Lead
Mumps carries a reputation for being mild. But one of its complications — hearing loss — can be permanent, unilateral, and irreversible. There is no treatment; once it occurs, it cannot be undone. That quality sets mumps-related hearing loss apart within the spectrum of mumps sequelae.
In Japan, the mumps vaccine is not part of the routine immunization schedule. That means the decision whether to vaccinate falls entirely to the parent. This article reviews the real rates of mumps complications and what is currently known about the vaccine.
The Real Complications of Mumps
Mumps virus infection extends well beyond the classic parotid gland swelling. A comprehensive review by Hviid et al. reports aseptic meningitis: inflammation of the membranes around the brain caused by a virus rather than bacteria, usually self-resolving in 1–10% of cases, orchitis: painful inflammation of one or both testes, a serious mumps complication in post-pubertal males (in post-pubertal males) in 20–38%, and sensorineural hearing loss: permanent hearing impairment from damage to the inner ear or auditory nerve, not the middle ear in 1 in 15,000 to 1 in 20,000 cases [1].
The hearing loss rate may look small in absolute terms, but the key issue is its nature: unilateral and essentially permanent. Direct viral invasion of the inner ear combined with vascular damage is thought to be the mechanism, and there is almost no effective treatment after onset [1,3]. Mumps virus is a non-trivial cause of acquired unilateral hearing loss in childhood.
Aseptic meningitis generally resolves without serious sequelae, but when encephalitis develops as a complication, outcomes can be severe.
Why Is the Vaccine Still "Optional" in Japan?
While most countries include a mumps vaccine as part of the MMR (measles-mumps-rubella) combination given in routine childhood schedules, Japan has kept mumps vaccination optional since the early 1990s.
The historical context: from 1989 to 1993, the MMR vaccine in Japan (containing the Urabe strain) was associated with a high rate of vaccine-induced aseptic meningitis, leading to the discontinuation of MMR. Since then, monovalent mumps vaccine has continued on an optional basis, and the debate over reintroducing routine mumps vaccination has been shadowed by the question of balancing vaccine-strain meningitis risk against the risk of disease-related complications [1].
The vaccines currently available in Japan, however, use different strains from the problematic Urabe strain (such as RIT4385). The Japan Pediatric Society has formally recommended routine introduction of the mumps vaccine and expressed support for vaccination [5].
Why One Dose Is Not Enough
Breakthrough infection: infection that occurs in a vaccinated individual when the vaccine does not provide complete protection after a single mumps vaccine dose — infection despite vaccination — occurs in roughly 20–30% of exposed individuals, depending on the study [2,4]. In outbreak settings, single-dose recipients have been documented as transmission sources [2].
For this reason, two doses are recommended — timed to align with the MR (measles-rubella) schedule: once around the first birthday and once the year before primary school entry. If the vaccination record in your child's Maternal and Child Health Handbook (boshi techo) shows only one dose, it is worth asking your pediatrician about the second.
Making the Decision
- The mumps vaccine can be given at the same visits as the routine MR vaccine (first dose around age one, second dose the year before school entry). It involves an out-of-pocket fee, but weighing infection risk, cost, and logistics is a reasonable place to start.
- If a child develops "one ear seems not as sharp after mumps," or seems to frequently ask for repetition in noisy environments, an audiological evaluation by an ENT specialist sooner rather than later is worth pursuing. Children often don't notice unilateral hearing loss themselves.
- Vaccination doesn't reduce the risk of infection to zero, but it substantially lowers the probability of severe illness and complications.
Summary
Mumps-related hearing loss is infrequent but irreversible when it occurs. Single-dose protection is incomplete, and multiple studies support the value of two doses. In Japan, "optional" means the decision is the parent's to make — not that it does not need to be made. Knowing the actual complication rates and the current state of the evidence is the starting point for that conversation with your pediatrician.
References
- Hviid A, Rubin S, Mühlemann K. Mumps. Lancet. 2008;371(9616):932–944. doi:10.1016/S0140-6736(08)60419-5. PMID: 18342688.
- Barskey AE, Schulte C, Rosen JB, et al. Mumps outbreak in Orthodox Jewish communities in the United States. N Engl J Med. 2012;367(18):1704–1713. doi:10.1056/NEJMoa1202865. PMID: 23113481.
- Vuori M, Lahikainen EA, Peltonen T. Perceptive deafness in connection with mumps. Acta Otolaryngol. 1962;55:231–236. PMID: 13901779.
- Yoshida N, Fujino M, Miyazaki C, et al. Mumps virus reinfection is not a rare event confirmed by viral isolations in mumps patients with previous mumps vaccination. J Med Virol. 2008;80(3):517–523. doi:10.1002/jmv.21091. PMID: 18205197.
- Japan Pediatric Society. Recommendation for mumps vaccine (2015). [Society statement on optional vaccination]