, the first author on the paper, a Wellcome Trust researcher from Royal Holloway, University of London in Egham, Surrey in the United Kingdom.
"We are approaching the danger zone where measles could once again become an endemic disease in the United Kingdom. We are not yet there, but it may be going that way," said Jansen.
"We are reporting a correlation between the drop in vaccinations and the increasing size of measles outbreaks. Of course the coincidence is suggestive of a causative connection, but we can not draw this conclusion from our data," Jansen explained.
However, they do conclude that the reproductive number has increased; and they quantified this increase.
The reproductive number is the average number of new infections that an infected person causes. The authors report that the reproductive number rose from a value of 0.47 for the years 1995-1998 to a value of 0.82 for the years from 1999-2002. They came to this conclusion after comparing the distribution of outbreak sizes before 1999 to the distribution for the years 1999-2002.
Currently in the U.K., cases and small outbreaks occur when people bring measles into the country from other parts of the world. If the reproductive number increases further to exceed one, measles will not disappear as it does now after each case or small outbreak.
"These findings re-enforce how important it is to vaccinate your child with MMR," said Mary Ramsay, an author on the paper from the Health Protection Agency in London, U.K.
"Measles is becoming harder to control due to the lack of confidence in the MMR vaccine and because of this the United Kingdom has the potential to have a resurgence of measles," said Ramsay who noted that the obvious question of when a possible resurgence could occur is impossible to answer.
"We do know, however, that some parts of the country, such as London, where MMR vaccination rates has been low for longer periods of time are at more risk of seeing larger outbreaks in the future," Ramsay explained.
Vaccination rates in the United Kingdom rose from around 50 percent in 1968 to 76 percent in 1988. After the introduction of the combined measles, mumps, and rubella (MMR) vaccine in 1998, vaccination numbers rose rapidly to a national average of 91 percent by the end of that year.
However, as vaccination rates peaked in 1998, the alleged side effects of the MMR vaccine began to be discussed. Although all the claims of serious side effects have been refuted, there has been a small decline in MMR vaccination rates in the U.K. leading to a growing pool of individuals susceptible to measles.
Caroline Ash, an editor for Science International in Cambridge, UK said, "This paper is an important contribution to Science. It offers a valuable tool to help us anticipate the risks of epidemic infections. Their findings, gleaned from recent data for measles, highlight the risks of falling vaccination rates to the wider population."
In their Science paper, the authors write, "In their attempt to help a child avoid the perceived risk associated with vaccination, parents' behavior collectively results in a substantial increase in the real risk of exposure to measles."
Measles is a highly contagious viral illness that is usually spread through large respiratory droplets. Measles is characterized by a fever that usually peaks at 103-105 degrees Fahrenheit (39-41 degrees Celsius) and a rash that begins at the hair line and spreads downward and outward to the hands and feet. Complications from the measles infection can cause permanent damage or death. Pneumonia is the most common measles-linked cause of death.
Measles outbreaks usually occur in late winter or spring. Prior to widespread vaccinations beginning around thirty years ago, nearly all children suffered through a case of the measles.
Today, children are usually first vaccinated against measles, together with mumps and rubella between their first and second birthdays. A second vaccine is administered later in childhood, typically before entering school. The goal of the second dose is to produce immunity in children who failed to respond to the first dose.
Vincent Jansen, Nico Stollenwerk, and Chris Rhodes are from Royal Holloway, University of London in Surrey, UK; Henrik Jensen is from Imperial College London in London, UK; Mary Ramsay and W. John Edmunds are from Health Protection Agency, CDSC in London, UK.
Funding for this research was provided in part by The Wellcome Trust through a grant to Vincent Jansen and Nico Stollenwerk. (grant no. 063134)
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