ARCHIVED - Guidelines for the Prevention and Control of Mumps Outbreaks in Canada

 

7.0 Immunization

7.1 Mumps-Containing Vaccine and Immunization Programs in Canada

The mumps vaccine is a live, attenuated virus vaccine and is available in the combined form with measles and rubella vaccine. The Merck MMR vaccine, using the Jeryl-Lynn mumps virus strain, has been used in Canada since its approval in the 1970s. There are two different mumps-containing vaccines currently available in Canada: M-M-R® II, manufactured by Merck Frosst Canada Ltd., and Priorix®, manufactured by GlaxoSmithKline Inc. Both products contain the Jeryl-Lynn mumps virus strain. There is currently no single-component mumps-containing vaccine available in Canada(1,16).

Trivirix®, manufactured by Institut Armand Frappier/Smith Kline, which uses the Urabe Am9 mumps strain, was licensed in the mid-1980s, but it was withdrawn in the late 1980s because of an association between the Urabe Am9 strain and aseptic meningitis.

By 1983, all provinces and territories were routinely immunizing infants with MMR vaccine. To eliminate measles, a two-dose MMR schedule was implemented in 1996 to decrease the proportion of children susceptible as a result of primary vaccine failure. Most provinces and territories conducted measles catch-up campaigns in 1996– 1997. Some jurisdictions used a measles-only vaccine, whereas others used a measles-rubella vaccine for catch-up. Two provinces (Nova Scotia and New Brunswick) did not conduct a measles catch-up campaign. All provinces/territories now use an MMR vaccine in their routine two-dose programs. The immunization schedule in all provinces and territories offers a first dose of MMR vaccine at 12 months of age. Ten provinces/ territories offer the second dose at 18 months of age, and the other three (Nova Scotia, Manitoba, and Alberta) offer it at 4–6 years of age.

While there are several genotypes (strains) of the mumps virus, it is traditionally accepted that mumps viruses belong to a single serogroup. There is evidence that the immunity induced by one mumps virus strain protects against infection by other strains(7). Rubin and colleagues(29) showed that sera from 74 different people who tested positive for mumps antibodies by a commercial enzyme immunoassay were able to neutralize mumps viruses belonging to two different genotypes. Only 10% of sera could neutralize only one virus and not the other. These results suggest that although the differences in neutralization titres provide evidence for some antigenic variation, the fact that 90% of the sera could neutralize both viruses supports the historical view that all mumps viruses belong to a single serotype.

Conversely, there are some data to suggest that the immune response directed against one genotype of mumps may not provide absolute protection against infection with mumps viruses of other genotypes(30). These results have shown that neutralization antisera generated by the vaccine containing the Jeryl-Lynn vaccine strain (genotype A) may not protect against infections with mumps virus from the C and D genotype lineage(31).

As described in Section 3.0, the viral strain in the two 2007 Canadian outbreaks (Maritimes and Alberta), the 2005–2006 Nova Scotia outbreaks, the U.S. multi-state outbreak in 2006, and the 2004–2006 U.K. epidemic was identical. This G genotype is not unusual or rare and, like the rest of known genotypes of mumps, it has been circulating globally for decades or longer. Genotypes currently identified include A to L(32).

The MMR vaccines are safe, immunogenic, and effective and are recommended by NACI and PHAC for primary immunization against measles, mumps, and rubella. The combined MMR vaccine should be used even in individuals who may have prior immunity to components of the vaccine, and it can be used to immunize susceptible adults against mumps.

Mumps immunization after exposure to mumps virus may not prevent the disease. Should the exposure not result in an infection, the vaccine should confer protection against future exposures. If indicated, a second dose of MMR vaccine can be given 1 month or more after the first dose(1).

It is unknown whether primary vaccine failure or waning immunity has been the major risk factor for mumps in vaccinated individuals and mumps outbreaks in the recent past(16). In controlled clinical trials, one dose of mumps vaccine was 95% efficacious in preventing mumps disease(33). However, observational studies conducted during mumps outbreaks have demonstrated lower estimates of vaccine effectiveness, usually around 70% to 80% with single-dose regimens(34-39).

Mumps outbreaks have been reported in school populations in the United States with very high (greater than 95%) coverage with single-dose mumps-containing vaccine, suggesting that one dose of mumps-containing vaccine is not sufficient to prevent mumps outbreaks in the school setting(16,39,40). A two-dose measles, mumps, and rubella immunization schedule used in Finland resulted in higher mumps-specific antibody levels, a higher seropositivity rate, and slower decline of antibody levels(41).

The duration of vaccine-induced immunity is unknown. There are many studies demonstrating a drop in antibody levels over time (i.e., waning immunity)(16,38-40,42-44). The length of antibody persistence is unknown in settings with high vaccine coverage but low or no circulating wild virus, and no data are currently available correlating specific antibody titres with susceptibility to mumps.

Immunization history was known for less than half of the mumps cases reported in Canada in 2007 (n = 586). Of those, 45 (8%) had received two or more doses, 430 (73%) had received one dose, and 111 (19%) had received no doses of mumps-containing vaccine.

7.2 National Advisory Committee on Immunization: Statement on Mumps Vaccine

NACI publishes detailed recommendations pertaining to the use of vaccines in Canada(1). These recommendations are contained in the Canadian Immunization Guide and are updated as new information becomes available. Updated and recent immunization statements are available at http:// www.phac-aspc.gc.ca/naci-ccni/index-eng.php.

NACI issued a revised statement for mumps- containing vaccine in August 2007 as a result of recent mumps outbreaks in Canada and internationally, and after reviewing data on vaccine effectiveness and waning immunity. Its previous recommendation for routine, one-dose mumps immunization was changed to two doses for infants and children. In addition, two-dose mumps immunization is now recommended for certain adult high-risk groups, including secondary and post-secondary students, military personnel, and health care workers(16). NACI suggests consideration of a single dose of MMR vaccine for high-risk adults like health care workers and military personnel born before 1970.

It is expected that large outbreaks of mumps will occur less frequently as the NACI two-dose recommendations are implemented. Cases that do occur may result in transmission of mumps, most likely among children and young adults who have not received two doses of mumps-containing vaccine or who have not had natural mumps disease. A dose of mumps-containing vaccine is therefore recommended for susceptible (i.e., those born in or after 1970 who received only one dose of a mumps-containing vaccine), at-risk populations during outbreaks. At-risk populations will need to be defined by the age groups and settings involved in the outbreak. No more than two doses of MMR vaccine given after the first birthday are currently recommended(16).

7.3 Outbreaks

7.3.1 Immunization of Susceptible Populations

In response to recent mumps outbreaks in Canada, several jurisdictions have undertaken immunization campaigns targeting populations that may be susceptible to mumps. Susceptible populations include those born in Canada in 1970 or later who did not receive two doses of mumps-containing vaccine (at least 4 weeks apart) after their first birthday and who have not had laboratory- confirmed mumps or who do not have documented immunity to mumps. The exact age of the cohorts varies by jurisdiction, depending on when one- and two-dose mumps containing immunization programs were introduced (Figure 1). For those individuals not born in Canada, susceptibility may be determined on the basis of immunization documentation. Those who do not have documented receipt of two doses of mumps-containing vaccine should be considered susceptible.

Jurisdictions contemplating an immunization campaign for susceptible groups as part of their outbreak control strategy should consider using the epidemiology of the outbreak to define their target susceptible group. In previous outbreaks, uptake has been found to be low, particularly among post-secondary student and young adult populations. If the outbreak strategy includes offering immunization to this group, specific approaches to increase coverage should be considered. These might include partnering with student health centres, offering incentives, and providing vaccine in settings where students congregate, such as residences, student centres, bars, and clubs(10).

As with any new immunization campaign, supply of vaccine should be coordinated in consultation with FPT counterparts (see Section 7.4 on Vaccine Supply).

7.3.2 Community Contacts of Cases

Immunization of susceptible contacts of cases may not prevent disease if an individual is already infected(10). It may be considered if repeated exposure to mumps is expected.

However, experience during recent outbreaks has been that public health capacity was quickly overwhelmed by the resources required for individual contact tracing and management. The logistics of providing immunization to susceptible contacts and population groups should be carefully considered. Some of the issues encountered in managing previous outbreaks included vaccine supply and acquisition costs, low uptake by the university-aged cohort, accurate determination of susceptible groups complicated by poor or non-existent immunization records, vaccine administration, and associated costs.

7.3.3 Health Care Workers Who Are Contacts of Cases

Health care workers who are the contacts of a confirmed case should have their immune status reviewed. If they have two documented doses of mumps-containing vaccine or documentation of antibody to mumps, then they can be considered immune and can return to work immediately. If they have one documented dose of mumps- containing vaccine, a dose of MMR vaccine should be provided and they can return to work immediately. If they have an undocumented immune history, it is recommended that mumps IgG be tested and one dose of MMR vaccine be provided. While waiting for the serology results, the health care worker should be excluded from work for the period of communicability, which starts on day 10 after the first exposure. If IgG serology is positive, then the health care worker can be considered immune and return to work. A second dose of MMR vaccine should be administered 28 days after the first for adequate measles protection. If IgG serology is negative, then the health care worker should be considered susceptible. A second dose of MMR vaccine should be provided 28 days after the first was given, and exclusion from work should continue from day 10 after the first exposure until day 26 after the last exposure.

7.4 Vaccine Supply

The status of MMR vaccine supply should be considered before undertaking immunization initiatives as part of the outbreak response. As with any new immunization initiative, vaccine supply should be coordinated in consultation with FPT counterparts through the Vaccine Supply Working Group and the CIC. While MMR vaccine supply has been stable in recent years, factors such as introduction of mumps-containing immunization catch-up programs in some jurisdictions, measles immunization programs, and possible introduction of MMR-V (measles, mumps, rubella, and varicella) vaccine could all affect the availability of MMR vaccine.

In the event of an actual or projected shortage of MMR vaccine during an outbreak, the identification of priority groups may be necessary. In the United Kingdom, the following priority order was considered: routine immunization for infants and children, immunization of rubella-susceptible women of child-bearing age, and immunization of susceptible (measles, mumps or rubella) health care workers, followed by immunization of other susceptible individuals as defined by the epidemiology of the outbreak(45). Prioritization should take place in consultation with the Vaccine Supply Working Group.


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