ARCHIVED - Guidelines for the Prevention and Control of Mumps Outbreaks in Canada
3.0 Epidemiology of Mumps in Canada
3.1 Prior to 2007
The number of reported mumps cases has decreased from an average of 34,000 cases reported per year in the early 1950s to fewer than 400 cases per year in the early 1990s. During the period 2000–2006, an average of 79 cases were reported annually, ranging from 28 in 2003 to 202 in 2002(1). From 1996 to 2006, Canada had five outbreaks, with the number of cases ranging from 13 to 193 (Table 1). These outbreaks primarily involved pre-school or school-aged children, adolescents, and young adults(2-5).
Over time, the age distribution of mumps cases in Canada has changed. The proportion of reported cases aged 20 years and older increased from 14% in 1988–1990 to 64% in 2003–2005(6). Conversely, the proportion of cases aged 1–9 decreased from 49% to 17% during the same period(6).
On the basis of the community epidemiology of mumps, most people born in Canada before 1970 are immune to mumps, as they were likely exposed to the wild mumps virus that was circulating during their childhood. In the majority of jurisdictions, most people born between 1990 and 1994 (depending on the province/territory of residence) have been offered two doses of mumpscontaining vaccine following the introduction of a second dose of MMR vaccine for measles control in 1996 and1997, either during a mass campaign or as part of the routine schedule. This left a possibly susceptible cohort of people born between 1970 through 1990 (to a lesser extent through 1994) who were offered only one dose of mumps-containing vaccine and who are not assumed to have natural immunity (Figure 1). It is important to note that the age at which natural immunity tomumps can be assumed to have been acquired is not known with certainty and that some individuals born before 1970 may still be susceptible to mumps. See section 7.1 for more information on mumps vaccine and immunization.
3.2 2007 Outbreak
As of March 5, 2008, 1,284 confirmed cases of mumps had been reported in Canada with symptom onset in 2007. The vast majority of cases (1,159; 90%) were from Nova Scotia, New Brunswick, and Alberta (Figure 2 and Table 2). The majority (58%) of cases occurred in persons aged 20–29 (Figure 3), many of whom were college or university students (50% when age information was known). Both sexes were equally affected.
The particular susceptibility among those who are college/university-aged is multifactorial. They are too young for natural immunity and too old for inclusion in routine two-dose MMR immunization programs. Mumps has a fairly long infectious period (up to 16 days) and a long incubation period (14 to 25 days), and 20% to 30% of infectious cases show no signs or symptoms.
In addition, the very social and mobile lifestyles of this age group appear to be facilitating disease transmission and interfering with control measures. Young people in this age group tend not to adhere to isolation requests, and they generally do not participate when immunization is offered. Furthermore, post-secondary students often share living/ sleeping arrangements, many are involved in competitive sports, and many frequent bars/pubs/ nightclubs, as well as travel during school holidays and breaks. Additional cases in this demographic group, and possibly other jurisdictions, would not be unexpected.
Immunization history was known for less than half of the mumps cases (586; 46%) reported in 2007. Of these, 45 (8%) had received two or more doses, 430 (73%) had received one dose, and 111 (19%) had received no doses of mumpscontaining vaccine.
* A confirmed case is either a laboratory-confirmed case OR clinically compatible and linked to a laboratory-confirmed case as of 5 March 2008
** Remainder of the 1,284 confirmed cases that were reported are missing onset dates.
Figure 3. Proportion of reported mumps cases by age, Canada; onset December 31, 2006, to December 29, 2007 (n = 1,284)
Source: Centre for Immunization and Respiratory Infectious Diseases, Public Health Agency of Canada.
The data on mumps hospitalizations and complications are incomplete. Complications were noted in approximately 8% of reported cases in 2007. There were reports of orchitis (76), oophoritis (9), hearing loss (8), mastitis (3), meningitis (1), encephalitis (1), pancreatitis (1), and nephritis (1). Less than 2% of cases resulted in visits to hospital emergency departments, overnight observation, or hospital admission.
Identifying the virus strain is useful for differentiating vaccine and wild-type strains, linking cases, linking outbreaks, tracking importations, and documenting the elimination of a particular strain from a geographic area. The viral strain in the two 2007 Canadian outbreaks (Maritimes and Alberta) is identical to the strain (genotype G) detected in the 2005–2006 Nova Scotia outbreaks, the U.S. multi-state outbreak in 2006, and the U.K. epidemic in 2004–2006. In the U.S. outbreak, there were over 6,500 cases reported in 45 states.
In the U.K., the epidemic peaked in 2005 with more than 50,000 mumps notifications, the majority being 15–24 years old.
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