Mumps

===Resurgence===
{| class="wikitable" align=right style="margin:1em"
|+ Select mumps outbreaks exceeding 1,000 cases in vaccinating locations<ref name=ramanathan />
! Year(s) !! Location !! Number of cases
|-
| 2005–2006 || Czech Republic || 5,998
|-
| 2006 || U.S. || 6,584
|-
| 2009 || New York (U.S.) || 1,521
|-
| 2009–2011 || Jerusalem || 3,130
|-
| 2012–2013 || Belgium || 4,061
|-
| 2013 || Poland || 2,436
|-
| 2014 || U.S. || 1,151
|-
| 2016–2017 || Arkansas (U.S.) || 2,706
|-
| 2017 || U.S. || 5,629
|}

In the 21st century, mumps has reemerged in many places that vaccinate against it, causing recurrent outbreaks. These outbreaks have largely affected adolescents and young adults in densely crowded spaces, such as schools, sports teams, religious gatherings, and the military, and it is expected that outbreaks will continue to occur. The cause of this reemergence is subject to debate, and various factors have been proposed, including waning immunity from vaccination, low vaccination rates, vaccine failure, and potential [[antigenic variation]] of the mumps virus.<ref name=su /><ref name=rubin /><ref name=ramanathan /><ref name=beleni />

Waning immunity from vaccines is likely the primary cause of the mumps resurgence. In the past, subclinical natural infections provided boosts to immunity similar to vaccines. As time went on with vaccine use, these asymptomatic infections declined in frequency, likely leading to a reduction in long-term immunity against mumps. With less long-term immunity, the effects of waning vaccine immunity became more prominent, and vaccinated individuals have frequently fallen ill from mumps. A third dose of the vaccine provided in adolescence has been considered to address this as some studies support this. Other research indicates that a third dose may be useful only for short-term immunity in responding to outbreaks,<ref name=latner /><ref name=ramanathan /> which is recommended for at-risk persons by the [[Advisory Committee on Immunization Practices]] of the [[Centers for Disease Control and Prevention]].<ref name=su />

Low vaccination rates have been implicated as the cause of some outbreaks in the UK, Canada, Sweden, and Japan, whereas outbreaks in other places, such as the U.S., the Czech Republic, and the Netherlands, have occurred mainly among the vaccinated. Compared to the measles and rubella vaccines, mumps vaccines appear to have a relatively high failure rate, varying depending on the vaccine strain. This has been addressed by providing two vaccine doses, supported by recent outbreaks among the vaccinated having primarily occurred among those who received only one dose. Lastly, certain mumps virus lineages are highly divergent genetically from vaccine strains, which may cause a mismatch between protection against vaccine strains and non-vaccine strains, though research is inconclusive on this matter.<ref name=su /><ref name=ramanathan />
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