A third dose of MMR is safe but do we need it?

It was recently reported - at the National Foundation for Infectious Diseases 15th Annual Conference on Vaccine Research - that the rate of adverse effects from a third dose of the measles, mumps and rubella (MMR) vaccine is the same as those of the second dose. This was conducted as part of a Centres for Disease Control study and led by Glen Abedi, an epidemiologist at the CDC and Masters student. I'm basing this on a media report of the conference as the paper has not yet been published. You can find more results here.


This is the first study to look at the safety of receiving a third dose of the vaccine in school children during an outbreak. The study has some obvious caveats but what it brings up is the question of whether we should extend 3-dose coverage to the population as a whole?


A mumps outbreak from 2009 to 2010 in and around New York City offered the CDC the chance to specifically look and see just how safe the administration of the third dose would be. In this outbreak - and in others - a very large percentage of those with clinical mumps had received two doses of MMR. We don't really know why, maybe it's a question of them getting a large dose of mumps, or maybe it's the vaccine not being perfect and inducing waning immunity.


They used this as a booster shot against the mumps virus in order to prevent further infection and spread from the community and to do so, they set up vaccination clinics in a number of school in the Orange County area of NYC. Those immunised were between 11 and 17 years old. These booster clinics have been set up before for mumps. These clinics are very money intensive and have reported to cost around $500/person. 


As a means to stem the tide of the outbreak, this three dose schedule worked and the rates of mumps infection dropped from 4.93/1000 to 0.13/1000. Although there was no reporting of antibody levels before and after the 3 doses. And we can't be sure whether the outbreak would have abruptly ended like this without vaccine intervention. One caveat with these kinds of studies is that they usually administer the dose late into the outbreak. If done earlier they may have completely controlled it's spread.


But to determine whether this was safe they had to send out questionnaires to the families of the kids, they consulted local clinics and they looked up the Vaccine Adverse Events Reporting System (VAERS). Over 90% of those immunised responded and only 115 reported adverse effects 2 weeks after vaccination. Those effects were only local pain/swelling at the injection site, muscle pain and dizziness/light-headedness. All the kinds of things that suggest that your immune system recognised the vaccine. Note that no cases of meningitis, glandular swelling  or orchitis were recognised, somewhat more serious effects of mumps vaccination in some cases.


This research highlights that in certain situations (a relatively small localised outbreak, with very targeted vaccination of schoolchildren) a third dose of MMR is safe. Of course if they wanted to definitively test this I think they would need a bigger sample size/diversity than the 1755 religious school kids. Remember also that they didn't report looking into levels of mumps immunity so time will tell whether these children were really protected and whether they may still get mumps in the next couple of years.


But the question now remains is whether or not we should extend three doses to the general population or even in cases of a localised outbreak. Mumps is a very infectious virus and hence you need very high levels of population protection to achieve herd immunity (estimated as high as 92%). In the U.S, those between 13 and 17 have an estimated 2-dose MMR coverage of 90.5% and the MMR vaccine uptake percentage in colleges etc is just shy of 90%. So maybe what this data says is that in general we should really focus on achieving very high levels of 2-dose MMR but in the cases of a mumps outbreak we could use a targeted third dose. If we didn't mind the cost. Maybe in the future to lower the costs, all schools/campuses will introduce a MMR catch-up when the new students start.


Another issue apparent is whether we need a new mumps vaccine. Clearly our current mumps vaccine has been amazingly effective to date but maybe it is not enough to completely eradicate the virus. This is something we will have to consider now that people are developing newer vaccines against the virus.



Centers for Disease Control and Prevention (CDC) (2010). Update: mumps outbreak - New York and New Jersey, June 2009-January 2010. MMWR. Morbidity and mortality weekly report, 59 (5), 125-9 PMID: 20150887