Mumps at Michigan…

If you’re a UofM student, you got an email yesterday from the head of University Health Service that there have been 3 cases of mumps on campus.

So. What exactly is mumps?

Mumps is a viral infection that mostly affects the salivary glands.  Some of those infected have either no symptoms or very mild ones. In those who get symptoms, they typically appear 2-3 weeks after exposure.

The classic sign of mumps is swelling of the salivary glands that results in a puffy, swollen cheek. There might also be pain in the salivary gland, pain while chewing/swallowing, or fever, muscle aches, and fatigue.

Ok. Not very desirable. Why haven’t I really heard of mumps?

Mumps is included in the classic vaccine that we all get as infants – MMR (measles, mumps, and rubella). Beginning in 1977, the CDC began recommending children receive 2 doses of MMR. Since then, incidence of mumps has drastically decreased in the US.

Interestingly, mumps outbreaks have become more common in the past decade, with a notoriously large outbreak on a college campus in 2006.  Interestingly, most of those college students who contracted mumps had been vaccinated with MMR. Twice. Getting a disease despite vaccination is called vaccine failure.

Vaccine failure is especially worrisome in this context because while the disease is not very serious in childhood, like Chickenpox, cases in adolescents and adults can be much worse. Up to 10% of mumps infections in teenagers or adults may cause complications. Some of these, in rare cases, can be severe – including deafness, meningitis, and inflammation/involvement of the reproductive organs.

The question is – why was the vaccine failing?

Lewnard & Grad, in a 2018 study, tried to answer this question by looking at different vaccine effectiveness studies to figure out what was driving vaccine failure. They found that while immunity to measles and rubella is long lasting, the immunity from the mumps component of the vaccine wanes over time.

Their analysis estimated that immunity persists, on average, 27.4 years [95% confidence interval 16.7 to 51.1 years] after receipt of a dose. They modeled how the immune response wanes, and estimated that in about 25% of people, immunity will wane in ~8 years, about 50% will be susceptible/at risk in ~19 years, and about 75% of people will be at risk within 38 years. While immunity doesn’t disappear at the same time for everyone, they believe adding a booster dose could help to alleviate these outbreaks occurring in older populations.

Through mathematical modeling, Lewnard & Grad predicted that the current practice of two doses of MMR will probably not eliminate endemic mumps transmission in the US. These calculations hinge upon the use of a metric called the  effective reproductive number (RE). RE corresponds to the number of new cases which emerge from a single case under existing immunity. (This is slightly different from the reproductive number, Ro, which is the number of new cases from a single case in a fully susceptible population.)

The authors estimate that the best option is to administer a third vaccine dose by 18 years of age. While this wouldn’t necessary confer lifetime protection, it would extend immunity through young adulthood, protecting the college-aged students at risk in recent outbreaks.

Why are college students at high risk?

College campuses are especially common sources of infectious disease outbreaks because of the high degree of social contact, food and drink sharing, and shared living space among students.

Lewnard & Grad’s findings concur with a federal expert panel recommendation from October of last year, which recommended a third MMR dose for those at highest risk of contracting mumps. Those entering college are a high risk group, and thus this recommendation could affect them. Nonetheless, the CDC still recommends just 2 doses of mumps: one at 12-15 months and one at 4-6 years of age.

While adding an 18 year dose would represent a major shift in the US adolescent/adult vaccine schedule, a dose of MMR has been administered regardless of vaccine history to anyone who joins the military since 1991. Additionally, amidst mumps outbreaks, US college campuses have administered a third dose of MMR to try and control the spread of the virus, indicating a precedent for vaccination in this age group.

Perhaps recommendations will change as evidence mounts that mumps immunity wanes, reducing the probability of outbreaks on college campuses.

Until then – UM students especially – check your MMR vaccination status, practice good hygiene, wash your hands, don’t share drinks, and report any symptoms involving your salivary glands to UHS!

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