- A 700 percent increase in incidence of mumps has been reported in Northern Ireland, reflecting a similar surge in cases in other developed nation.
- Most cases have occurred in fully vaccinated young adults, for whom the potential complications of mumps are more serious than they are in children.
- The increase in cases has been attributed to waning effectiveness of the vaccine portion of the measles/mumps/rubella vaccine over time.
The incidence of mumps has increased by 700 per cent in Northern Ireland, from 67 cases in 2018 to 534 in 2019, a change attributed in part to waning effectiveness of the mumps portion of the measles/mumps/rubella (MMR) vaccine. There have already been 79 cases reported in 2020.1
According to Dr. Jillian Johnston of the Public Health Agency in Belfast,2 most of the cases occurred in young people between the ages of 15 and 24 who had had two doses of the MMR vaccine. This type of spike in mumps cases has followed a trend, with the number of cases tending to rise and fall over the course of a few years. “We know that the mumps component of the vaccine decreases and becomes less effective over time, so every two or three years the number of people who are susceptible to getting mumps increases and that’s what’s happening,” said Dr. Johnston.”3
Similar surges were also reported for 2005, 2009, 2013 and 2016.4 Outbreaks in England, Scotland and Wales have been similarly elevated in 2019 compared with 2018. College and university students have been most affected by the uptick in mumps outbreaks, with England reporting 5,042 cases in 2019, four times the 2018 statistic and the highest level seen in a decade.5 An increased incidence also has been seen in the U.S., with 2,251 cases reported to the Centers for Disease Control and Prevention (CDC) for the year 2018.6
Waning Effectiveness of the Mumps Vaccine Component
Waning effectiveness of the mumps vaccine itself has been recognized as the primary driving force behind the resurgence of mumps cases in developed countries.7
In a study designed to evaluate whether the surge was attributable to a change in the circulating strain of mumps or to a diminishing effectiveness of the vaccine over time, researchers concluded that the increase did not reflect a change in viral type but was consistent with waning vaccine acquired artificial immunity to mumps. “Outbreaks from 2006 to the present among young adults, and outbreaks in the late 1980s and early 1990s among adolescents, aligned with peaks in mumps susceptibility of these age groups predicted to be due to loss of vaccine-derived protection. In contrast, evolution of mumps virus strains escaping immune pressure would be expected to cause a higher proportion of cases among children, not adolescents and young adults, as observed.”8
Mumps Complications More Serious in Adults
The increased susceptibility of young adults to mumps infection is of concern because of the increased likelihood of serious complications in that population compared to children, who made up the majority of cases in the pre-vaccination era.
Mumps is a highly contagious viral infection typically associated with headache, muscle aches, tiredness, and loss of appetite. “Parotitis,” or swelling of the parotid gland on one or both sides of the neck right under the ears, is the most widely recognized feature of mumps, giving patients the “hamster face” associated with the disease. Many people with mumps exhibit no symptoms at all and as many as half may only show signs of a mild, nonspecific illness.9
Some possible consequences of mumps infection may affect both children and adults, but there are more frequent serious complications among adults. Inflammation of the testicles (orchitis) in post-pubertal males, for example, may lead to testicular atrophy and, rarely infertility.10 Inflammation of the ovaries (oophoritis) and/or breast tissue (mastitis) also may occur. Neither testicular nor ovarian inflammation is thought to cause infertility in most cases.11
Other possible complications of mumps include inflammation of the pancreas (pancreatitis), the brain (encephalitis) or the tissue covering the brain and spinal cord (meningitis), and deafness.
The vaccine against mumps is only available as part of the combination live virus MMR or MMRV vaccines. Although some health authorities are recommending a third booster dose of the mumps portion of the combination vaccine to counteract waning effectiveness over time, there is no option for only supplementing the mumps segment of the vaccine. In order to boost vaccine-acquired artificial immunity to mumps, one must accept exposure to a third dose of both the measles and the rubella vaccines as well.
1 Mumps: Northern Ireland Cases 700% Rise Down To Vaccine Cycle. BBC News Feb. 19, 2020.
2 Annual Immunisation and Vaccine Preventable Diseases Report for Northern Ireland, 2016-17 (PDF).
3 See Footnote 1.
4 See Footnote 2.
5 England Sees Mumps Cases Soar To Highest Number In A Decade. The Irish Examiner Feb. 14, 2020.
6 National Vaccine Information Center. What Is The History Of Mumps In America And Other Countries? 2020.
7 Lewnard JA, Grad YH. Vaccine Waning And Mumps Re-Emergence In The United States. Science Translational Medicine Mar. 21, 2018.
9 NVIC. What Is Mumps? 2020.
10 Mumps. CDC Apr. 15, 2019.
11 Centers for Disease Control and Prevention. Complications of Mumps. Mar. 15, 2019.