Nov. 12, 2003 -- A major measles outbreak in England? With vaccination rates dropping across the U.K., it may be only a matter of time.
Measles vaccine is part of the MMR (measles-mumps-rubella) combination vaccine. It's extremely safe, agrees every major health agency in the world. But now-refuted claims that the vaccine is linked to autism persist. In the U.K., that's led to a drop in MMR coverage to 82% from a high of 92% in 1995-1996.
A 90% coverage rate ensures that outbreaks will quickly burn themselves out. The U.K.'s low MMR vaccination rate means that a single case of measles could explode into a major outbreak, says Steve Cochi, MD, MPH, deputy director of the CDC's National Immunization Program.
"Formerly, a case of measles imported into the U.K. was like throwing a match into a wet forest," Cochi tells WebMD. "Now it's like throwing a match into California."
The Criticality Threshold
A vaccine coverage rate above 80% seems pretty high. But that's just a national average. It means that two in 10 people are vulnerable to this highly infectious disease. And it suggests that there must be many pockets of unprotected people. In some areas of London, MMR coverage has dropped to 64%.
The key is something called the reproductive number, says Vincent A.A. Jansen, PhD, a professor at Royal Holloway University of London.
"The reproductive number is the typical number of cases an infected individual would cause," Jansen tells WebMD. "If it's less than 1, the outbreak would fizzle out. If it is greater than 1, the epidemic can take off. So 1 is the threshold of criticality."
The U.K. is now close to that threshold. It's climbed from 0.47 in 1998 to 0.82 in 2002. At that rate, England will soon cross the threshold of criticality.
Could It Happen Here?
Even at 91%, the U.S. measles vaccination rate isn't as high as experts would like. Especially if there's an epidemic in England, imported cases could cause relatively large outbreaks in some areas. Across the nation, however, more than nine in 10 2-year-olds have had their first dose of measles vaccine, making a large U.S. outbreak extremely unlikely.
Who's at risk? Pockets of unvaccinated children and adults can be found in various parts of the U.S. And some areas are well under the 90% protection threshold, according to 2002 figures:
- Arizona (excluding Phoenix) has 84.8% MMR coverage.
- Houston has 84.7% MMR coverage.
- Montana has 85.3% MMR coverage.
- Oklahoma has 86.4% MMR coverage.
- Oregon has 86.6% MMR coverage.
- Idaho has 86.9% MMR coverage.
- Other states under 90% MMR coverage: Alaska, California (outside Los Angeles, San Diego, and Santa Clara), Kentucky, Louisiana, Nevada, Texas, Washington, and Wyoming.
Even the 90% coverage threshold is not ideal.
"With measles it would be nice to get above 95%, but if we are above 90% everywhere -- that's our goal by 2010 -- we'd feel pretty good about that," Cochi says. "Every percentage point reduces the chance that a case of measles will cause a disease cluster. With more importations from the U.K., there will be more small outbreaks. But unless coverage drops substantially we won't have a major epidemic."
Unless, of course, Americans lose faith in MMR safety.
"With this misinformation out there, there is always the chance we might see that happen," Cochi worries. "Right now we have high coverage, so we won't see any kind of epidemic unless our national vaccination rates drop."
Measles Kills and Blinds
Measles is only one part of the MMR vaccine. Mumps and rubella are also serious illnesses. But measles is a lot scarier.
Measles is one of the most contagious diseases in the world. It's spread by airborne droplets that can linger in the air for hours. A very small dose of measles virus is all it takes for an unprotected person to get ill.
Before the vaccine became available in 1963, everybody got measles. There were as many as 4 million cases per year in the U.S., with 500 deaths. Even a healthy person gets seriously ill. But when a person is poorly nourished, measles can be a disaster. Among the poorer nations of the world, measles is still a major plague.
"Worldwide measles deaths in 1999 were 875,000 people," Cochi says. "Almost a million children died of measles at the end of the last century. Last year we think the deaths were down to about 700,000."
The decline in deaths is due to a major vaccination effort by the World Health Organization, the CDC, and other health organizations and charities. Worldwide vaccine coverage now is about 70%. In sub-Saharan Africa, it's just over 50% -- and rising, thanks to the huge international effort.
Still, the battle is far from over. In some nations, one in four children under 5 dies of measles complications. Measles is the leading cause of blindness in African children.