Immunity, Breastfeeding, and the Timing of Measles Vaccine

Leave it to Disney to make a splash—any day now, we’ll hear that Anna and Elsa have caught the measles themselves (imagine a link to the sisters all covered with spots, looking miserable in the hospital, with a worried snowman and moose cowering in the background. “For the first time in forever… measles is back….”)

I’ve already covered the outbreak in detail. Briefly: over the December holiday someone at one of Disneyland’s theme parks in California brought in measles. At least 5 employees and probably about 40 park visitors caught it, almost all of whom were unvaccinated. Since then, despite a massive public health effort to identify and isolate potentially infectious contacts, the outbreak has spread to about 100 cases in 6 states. Again, and this can’t be repeated too much, almost all of the cases are occurring in people who have not been fully vaccinated, either because they’re babies who are too young, or for other reasons. It’s not yet clear exactly what that breakdown is. Some of the cases could have and should have been vaccinated; it’s likely that others had health issues that prevented timely vaccination. In any case, since measles is super-contagious, it will likely continue to spread, especially among communities with poor immunization coverage. Sadly, this has been an entirely predictable and avoidable outbreak.

A few comments and notes sent in—thanks especially Emily and Jennifer–have asked for more details about the MMR vaccine and how immunity affects how it works. I feel another Q&A coming on…. 

Aren’t newborns pretty well protected against measles, from mom’s antibodies? 

The placenta sends lots of important things to baby—oxygen, nutrition, growth factors, love, and what’s called “passive immunity” via maternal antibodies. These are large molecules, a kind of immunoglobulin called “IgG” which mom had made previously after exposures to diseases or vaccines. Good maternal immunity to things like influenza and measles does provide good protection for their newborns. That’s why it’s important for pregnant women to get flu vaccines, and for all girls to get all of their vaccines—so later, when they’re pregnant, their little babies get protection, too.

But those IgGs from momma, they don’t last so long. The “titers” drop off fairly rapidly, and the protection falls quickly. Best protection probably lasts weeks, with some protection falling off over months. By six months of age, there’s probably no protection from maternal IgGs.

However, there’s still some small amount of IgGs circulating. Though they’re not protective, they can interfere with some kinds of vaccines (especially live, attenuated vaccines like MMR and chicken pox.) That’s why these vaccines are ordinarily given at 12 months of life or later. It’s not dangerous to give them early—it’s just that they probably won’t work as well to provide strong, lasting protection. Maternal IgGs do not interfere with the effectiveness of many other vaccines, like the Hepatitis B, DTaP, polio, and the other vaccines given in the first year of life. 

Can you give MMR vaccine earlier, say if exposure risk is high?

Yes, though it may not work as well or provide protection that’s long-lasting. Current recommendations are to give the first dose of MMR routinely at 12-15 months of life. It should be given early (as early as 6 months) if the risk of exposure is high. For example, the CDC currently recommends early MMR for international travel to Europe, Asia, the Pacific, and Africa. I think it would also be prudent to vaccinate early for travel to California, especially if your baby will be in crowded places like airports or theme parks (California officials have said that these places are safe—IF you’re vaccinated.)

A dose of MMR vaccine given in the 6 – 11 month window will provide some protection, but since the lingering maternal IgGs will prevent it from being fully effective the dose doesn’t “count.” Two further doses will still be needed, following the typical schedule at 12-15 months and at 4-5 years of age.

Doesn’t breastfeeding give baby antibodies? Wouldn’t that prevent measles? Or can breastfeeding interfere with the MMR vaccine?

Breastmilk does contain antibodies, but they’re a different kind of antibodies. They’re not the IgG antibodies that circulate in the blood, they’re IgA antibodies that concentrate more in body secretions, including nasal mucus and breast milk. These IgA molecules don’t interfere with vaccines. They provide modest protection against mostly gastrointestinal infections (think diarrhea and vomiting illnesses)—which makes sense, because the breastmilk IgA molecules are swallowed. They don’t make their way into the blood, or at least not very much—like other proteins, if you swallow them they’re mostly torn apart during digestion. Breastmilk IgA provides just a little protection against infections that are caught via the respiratory tract, including the common cold and measles. For instance, a breastfed baby on average statistically will likely get one half of an ear infection fewer in the first year of life. Not a huge impact, at least not in respect to those kinds of infections.

Is there any way to test for those maternal measles IgG antibodies? I mean, if my baby’s antibodies are low enough at 9 months of age, could I get him vaccinated then?

Well, you can test for them, but the exact amount doesn’t perfectly correlate with whether the baby will become immune after the vaccine. You won’t know if the vaccine given at 9 months worked well unless you test your baby afterwards—and even then, there’s a grey zone in the measurements.

Maybe we should test for immunity? I mean, should we be testing children after the MMR to make sure it worked?

After one dose of MMR, about 85% of children will get complete, lifelong protection against the three components: measles, mumps, and rubella. The second dose, traditionally given at age 4-5, will pick up almost all of the remaining unprotected 15%, leaving only 1% non-immune. Those odds are really, really good—and if a community has high vaccination rates, that 1% of kids whose MMR didn’t take are still well protected by herd immunity. Of course, if vaccine rates fall, it all falls apart. The 1% who didn’t respond are vulnerable, as are babies too young to vaccinate and people with health conditions that preclude vaccination.

Testing for immunity can done under special circumstances, sometimes to help control an outbreak, or in people at risk for losing immunity after chemotherapy, for instance. But the testing is expensive and kind of a hassle (it’s not always easy to draw blood from children, and they don’t like it very much.) Because the vaccine is so safe, it makes more sense to just give the two doses than to test everyone.

This blog was originally posted on The Pediatric Insider.

© 2015 Roy Benaroch, MD