In this week's New England Journal of Medicine, the cause of a very peculiar outbreak of mumps was explained.
In the year June 2009- June 2010, there were significant outbreaks of mumps, mainly in the NYC area, but some cases erupted in Cleveland too. Two aspects of the mumps outbreak were striking and unusual- most of those who caught the mumps were already well immunized, and most were in the Orthodox Jewish community. Nothing we knew about mumps could explain either of these observations. After all, the whole point of being immunized to make sure you cannot catch the disease even if you are exposed. And, there is no reason a virus like mumps should only infect one group in a society.
After 2 years of painstaking epidemiologic detective work, the CDC scored another triumph of science in finding out how this happened.
The key elements turned out to be European immunization rates, the nature of a virus that spreads in the air, and how people study in class.
First the European immunization rates. Many countries in Europe do not require parents to immunize their children as a condition of attending school, and thus have much lower immunization rates than in the US.
This leads, of course, to epidemics of illnesses not seen in countries where children are immunized at high rates.
In the spring of 2009, England experienced serious outbreaks of mumps, presumably due to a drop in the number of kids immunized for mumps. That set the stage for the story that unfolded. The English mumps epidemic was the hot fire that kindled very wet wood.
The next factor to come into play was the nature of the classroom in a traditional Orthodox Jewish school. For boys, much of the day can be spent in paired study sessions, where two boys face each other and engage in intense study and verbal discussion of the texts at hand. A table can often hold several pairs of studying boys.
And the final factor is that the mumps virus is spread by breath, and it turns out that many respiratory viruses can overwhelm body defenses if enough virus is transmitted. In this case, being in close proximity to each other's breath for 4-10 hours a day can deliver enough mumps virus to cause an infection, even in someone immunized whose antibody levels to mumps would normally protect them from infection.
So what happened is that an 11 year old, American, Orthodox Jewish boy was studying in England and happened to sit across from an English boy who had caught mumps as part of that country's mumps epidemic. Although fully immunized, the contact with mumps overwhelmed the 11 year old's defenses and he got mumps. While contagious, he returned home in June 2009 and went to his traditional school in NYC.
At that school he spread mumps to study partners who also tended to be fully immunized, and then they spread it to their study partners. By the time a year had passed, about 3,500 children came down with mumps. Over 95% of them had confirmation of the infection by a lab test. Over 75% of them were male, and over 95% of them were from the Orthodox Jewish community. A number of the boys who got mumps lived in Cleveland, and nearly 90% of them had been fully immunized. Very few were girls, almost none were outside the Orthodox Jewish community.
This story is a dramatic example of a very unusual circumstance opening the door to a very selective pattern of infection. In the US, so many children are immunized against mumps that we do not see mumps very often if at all. In 2008, the whole country reported only 400 cases, mainly in unimmunized families.
It turns out that this outbreak could only occur in the special circumstance of a child getting mumps and then sitting in very close contact with one other child for extended hours every day. This special educational circumstance, limited to boys, and to boys in traditional Orthodox Jewish schools, created the unusual situation of a respiratory virus being able to accumulate enough numbers in someone to overcome good protection from immunization. The fact the children were immunized made their mumps far more mild, and limited spread mainly to boys in close contact. The outbreak did not spread very much to girls in these schools, or outside the schools.
The outbreak was mild, no deaths occurred. But it was an interesting window into the nature of viruses, and how our behaviors can influence our epidemiology.
Dr. Arthur Lavin
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