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Last Updated: Apr 16, 2008 - 5:52:06 PM |
THURSDAY APRIL 10, 2008 (foodconsumer.org) -- The mumps outbreaks among college students that occurred in 2006 suggested that the second dose of mumps vaccine may not be as effective as expected, according to a study published in the recent issue of the New England Journal of Medicine.
But authors of the study from the Centers for Disease Control and Prevention and other organizations said the resurgence of mumps could have affected tens of thousands of people should the mumps vaccine were not given to schoolchildren.
Mumps, an acute viral illness causes fever, headache, muscle aches, tiredness, and loss of appetite, followed by swelling of salivary glands. The parotid salivary glands are most frequently affected. Severe complications are rare, but it may potentially cause inflammation of the brain, spinal cord, the testicles, the ovaries and or breasts, spontaneous abortion, and permanent deafness.
Prior to the introduction of the mumps vaccine in 1967, cases of mumps were as high as 152,000 each year. The number decreased to 2982 in 1985. Then mumps resurged in 1987, prompting The Advisory Committee on Immunization Practices to recommend second dose of mumps vaccine.
Ever since, mumps had become even rarer. No more than 350 cases were reported each year from 2000 through 2005. The steady decline led the federal government to propose to eliminate the disease by 2010.
The unexpected resurgence of mumps caught attention from the federal and state health officials. Gustavo H. Dayan from the CDC and colleagues from other organizations assessed the data of the outbreak to see whether mumps elimination would be possible by 2010 though the current two-dose childhood-vaccination strategy.
According to the authors, the 2006 outbreak involved a total of 6584 cases of mumps with 85 hospitalizations, but no deaths. Eighty-five percent of the patients lived in eight contiguous Midwestern states including Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota and Wisconsin.
Dayan and colleagues found "The incidence was highest among college-age youths between the ages of 18 and 24 years and recipients of two doses of mumps vaccine." Nationwide in the U.S., the two-dose coverage among adolescents was 87%, the highest in U.S. history compared to 86% or more in the highly affected states.
They said a number of factors may have attributed to the outbreak including waning immunity, high population density and contact rates in colleges, and incomplete vaccine-induced immunity to wild virus.
According to the authors, the vaccine is 95% effective. So even if the vaccination coverage rate is 95%, the population immunity would be about 90%, a value near the herd-immunity threshold, 88 to 92% in the case of mumps as one study estimated. This means even if both the coverage rate and efficacy of the vaccine are high enough, the population is not 100% immune against the disease.
Mumps occurred most likely in those who received the second dose of MMR vaccine 10 or more years previously than did their roommates without mumps, one early study showed. Another study showed lower levels of antibodies among students who had been vaccinated with a second MMR dose 15 or more years previously than among those who had been vaccinated 1 to 5 years previously.
Also, the 2006 outbreaks on college campuses resembled those among schoolchildren who received one dose of vaccine during the 1980s, the researchers found, meaning the second vaccine may not render persistent immunity after one dose of mumps vaccine.
Dayan and colleagues wrote "Despite a high coverage rate with two doses of mumps-containing vaccine, a large mumps outbreak occurred, characterized by two-dose vaccine failure, particularly among Midwestern college-age adults who probably received the second dose as schoolchildren."
They suggested "a more effective mumps vaccine or changes in vaccine policy may be needed to avert future outbreaks and achieve the elimination of mumps."
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