Mumps Vaccine: Perceptions and Emerging Realities
Mumps outbreaks are occurring in highly vaccinated populations and this has led to differences in opinion amongst scientists around the efficacy of the vaccine, with various mechanisms of failure being put forward. It has been suggested that the number of vaccinated young adults (18-24 yrs) who contracted mumps in the US in 2006 form a small percentage of the overall vaccinated population. The fact that 84% were vaccinated with two doses is reduced to a minor detail when numbers are crunched in the “right” way. However, this is not the view of all scientists. There is concern as to why highly vaccinated populations are having mumps outbreaks. By choosing to avoid the issue, essentially ignoring the reason why young adults who have been vaccinated twice with the MMR are coming down with mumps, the evidence on the real efficacy of mass vaccination against a benign childhood disease is not discussed.
Back in 1991, the occurrence (1861205 PMID) of mumps illness in a vaccinated population that had received either a single or a double dose of vaccine was studied. The study found a slightly higher incidence of mumps in children with only one vaccine dose and found that children who had had their last vaccine more than 4 years previously were more at risk of contracting mumps. The addition of a 2nd dose to the vaccine schedule was introduced to rectify this problem.
Again in 1995, this study (7795768 PMID) concludes that vaccine failure is resulting in a vaccinated population being at risk for outbreak of mumps. It appears that the vaccine itself does not always work, not that immunity wanes. The plan to accommodate this problem was once again the recommendation of a two-dose vaccine schedule, with the assumption that the problem was one dose not protecting the population.
Fast forward to 2007 and there is mounting evidence that the two doses of the MMR are still not protecting the population from outbreaks of the mumps disease. Scientists are theorizing as to how this occurs in a highly vaccinated population, offering various explanations:
Primary vaccine failure (where the vaccine itself fails to produce antibodies). There is evidence (9592852 PMID) that some strains of the vaccine are better at creating an immune response than others. Unfortunately, even vaccine strains with a better rate of conferring measurable immunity are not protecting populations from outbreaks of the mumps disease. For instance in the USA the Jeryl Lynn strain has been in widespread use since 1967 and enjoys favorable evidence of efficacy from the initial studies done. However, this is not being translated into the same level of efficacy in the population at large, with an estimated efficacy of 61.6%.
Other vaccine strains such as the Japanese Urabe Am9 or the Russian Leningrad-3, neither of which are used in the USA, show a better rate of conferring immunity. However, they also show (10707922 PMID) a higher incidence of aseptic meningitis post vaccination. For a full breakdown on which strains are used in which parts of the world look here .
The two dose vaccine strategy is failing with regard to mumps and has been identified as not being effective for the mumps portion of the MMR
Two-dose vaccine effectiveness was 76–88% with no significant difference for attack rates between one and two doses.
Secondary vaccine failure (where the immunity initially derived from the vaccine wanes, requiring boosters). There are calls for a booster shot for college entry students in an attempt to catch any issues of waning immunity. The data on waning immunity is far from conclusive. The difference between vaccine-acquired immunity and immunity from exposure to the wild virus is complex. The measure of immunity used in science (antibody titres) is not always predictive of whether a person will develop an illness or not. The shift in the age group who are sick with mumps suggests waning immunity, with 19-24 year olds now most likely to contract mumps. The cause for this waning immunity is not well understood, with the possibility that a reduction in circulation of the virus could result in a reduction in the natural ‘boosting’ effect when there is already immunity to mumps virus.
“It might not be cost effective to give everyone a third shot, but it should be considered for college students, said Dr. Stephen Marcella, an epidemiologist at University of Medicine and Dentistry of New Jersey’s School of Public Health.”
There is increasing pressure on college-aged young adults to produce proof of vaccination, when this is not necessarily the most predictive factor when it comes to immunity.
It also issued lists of affected students to professors, directing them to send the students home if they came to class. Officials said the students could be escorted off campus by the university police. “The students are essentially banned from campus until they show they have received the immunizations,” said Robert S. Caswell, the university’s director of public relations. (NYT)
The strain of circulating virus not matching the strain in the vaccine. Evidence from different parts of the world are beginning to question whether different strains of the mumps virus may circulate in a population all at once, and whether individual vaccines cover all the strains or not. Some scientists wonder if perhaps a different mumps vaccine is needed. The assumption that all types of mumps viruses would be susceptible to the vaccine antibodies of just one strain of vaccine is questionable.
There was evidence of waning immunity, which is a likely factor in mumps outbreaks, aggravated by possible antigenic differences between the vaccine strain and outbreak strains
Subclinical disease (where vaccinated individuals can carry the disease without presenting with a clinical symptoms) has also been put forward as a possible explanation for the outbreaks in mumps disease.Furthermore, prior immunization may result in more subclinical disease, which increases the potential for spread. Young adults with mumps who are feeling well may not seek medical assessment, and even if they do, physicians unfamiliar with mumps may miss the diagnosis.
This article highlights the emerging problem with mass vaccination against mumps:
We are really defining right now how long the immunity from the mumps vaccine lasts, how durable it is, how good it is,” said Leigh Grossman, head of the division of pediatric and infectious diseases at UVa. “I think what we’re learning, much like what we’ve learned with pertussis, measles, and now mumps — this is a group of kids who never had this disease, were properly vaccinated, and are again susceptible in their late teens/early adult years.
So what is causing some recipients of two doses of MMR to contract mumps? How many boosters will be required to protect the public health? Can this be considered a successful public health strategy with so many unknowns?
And why in the world is there only one solution ever considered for vaccine failure…more vaccination with the same vaccine that already failed?