The Centers for Disease Control and Prevention's summary of the alarming 118 U.S. cases of measles in 2011 reports that nearly all were caused by scattered inadvertent measles introduction from measles-endemic countries. This importation resulted from U.S. residents returning or immigrants coming from endemic countries. A dozen or so imported cases of measles are not unexpected or new. Every year, cases of imported measles occur.

So why is there an increase in the number of transmitted cases in the United States? Increased vulnerability to ongoing transmission is now possible because herd immunity has become unreliable.

Herd immunity in the past was often discussed in the context of protecting the less than 5% of the community who are too young (less than 12 months old) to receive MMR vaccine or who have true contraindications to vaccine. For measles, reliable herd immunity requires approximately 90% of the community to be immune to measles. To achieve this, we need 95% of the community immunized because approximately 5% of immunized children fail to become immune from a single immunization.

This became clear during the 1990s measles outbreaks and led to a recommendation for two doses, the second dose at 4–6 years of age. That controlled measles outbreaks until the past 2 years, when measles has been increasingly reported. Partly, this is due to the increase in the number of countries with endemic measles, including developed countries, most notably France, as reported in MMWR (2011:60;666-8). So the number of imported cases likely increased. But if herd immunity is strong, secondary cases should not be frequent.

What is new, and has directly led to many cases, is an increase in the number of geographic clusters of unvaccinated children due to parents delaying or refusing measles vaccine. In those areas, secondary cases are occurring at a rate not seen in decades. It's not that the overall national measles immunization rate is that much lower. The overall rate of one dose of MMR vaccine is near 90%, and two-dose coverage is around 80%.

The problem is that the extra geographically clustered 5% who choose to delay or avoid MMR vaccine permit transmission from imported cases mostly among unvaccinated children. In those areas, herd immunity is broken. Just this relatively small shift in local immunization density breaks down herd immunity to measles.

Why is it so? Measles is one of the three most contagious infections, being transmitted via airborne particles. Further, during the first 3 days of measles the presentation is a nonspecific febrile upper respiratory tract infection. So persons with measles often do not restrict their activity and may expose many people via normal activities or even in the reception rooms at medical facilities.

It is not until the third day or perhaps the fourth day of contagion and fever that the classic symptoms of cough, coryza, and nonpurulent conjunctivitis (the 3 C's) begin to appear, along with the beginnings of a maculopapular rash starting on the head. Because most clinicians under 60 years of age have little if any experience with measles, measles may go initially undiagnosed. This leads to additional exposures.

So imported measles added to focal weak spots in herd immunity to measles is the mechanism for increasing measles cases in the United States. Now children whose parents choose to avoid measles vaccine because the disease “is gone” or because of unfounded fears of adverse effects are no longer safe from disease. Note that 105 of the 118 (89%) cases were unvaccinated. And parents who would wish to vaccinate their children but cannot because of age or true contraindication also can no longer rely on herd immunity to protect their children.

This is a call to action. First, we should continue to be strong advocates for on-time MMR vaccination. We are unlikely to convince adamant antivaccine parents, but perhaps we can sway those who are merely conflicted by the false and discredited information promulgated by antivaccine groups.

Second, each of us needs to be aware of whether measles has occurred in our practice area, or in areas where our patients are planning to travel. If there is an expectation of possible exposure, consider administering the second MMR dose anytime more than 1 month after the first dose. And if the child is 9–12 months of age, consider giving a first dose prior to the usual 12 months of age. This will not be a valid dose per current Advisory Committee on Immunization Practices (ACIP) and American Academy of Pediatrics recommendations, but it may save the child from an illness with risks for both immediate and long-term severe pulmonary or neurological complications.

Third, don't miss the disease if it shows up in your patient. Be hypervigilant for the three C's plus high fever, and classic morbilliform rash.

Measles is in the air and until herd immunity is restored, expect cases in every major city in the United States. Hopefully, we as pediatric clinicians, in partnership with our local health departments, can make a difference and minimize these outbreaks.

Unreliable Herd Immunity Leads to More Measles