Mumps outbreaks have been making headlines for several years now, with some pointing to declining vaccination rates as a possible cause. Now a California outbreak apparently started by an unvaccinated young man reveals that even people who have had the vaccine may be at risk.
The outbreak, documented in a CDC study released last week, ran from August 2011 to January 2012 and eventually sickened 29 people. All were connected in some way to a California university, which is unnamed in the study but appears to be UC Berkeley.
The CDC study traces the California outbreak back to an unvaccinated 21-year-old man who had recently been to western Europe, where mumps is more common. However, many of those who later got sick had actually been vaccinated — 22 of the 29 had gotten the recommended two doses of vaccine.
Mumps outbreaks are becoming more common, says epidemiologist Dr. Mark Dworkin, who specializes in vaccine-preventable diseases. One possible reason is that while two doses of mumps vaccine are now recommended, many people today were vaccinated at a time when just one was the standard, and those people may have lower immunity. The drop in vaccination rates sparked by fears of vaccine-related autism could be a factor as well — one report found a nearly 4% drop between 2008 and 2009 in the number of 2-year-olds with private insurance who were fully vaccinated. Says Dworkin, "People who aren't vaccinating are maintaining a pool of [susceptible people] who are fueling these outbreaks."
Dr. Patricia Quinlisk, an epidemiologist in Iowa, where a mumps outbreak sickened thousands in 2006, concurs that vaccination levels are crucial: "Once a disease gets into a population, the more people who are immune, whether because of natural disease or vaccine, the more likely it is for that disease to hit a brick wall and not go any further." She cites a recent outbreak of whooping cough in California, where opposition to vaccines has lowered vaccination rates. "Once it got going," she says, "there were no brick walls to stop it."
Quinlisk notes that two doses may no longer be enough. Initial efficacy tests of the vaccine were done when naturally occurring mumps was still circulating in the population, and exposure to the disease can boost immunity even if someone doesn't get sick. But now most people have never been exposed, and the natural boost is gone, so they may need more doses to be fully protected. She says giving a third dose of the vaccine appeared very effective in combating recent mumps outbreaks in Brooklyn, but state and federal authorities aren't officially recommending a third dose yet.
Even if it's not perfect, the vaccine in its current form may confer important benefits. Severe cases of mumps can cause infertility and deafness, but Quinlisk says that in Iowa, vaccination appeared to protect people from those complications, even if they did get the disease.
Dr. John Talarico, chief of immunization for the California Department of Public Health, says that while there's no documented evidence that mumps cases are on the rise nationwide, the recent California outbreak and others of its kind demonstrate the need for colleges to make sure all students are vaccinated and to require documentation to prove it.
The purported link between autism and the measles, mumps, and rubella vaccine has been thoroughly discredited, but many parents remain fearful of vaccinating their kids. This can create clusters of low immunity where an outbreak can begin. And the recent CDC study shows that if mumps enters a community through an unvaccinated person, even the vaccinated are at risk.