|Two Jewish men in New York (Flickr by Kynan Tait). The site of 2009/10's near-4,000 large mumps outbreak|
It started during the middle of 2009 when an eleven-year old boy returned home to the U.S from a holiday in the UK. The UK was just experiencing an exceptionally large (about 7,500 people) outbreak of mumps that year but don't worry, that boy had been twice vaccinated with the MMR immunisation. He should be OK. Shouldn't he?
As it turns out, he wasn't and he became infected with the mumps virus. And so our three year-long story begins.
The problem was that by the time this now mumps-infected boy realised he was ill (fever, swollen glands, potentially inflammation of the testicles and meningitis) he was attending a youth camp in New York along with 400 other orthodox Jewish boys. Unluckily for us, the mumps incubation period can be over 2 weeks and you can be infectious up to one week before this, making it particularly hard to contain as we will see.
A couple of days later the camp ended and each one of those mumps-exposed, potentially infected children were seeded back into society. By the time it had subsided nearly one year later, 3502 cases of mumps had been observed across the state of New York, the biggest outbreak the US had seen in decades and it was most likely attributable to this single index patient who brought the disease in from the UK.
But what was most worrying was that 76% of them had been immunised with two doses of the MMR vaccine, our mainstay of protection against the virus. So just how could mumps get passed this defence and cause such a massive outbreak? It took the U.S Centers For Disease Control and Prevention nearly 3 years to find an answer. The investigation is published in the New England Journal of Medicine here.
What caused the outbreak?
Firstly rather counterintuitively, this story provides evidence that says that our vaccine works but also that it doesn't. This is because of the kinds of people who were infected with the virus. If you look at the individual cases carefully you begin to see that the outbreak predominantly affected young Jewish males. Although later on in it spread within family groups, it barely made an impact on the wider community across New York. Of course this was helped by the effective countermeasures in place, such as strict isolation and booster vaccinations but even these can't explain why the outbreak spread so well among the initial population. What was different about these young Jewish men that made them so susceptible to the virus?
The CDC put forward the idea that it was the orthodox Jewish schooling in Yeshiva's (I am not an expert in this so don't shout if I get it wrong), which involves intense, one-to-one learning, contributed to the outbreak, facilitating mumps spread by virtue of amount of virus transmitted. I.e the long hours spent in front of someone infected with mumps allowed a relatively large amount of virus to be spread to you which was able to overwhelm your vaccine-induced immunity.
The opposite of this being that the vaccine-induced immunity of non-Yeshiva attendees (wider families and non-Jewish community) was able to protect from the virus. Although the investigation never looked at the amount of antibody against mumps in these cases, they can't be certain of exactly what contributed to the spread.
Any public health implications?
Now this theory fits in nicely with other mumps outbreaks, especially those within university/college settings where outbreaks are often kindled by parties. So what does this mean from a public health perspective? Well it shows the power of two-doses of MMR vaccine has in protecting against mumps disease and also how an extra dose can provide that extra bit of protection against mumps ( reported in a separate paper here). It also, as have many other outbreaks, shown us that situations where there can be periods of intense virus-transmission are likely to catalyse outbreaks. We should therefore be aware of the danger of such situations. It would be impossible to put an end to such gatherings.
What's new in this paper is the epidemiological evidence it provides for the theory that the level of virus inoculum (that is the amount being secreted in respiratory fluids like saliva) has on infection and in particular in overcoming vaccine-induced immunity (although they never measured virus shedding, they only presume).
Looking forward, we should investigate exactly what immune barriers are being broken down here. Is it antibody levels in the upper respiratory tract? How about levels of memory T cells? Does innate immunity have a role? Maybe the vaccine doesn't protect against infection but only the symptoms of mumps? If we investigate these we may be able to develop new vaccines that specifically boost these defences and can therefore halt future outbreaks. Sadly very little is understood about how mumps virus causes the disease and is spread between people.
Barskey, A., Schulte, C., Rosen, J., Handschur, E., Rausch-Phung, E., Doll, M., Cummings, K., Alleyne, E., High, P., Lawler, J., Apostolou, A., Blog, D., Zimmerman, C., Montana, B., Harpaz, R., Hickman, C., Rota, P., Rota, J., Bellini, W., & Gallagher, K. (2012). Mumps Outbreak in Orthodox Jewish Communities in the United States New England Journal of Medicine, 367 (18), 1704-1713 DOI: 10.1056/NEJMoa1202865