In January, I discussed the first measles outbreak of 2024 here in the United States, specifically in Philadelphia. That outbreak was officially over in late February and ended up involving six children and two adults living in the city with one additional case diagnosed in someone who was visiting. Six of the cases resulted in hospitalization but there were no deaths…yet.
Nine cases of measles in a city with a population of more than 1.5 million might sound unimpressive. I get it. It’s a tiny percentage. Measles, to risk anthropomorphizing a deadly virus, doesn’t care what your or I think. Because of the S-tier contagiousness of the measles virus, even a city with an overall immunization rate of 93% is not protected against the possibility of significant outbreaks. This could have been much worse, as I explained in my earlier post:
That may sound good, but 95% is necessary in order to achieve herd immunity against measles in a community. With such a contagious virus, having 7% of children unprotected means that there are enough for outbreaks to occur, particularly if social factors result in clumping of unprotected children in specific schools, churches, or other groups. It is important to remember that even with higher vaccination rates, any unlucky at-risk individual could still be at the wrong place at the wrong time.
We are now over the halfway point for 2024 and measles has continued to be an issue, which should come as no surprise. Childhood immunization rates have yet to return to prepandemic levels, a problem that is not isolated to the United States. This means that there are still millions of children around the world who are at risk of catching measles and who could serve as an index case for a severe outbreak that results in large numbers of children being injured or killed pretty much anywhere they might travel.
So how are things going so far here in the United States?
As of last week, 188 measles cases have been reported to the CDC from 27 different jurisdictions, which is a 3-fold increase compared to the total for all of 2023. Even states with high immunization rates, like Massachusetts, have reported cases this year. There have been 13 clusters of 3 or more related cases, which is how the CDC defines an outbreak, which have accounted for two thirds of the total number so far.
Breaking the numbers down a bit more, and none of this is unexpected, most cases have occurred in the young (43% under age 5 years) and underimmunized (85% unvaccinated/unknown, 10% with one MMR dose). 5% of cases involved fully immunized (2 doses of MMR) individuals because no vaccine is 100% effective. This is why herd immunity is such an important concept, particularly with measles.
In order to achieve herd immunity, and to provide a meaningful barrier of protection for those who are too young to be fully immunized, or for those who have a legitimate contraindication, 94% of eligible people would need to be immune. Taking into account the small chance that two doses of MMR do not result in immunity, which is generally considered to be 4%, you need a higher percentage of eligible people to get the shot. But because people who intentionally avoid the MMR, whether because of vaccine hesitancy or firmly held anti-vaccine beliefs, often cluster together, it is exceedingly difficult to protect them fully via herd immunity because it only takes one mission trip to Uganda, for example, to bring measles back to a highly vulnerable community in, say, Tallahassee.
Of the 188 cases reported so far this year, half were hospitalized with 61% of patients under age 5 years spending some time as an inpatient. What is not clear is how ill these patients have been. Some were probably admitted out of an abundance of caution with the intention to quarantine them rather than because they had severe disease. No deaths have been reported so far, but again we need to give it a decade or two to say with certainty that none of these cases resulted in death because of the late-onset terror that is SSPE.
Measles is a dangerous infection. It can cause serious morbidity and result in death from acute injury to the lungs and brain, and a small but scary number of deaths from brain injury years after a seemingly uneventful recovery. Historically this amounted to about 1-3 deaths per 1,000 cases in the United States. That number was probably a significant underestimation, however, as we have only recently recognized the increased risk of bacterial infections in the months after measles as a result of immune amnesia.
Fatality rates would very likely be lower in the United States in the event of major outbreaks involving thousands of people, but there would be deaths. 188 cases and counting is the highest since we saw more than a thousand in 2019, a total that we are unlikely to see this year. Still, as immunization rates continue to decline or remain stagnant, it truly is only a matter of time before we see another major outbreak or three. We clearly need to do better, but frankly I don’t see the tide turning in the current climate. Fingers crossed.