written by Tara Haelle

Of all the childhood vaccines on the CDC’s recommended schedule, the MMR is one of the oldest, the most effective… and the most feared. That is, I hear more parents express concerns about the MMR, which protects against measles, mumps and rubella, than any other childhood vaccine, and it’s the vaccine once maligned by debunked concerns about autism (debunked literally dozens and dozens of times). I also hear a lot of hesitancy about the varicella vaccine, which protects against chickenpox, perhaps because parents don’t understand why it’s necessary and perhaps because of worries about possible but extremely rare breakthrough infections.

It’s true that both of these vaccines are associated with more reactions that parents would notice – and worry about – than most of the other vaccines on the schedule, but a recent study in Pediatrics should offer some reassurance to parents wondering whether there are rare, undetected side effects from the combined MMRV or from the MMR when given along with the varicella vaccine. Answer: there aren’t. The study confirmed the risk of the same adverse events that we already knew were linked to these vaccines, and the researchers ruled out the other studied conditions as possible outcomes and didn’t find anything new to worry about. (Though, it’s nice that they keep looking, just in case, right? This is what I love about vaccine research. It never ends, so we just keep gathering more and more data to support what we know or help us understand what we don’t. But I digress…)

First, you can read what side effects have been linked to the MMR, to the varicella vaccine and to the MMRV at the CDC website. The most serious ones for MMR and MMRV are high fever, a seizure with no lasting damage, low platelet count and, in the rarest cases, a severe allergic reaction. (The site lists other adverse events that have been reported to occur after the vaccine, but these haven’t been shown to be caused by the vaccine; they occur so rarely that it’s so far been impossible to determine. Given the millions and millions of MMR doses administered over the years, the fact of that rarity alone should be reassuring.)

This new study, published today in Pediatrics and funded through America’s Health Insurance Plans and the CDC, looked at seizures, fevers, low platelet count (resulting from a condition called immune thrombocytopenia purpura, or ITP) and allergic reactions (anaphylaxis), but it also looked for increased risk of ataxia (jerky muscle movements), arthritis, meningitis/encephalitis, acute disseminated encephalomyelitis (a neurological disease) and Kawasaki disease. The researchers compared the MMR and the MMRV from 2000 to 2012 in children aged 12 to 23 months old and enrolled in health care centers participating in the Vaccine Safety Datalink. The study involved more than 123,000 doses of MMRV and more than a half million doses of MMR plus varicella.

The good news: the researchers found no increased risk of any of these seven outcomes when comparing the MMRV to the MMR. In other words, the risk from the MMRV is no worse than the risk from MMR when it comes to the seven conditions they studied. I know, that doesn’t sound reassuring. But that’s what the point of the study was, to compare the MMRV to getting the MMR along with the varicella. And don’t worry – they also looked at the risk of these outcomes with each vaccine individually, and that’s where we get…

The better news: Other than the known link to ITP, no increased risk was seen for the other outcomes, and no new safety concerns were seen. “These estimates indicated that even if an increased risk for these outcomes exists, the risk is low and rare after either measles-containing vaccine,” the authors wrote. In fact, the risk of ataxia was actually significantly *lower* after both the MMR and the MMRV (though that’s probably a fluke and not a result of any protective effect from the vaccines).

The findings mathematically showed an increased risk for anaphylaxis (severe allergic reaction) after the MMRV, but this finding was based on only two cases, neither of which was confirmed as related to the vaccine and both of which occurred in children with a history of allergic reactions. So all this means is that two children with a history of severe allergic reactions had an allergic reaction following the MMRV that may or may not have been related to the vaccine. While the Institute of Medicine has established that the MMR and MMRV can cause a severe allergic reaction, it occurs in approximately 1.5 o 1.8 of every 1 million doses. An unvaccinated child has a much higher likelihood of catching the measles.

The bad news: The MMRV has about double the seizure risk of the MMR. But, we already knew this and the risk is still pretty low – 1 child out of 3,000 for the MMR and 1 child out of 1,250 for the MMRV, according to the CDC, which matches up with what was found in this study. The study also confirmed what we know in general: that the risk of fever and seizure is higher in the 7 to 10 days after getting either the MMR or the MMRV.

There was also an increased risk of ITP, particularly 14 to 28 days after the vaccines but also up to 42 days afterward. Yet again, we already knew that; the CDC estimates these low platelet counts occur in 1 out of every 30,000 doses. The condition goes away on its own within 6 to 12 months in 80 percent of kids and is rarely a serious or life-threatening condition.

So, today’s study is pretty unexciting, but then, that’s what we usually want from vaccine studies: nothing surprising and confirmation of what we already knew.