By the time the outbreak ended, 8 states had been heavily affected (and 45 reported at least one case), with a total of 6584 cases of mumps and 85 hospitalizations reported by the end of 2006. All told, this was the largest outbreak of the virus in approximately 20 years, after a 1986-1990 outbreak resulted in a change in the recommended vaccine schedule (adding a booster shot of MMR).
A paper out in today’s New England Journal analyzes the outbreak–what happened and why, and what public health professionals can do to prevent future such outbreaks. More after the jump.
The NEJM paper examined cases of mumps dating back to 2000 and continuing up to mid-2007 in order to determine variation in the virus and case populations (including vaccination status) pre- and post-resurgence. The central question is, why did a mumps outbreak occur in this population? Most of the individuals affected had received 2 doses of the MMR vaccine, the first in childhood and the second in adolescence. Theoretically, most of them should be protected against the virus–so why the outbreak?
The authors examined several possibilities, and didn’t conclusively rule out any of them. First, were records and reports of vaccine doses accurate? Could some of the cases have received the measles-only vaccine, rather than the MMR combination? The sheer numbers of each of those vaccines administered suggests that was an unlikely explanation.
Was the outbreak possible because the hardest-hit group–college students age 18-24–had little or no exposure previously to wild mumps virus, therefore not boosting their immunity? We already know that the mumps vaccine isn’t as good as the measles or rubella vaccines it’s given along with, so even with 2 vaccinations, there would be a small percentage of individuals who would lack immunity to the virus (as I explained here). In those in whom the vaccine “took,” does immunity start to wane in a fairly short time period, making them again vulnerable to infection? This could certainly play a role in an outbreak, and the authors note that “if population immunity is already near the herd threshold, even negligible waning immunity, particularly when combined with increased exposure, could potentiate an outbreak.”
A second explanation, and not necessarily mutually exclusive, would be that the strain of virus–serotype G–wasn’t a close enough match to the vaccine strain (genotype A)
to provide good cross-protection. However, outbreaks of the same strain in Europe responded to increased vaccination with the genotype A strain, suggesting that the vaccine does provide adequate protection. This is good news considering that, because a large population of the world doesn’t vaccinate against mumps, imported cases are likely to continue.
Finally, with lines like “CDC: Waning immunity blamed for outbreak of 6,600 cases”, I expect the anti-vaccine groups to be once again railing against the mumps vaccine, and ignoring the fact that prior to vaccine introduction, there were 152,209 cases of mumps a year–a rate that had dropped 98% by 1985. Still, the possibility of waning immunity coupled with more people avoiding the MMR shot for their children and a regular stream of imported cases means that outbreaks like this may become more common–and vaccination policy again my change.
Dayan, G.H. (2008). Recent Resurgence of Mumps in the United States. New England Journal of Medicine, 358 (15), 1580-1589. Link.
Image from http://www.mullhaven.co.uk/mumps.jpg