By Philip R. Fischer, MD, DTM&H
Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN; Department of Pediatrics, Sheikh Shakhbout Medical City, Abu Dhabi, United Arab Emirates
SYNOPSIS: Recent mumps outbreaks in the United States have involved vaccinated individuals without international travel. The genotype of the mumps virus circulating in North America and Europe is different than that of the virus used to manufacture the attenuated vaccine used in the United States.
SOURCE: Shepersky L, Marin M, Zhang J, et al. Mumps in vaccinated children and adolescents 2007-2019. Pediatrics 2021;148:e2021051873.
Mumps usually is a mild illness with inflammation of the parotid glands, but serious complications can result. With vaccination against mumps available since 1967 and with a two-dose series of vaccine recommended since 1989, the number of mumps cases dropped by more than 99%. Nonetheless, outbreaks continue to occur. However, the majority of pediatricians would not test for mumps in a vaccinated child with parotitis. To better understand the epidemiology of mumps and to highlight risk factors to aid in clinical diagnosis and management, Shepersky and colleagues at the Centers for Disease Control and Prevention (CDC) reviewed clinical data from reported mumps outbreaks in the United States since 2007.
Mumps is a reportable illness in the United States, and the CDC is informed of confirmed positive cases. These researchers reviewed epidemiological data from mumps-positive patients younger than 18 years of age from 2007 through 2019. They considered patients up to date with mumps vaccination if they were younger than 5 years of age with at least one dose received, and with two doses if ages 5-17 years.
There were 9,172 reported cases of mumps in children during the study period. These children represented 32% of all cases of mumps reported. Cases were reported from all 50 states and from the District of Columbia. The median number of pediatric cases per year was 349, but there were peaks in 2009-2010 and 2016-2017 with much higher numbers of cases. Overall, 71% of mumps patients had their illness in association with a known outbreak of mumps.
Of patients with medical records noting vaccine status (79% had vaccine status noted), 89% had received at least one vaccine and 84% were up to date with mumps vaccination. Only 2% of mumps cases were associated with international travel. Half of pediatric patients with mumps were adolescents; interestingly, the most affected adult age group during this period was the 18- to 22-year-old group. A complication was seen in 1% of mumps-infected children, and 3% of the post-pubertal males with mumps had orchitis. (Of those with orchitis, 66% were up to date on mumps vaccination.)
The authors pointed out that the occurrence of mumps in vaccinated individuals could be because of either waning immunity over time post-vaccination or an antigenic mismatch between the circulating mumps virus strain and the vaccine strain. The ages at most risk (adolescents and young adults, at least five years after the second and final routine mumps vaccination) supports the notion of waning immunity. However, it also is true that the vaccine strain (genotype A) prompts incomplete immune responses to the commonly circulating mumps strain (genotype G). Specifically, although the A genotype vaccine provides neutralizing antibody responses against the G genotype virus, the level of those cross-reactive antibodies is less than half that of the same-genotype responses.
Unlike measles and rubella, mumps still is endemic in the United States, with cases reported each year from almost every state. International travel or exposure to a traveler is not required to get mumps.
COMMENTARY
It is true that some past mumps outbreaks, and many measles outbreaks, result from contact with someone in or from another country and occur mostly in unvaccinated individuals.1 However, a huge take-home lesson of this new study is that the vast majority of mumps cases in the United States now are because mumps is endemic in the United States. Foreign travel is not necessary to start mumps outbreaks, and vaccination is not fully protective. Detailed virologic studies suggest that there is steady transmission of the same virus strains in the United States, mostly those with the G genotype.2 New outbreaks do not seem to be caused by the introduction of new viral variants.2,3
In 1963, Maurice Hilleman was a virologist with a pharmaceutical company.4 His 5-year-old daughter Jeryl Lynn became ill with mumps.4 He isolated the mumps virus from her respiratory secretions, attenuated the virus, and, by 1967, produced the Jeryl Lynn strain of mumps vaccine.4 A quick internet search reveals a 1968 photo of Jeryl Lynn holding her crying younger sister, Kristen, as Kristen received the mumps vaccine. Continued internet searching shows a 1991 photo of Jeryl Lynn with her son, Colin, as he received the vaccine. The Jeryl Lynn vaccine is an attenuated mumps virus strain with the A genotype and still is used in the United States.
Different vaccines are used in different parts of the world,5 but side effects are more common with some of the vaccines used in other countries. Currently, though, the G genotype strain of mumps virus that is most common in the United States also is most common in Portugal6 and Spain,7 while genotype F, H, and I strains have emerged in Korea during the past two decades.8
If waning immunity was the only issue leading to mumps outbreaks continuing, adding an adolescent vaccine dose could be effective in providing longer-term protection. But, the strain difference seems relevant. The current Jeryl Lynn A genotype-based vaccine triggers less immune response to the circulating G genotype viruses, so it could be incompletely protective or subject to more serious loss of immunity as seroprotection wanes (from a lower initial starting point).
Mumps is endemic in the United States, and outbreaks continue to occur. Hun-dreds of children are ill with mumps each year. It could be time to consider developing or using a different vaccine that provides better protection against the mumps virus strains with the G genotype.4,5
REFERENCES
- Centers for Disease Control and Prevention. Mumps outbreak on a university campus — California, 2011. MMWR Morb Mortal Wkly Rep 2012;61:986-989.
- Wohl S, Metsky HC, Schaffner SF, et al. Combining genomics and epidemiology to track mumps virus transmission in the United States. PLoS Biol 2020;18: e3000611.
- Bryant P, Caldwell H, Lamson D, et al. Streamlined whole genome sequencing of mumps for high resolution outbreak analysis. J Clin Microbiol 2021; Nov 10;JCM0084121. [Online ahead of print].
- Grose C. From King Nebuchadnezzar of Babylon to mumps genotyping and vaccination 26 centuries later. Pediatrics 2021;148:e2021052761.
- Connell AR, Connell J, Leahy TR, Hassan J. Mumps outbreaks in vaccinated populations – is it time to re-assess the clinical efficacy of vaccines? Front Immunol 2020;11:2089.
- Perez Duque M, San-Bento A, Léon L, et al. Mumps outbreak among fully vaccinated school-age children and young adults, Portugal 2019/2020. Epidemiol Infect 2020:149;e205.
- Barrabeig I, Antón A, Torner N, et al. Mumps: MMR vaccination and genetic diversity of mumps virus, 2007-2011 in Catalonia, Spain. BMC Infect Dis 2019;19:954.
- Won H, Kim AR, Yoo JS, et al. Cross-neutralization between vaccine and circulating wild-type mumps viruses in Korea. Vaccine 2021;39;1870-1876.
Recent mumps outbreaks in the United States have involved vaccinated individuals without international travel. The genotype of the mumps virus circulating in North America and Europe is different than that of the virus used to manufacture the attenuated vaccine used in the United States.
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