As of just a few days ago, over 400 confirmed and probable cases of mumps have been diagnosed in Washington state. To put this in perspective, in a full year the entire United States can expect to see roughly the same number of cases. We’re not even two months into 2017 yet.

Since the mumps vaccination program began in 1967 the United States has seen about 440 cases per year. Since 2005, however, there have been 6 years with over a thousand reported cases. Last year, in fact, there were 5,311 cases of mumps, about half of which came out of Arkansas alone. That outbreak started in August and peaked at 50 cases per day. In 2006, about 6,500 college students throughout the midwest were infected. 2009 and 2010 also saw large numbers of cases in New York and New Jersey, where about 3,000 students were infected. In 2014, there were several hundred cases in Columbus, Ohio, up from their typical yearly average of one.

Usually when there’s an outbreak of a vaccine preventable disease, the majority of cases are attributable to individuals that are unvaccinated or under vaccinated. But with the mumps, we’re seeing a much larger proportion of the cases from individuals whose vaccines are up to date. Why is this happening, why are the mumps on the rise, and why do we care about the mumps anyway? To answer all of those questions, we first need to answer what the mumps is and how it spreads.

What is mumps?
The mumps is a viral infection that only occurs in humans. Lucky us. Like the measles, it is a Paramyxovirus. Paramyxoviruses are a family of particularly nasty viruses. They include respiratory syncytial virus (RSV), which is the most common reason that children under two are admitted to the hospital, and parainfluenza, the virus responsible for most cases of croup. While only humans get the mumps, other animals are vulnerable to Paramyxoviruses, including dogs (canine distemper virus) and dolphins (morbillivirus). In short, Paramyxoviruses are no fun no matter what species you are.

The mumps is also like measles in that it is extremely infectious and easily, if not literally, jumps from susceptible host to susceptible host. It doesn’t care about the age, ethnicity, sex or religion of the host either. The most important factor in transmission, other than susceptibility, is proximity of potential hosts, which is why so many outbreaks have occurred where people are in close quarters: schools, camps, sports teams and military bases.

The virus is suspended in respiratory droplets but can also be transmitted by direct contact with infected patients or objects. Once in the body, it tends to incubate for a couple of weeks prior to causing symptoms but viral shedding takes place anywhere from a few days before symptoms to several days after they occur.

Those unlucky enough to develop classic mumps will still have nonspecific cold or flu-like symptoms initially, such as fever, malaise, significant appetite loss, and body aches. They will then develop inflammation of the parotid glands, which are salivary glands located on the cheeks over the angle of the jaw, within a couple of days. The degree of swelling can be substantial, very painful, and can last for up to 10 days.

One of the most famous photos of the mumps recently is of Pittsburgh Penguins star Sidney Crosby giving an interview before he received his mumps diagnosis. The virus spread through the NHL like wildfire that year, leaving dozens of players sidelined with the virus.

Unfortunately, mumps is known for more than simply self-limited involvement of the parotid glands. In postpubertal males, inflammation can occur in the testicles. This is known as orchitis, and occurs in roughly one out of three older boys, and can cause atrophy and infertility. Older girls can develop inflammation of the ovaries, but this isn’t nearly as common.

The most common complication after parotid swelling is meningitis. Thankfully this form of viral meningitis tends to be mild and doesn’t cause long-term injury. But in a small percentage of patients it can be extremely painful. A more diffuse inflammation of the brain itself, known as encephalitis, can also occur. It is rare, about one out of every 6,000 cases, and also generally doesn’t cause long term difficulties, but it is more likely to cause severe symptoms such as altered mental status, seizures, and muscle weakness or temporary paralysis.

Mumps is also associated with hearing loss that in some cases is permanent. Other rare but potential complications include Guillain-Barre syndrome, facial palsy, pancreatitis, and even a potentially fatal inflammation of the heart. Unlike rubella, mumps has not been linked to birth defects when a pregnant woman is infected, although as I mentioned before in some cases it can lead to sterility in men when the mumps are contracted as an adult.

In a small but significant percentage of infected individuals, as much as 15 to 20% in fact, there are no obvious symptoms. Some will only suffer nonspecific complaints that are similar to the common cold. But these lucky folks can still serve as a source capable of spreading the virus to other susceptible hosts.

How is mumps treated?
There is no treatment for mumps that is specific to the infection. Just like many other miserable viruses, there’s no treatment that can shorten the duration of the illness, but we can offer supportive care. The general approach is medications to target pain, fever, and inflammation. Ibuprofen is a good choice that targets all three concerns. Non-pharmaceutical interventions are equally important and can include warm compresses, ice, elevation of a swollen scrotum, and good old-fashioned TLC. It is rarely necessary for a patient to be admitted to a hospital, but not unheard of with involvement of the brain or pancreas.

Preventing spread of the infection to susceptible hosts is an extremely important aspect of mumps management. The most effective step in prevention is vaccination.  The mumps vaccine is part of a combination shot called MMR or sometimes MMRV (these are two different vaccines that provide protection against the measles, mumps and rubella, although one also protects against varicella (also known as the chicken pox)). The MMR/MMRV vaccine is a part of the recommended immunization series for all children without medical contraindications, with doses at 12 to 15 months and 4 to 6 years considered adequate. A third dose has shown some potential efficacy in helping to stop spread during an outbreak.

If an individual has a confirmed or suspected case of the mumps we generally recommend patient isolation for up to five days after symptoms first begin. We also recommend vaccinating anyone who might be at risk, although this may not prevent illness in those who have already been exposed.

Why are so many cases occurring in vaccinated individuals?
I said before that the mumps are a Paramyxovirus, which is in part why the mumps vaccine is given in the same combined shot as the measles vaccine, which is also a Paramyxovirus. The MMR/MMRV  consists of a weakened live virus and unfortunately, the mumps vaccine isn’t as good as its measles counterpart. Even with a second dose upon entering school there is only 80-90% effectiveness. In comparison, two doses of the measles vaccine component of the MMR/MMRV is 99% effective at conferring immunity.

To really quantify what that 80-90% efficacy looks like for the mumps vaccine you need to look at what’s called the attack rate. That sounds like something out of Game of Thrones, but I promise it is a legitimate scientific term. In epidemiology, the attack rate of a virus refers to the percentage of people within a population that contract a circulating virus. We know from studies done both during and after the most recent mumps outbreaks that the attack rate for the virus among individuals with two confirmed doses of the MMR vaccine is about 4% and anywhere from 25-43% for those who are unvaccinated. That’s a pretty big risk reduction, no matter how you look at it.

The other issue that skews the numbers on the mumps outbreaks is sheer volume. In recent and current outbreaks of mumps in the United States, most people exposed to the virus are vaccinated. The number of exposed people likely numbers in the many thousands. Only a small percentage of those vaccinated and exposed individuals are becoming ill, but 4% of thousands of people adds up. If this were an unvaccinated population, the number of cases would be significantly larger.

In order for herd immunity to play a role in preventing outbreaks of mumps, 92% of exposed individuals would need to be immune. But due to the lower efficacy rate of the mumps vaccine, we aren’t going to get “perfect” herd immunity even if 100% of people were vaccinated, which is why we’ve seen about 440 cases of the mumps per year since the vaccine was introduced. Until we have a better mumps vaccine, we won’t be able to eliminate the virus like we have smallpox or polio. By the way, those 440 cases a year are almost to the number we would expect, statistically speaking, if there was approximate herd immunity based on the attack rate.

So if the numbers for the last several decades have been almost spot-on what we would expect, why is there now a surge in mumps cases? As we’ve seen over the last 15 years there have been significant declines in vaccination coverage in specific geographic pockets as well as socio-economic groups, which has presented more opportunities for the virus to get a foothold, even if national vaccination rates remain high.  Those pockets provide more opportunities for exposure, for everyone else, including those who are vaccinated.

Finally, these outbreaks have provided concrete evidence of what we’ve suspected for a number of years: the efficacy of the MMR vaccine starts to wane after 10-15 years. This has helped build the case for the need for a third dose of the MMR/MMRV vaccine. The third recommended dose isn’t on the CDC schedule for adults yet, but if you have concerns about yourself or your college-aged child, it’s something you might want to discuss with your doctor.

So what’s the short version? The mumps isn’t the most serious vaccine preventable disease, but it is highly contagious and it’s pretty miserable. The mumps portion of the MMR vaccine isn’t as effective as the measles portion, but it still significantly cuts your child’s risk of contracting the disease. The vaccine is performing as well in the real world as we would expect it to in statistical models, and the volume of individuals fully vaccinated and exposed to the virus skews the data. Also, if it’s been more than 10-15 years since you or your child has last had their MMR or MMRV, you might want to talk to your doctor about receiving a third dose.

Categories: Infectious Disease + Vaccines