The ongoing mumps epidemic in the Midwestern US is a stark reminder of how fast and easily an epidemic can still occur. It serves as a warning that we need to remain vigilant in maintaining high immunization levels among our patients. The morbidity and mortality resulting from this outbreak will depend on the soundness and response of the public health infrastructure.
We family physicians need to be prepared to detect and prevent mumps among our patients and staff. While it may not be immediately apparent, if we fulfill our roles competently the outbreak will be less severe.
The start of an epidemic
As of May 15, 1765 cases of mumps had been reported to the Iowa Department of Public Health.1 The median age of those infected was 22 years, with more than one third of cases occurring in those aged 16 to 22 years (FIGURE). College students accounted for 20% of cases.
By the middle of April, at least 8 other states were also investigating possible mumps cases. More states will likely become involved: 2 people who were probably infectious traveled on 9 commercial airline flights in a 1-week period in late March.2 The number of fellow travelers infected is under investigation.
In Iowa, investigation has shown that 5% of those infected reported no history of mumps immunization, 13% reported only 1 MMR vaccine, and in 30% the vaccine status was unknown. A majority of cases (51%) had received the recommended 2 MMR vaccines.1 These numbers are consistent with what would be expected with a vaccine that has 90% to 95% effectiveness in a highly immunized population. In such conditions, most cases occur among the vaccinated even though their rate of infection is far lower than the unvaccinated.
The strain of mumps virus isolated in this outbreak is the same one responsible for a recent large outbreak in Great Britain, where there have been more than 70,000 cases in the past 3 years.3 In contrast to measles—which has resulted in few secondary cases and practically no prolonged transmission in the US after importation from abroad4 —this introduction of mumps has resulted in more cases and multiple generations. Given that mumps and measles vaccine are usally given together in MMR, the reasons for this difference are not clear and include several possibilities:
- The mumps vaccine appears to be less effective than measles vaccine. The mumps vaccine is reported to be 80% effective after the first dose and 90% after the second,3 where measles vaccine effectiveness approaches 98% after 2 doses.
- The number of persons with atypical or asymptomatic mumps presentations is significant and complicates control efforts.
- There is possibly a lower level of immunity to mumps in older age groups. The pre-vaccine epidemiology of measles and mumps were different, with virtually everyone born before 1957 having contracted measles naturally. This artificial cutoff date has also been used to set an age limit for mumps vaccine, although those born before 1957 may not be universally immune to mumps from natural infection.
- Later adoption of routine mumps immunization (1970s) vs routine measles immunization (1960s).
FIGURE
Age distribution of mumps cases in Iowa
Source: Data from the Iowa Department of Public Health website.1
The public health response: Controlling the outbreak
Mumps vaccine is not effective until 2 to 4 weeks after it is administered, so it is not useful as postexposure prophylaxis. However, it is effective in stopping population-based outbreaks after 1 or 2 generations. MMR vaccine should be given to all who cannot provide evidence of immunity, which includes laboratory evidence of prior mumps infection, birth before 1957, or history of 1 dose of mumps-containing vaccine. Health care workers and students (including college students) should receive a second dose of vaccine.
At schools with documented cases, students and staff without proof of mumps immunity should be excluded until they receive a mumps vaccine (MMR) or provide proof of immunity. Exclusion should be until 25 days after the onset of parotitis in the last person with mumps at the institution. Anyone with mumps should be isolated until 9 days after onset of symptoms.
Laboratory confirmation of mumps involves positive mumps immunoglobulin M (IgM), a 4-fold rise in mumps IgG titers between acute and 2-week convalescent serum, or detection of virus by culture or polymerase chain reaction (PCR) in urine or throat swab specimens. Laboratory results can be confusing in those previously vaccinated; mumps IgM titers may not be detectable, a 4-fold rise in IgG titers my not occur, and the acute IgG titer may be high. False-positive IgM results can occur because of infection with parainfluenza viruses.7