Two shots of the MMR vaccine are thought to give an individual a 97% chance of avoiding a measles infection when exposed.
And that number has a near-echo in the most recent measles outbreaks nationwide — 92% of the people infected have been unvaccinated or had an unknown vaccination status, according to the federal Centers for Disease Control and Prevention.
But, in Colorado, six out of 16 infections have been in people who are fully vaccinated — what are known as breakthrough cases. That’s more than a third.
So, what gives? Does the vaccine work worse at altitude or something?
No, said a bunch of experts The Colorado Sun spoke with.
“I wouldn’t want people to get discouraged,” said Dr. Michelle Barron, the senior medical director of infection prevention and control at UCHealth University of Colorado Hospital.
Instead, the experts offered up some possible explanations for why a higher percentage of Colorado’s infections have been in vaccinated people — and the answers involve airplanes, testing … and counterintuitive math.
More than half our cases are tied to an airport superspreader event.
Despite Colorado this year seeing more measles cases than it has in the entire prior decade, Barron noted that we’re still talking about small numbers and that data trends are less reliable in small sample sizes. Colorado is not so far seeing sustained transmission of the virus within the state.
Barron also pointed out that four of the six vaccine breakthrough cases were linked to an infectious person who was on a Turkish Airlines flight to Denver and who also traveled twice through the airport. The two other breakthrough cases also were linked to international travel.
Being crammed close to someone who is infectious for long periods may increase the chance that a vaccinated person gets sick.
Barron said she’s always astonished at how crammed DIA is — on the train, around the gates and in line at the stores. That’s why she wears a mask at the airport.
“You’re literally up against everybody,” she said.
Testing matters
Dr. Daniel Pastula, a professor of neurology, infectious diseases and epidemiology at the Colorado School of Public Health and the University of Colorado School of Medicine, said it matters how thoroughly a state is looking for measles.
Colorado has a robust public health system compared to other states, Pastula argued. And he said doctors and other medical providers here are on alert for measles cases.
“We are more likely to catch mild cases that other states may miss,” he said.
This may be especially true with breakthrough cases, which Pastula said tend to be milder — meaning people who are sick may not always be an obvious target for testing.
Epidemiology math can surprise
There’s also another explanation for why you might see more infections in vaccinated people than you might expect, and it has to do with the relative population sizes of the vaccinated and the unvaccinated.
Colorado has among the nation’s lowest rates of MMR vaccination among schoolkids — a measurement that is sometimes used as an analog to estimate a state’s risk for a measles outbreak. But even still, the number of people who are fully vaccinated far outnumbers those who are unvaccinated.
Think of what follows like this:
Imagine you have a high school with 1,000 kids. If that school’s vaccination rate matches the statewide rate:
- 93.3% of the school would be fully vaccinated,
- 0.3% would have vaccinations in progress — we’ll consider that to be vaccinated with one dose for this demonstration,
- 2.7% would have an exemption — let’s assume they’re unvaccinated — and
- 3.7% would be out of compliance with vaccination reporting rules. Since we don’t know what their vaccination status is, we’ll just toss them out.
That leaves us with 933 kids who are fully vaccinated, three who are vaccinated with one dose, and 27 who are unvaccinated.
Now let’s assume that everyone in the school is exposed to measles.
Dr. Ned Calonge, the chief medical officer at the Colorado Department of Public Health and Environment, said it is estimated that 90% of unvaccinated people who are exposed will become infected.
That means 24 of the unvaccinated kids in our school will get sick.
Meanwhile, 97% of the fully vaccinated kids and 93% of the partially vaccinated kids will be protected from illness. This leaves us with 28 fully vaccinated kids becoming sick and, likely, none of the partially vaccinated kids.
Put those numbers together, and you get more than half of the measles cases in our school being breakthrough cases — even though the risk of getting sick is much, much lower for the fully vaccinated kids.
“If you’re dealing with a pretty highly vaccinated population, then most of the cases will be in people who are vaccinated,” Calonge said.
To Pastula, this leads to a conclusion that stresses the importance of being vaccinated right now, as the United States continues to see measles cases ripple across the country.
“If we’re seeing this many (breakthrough) cases out there,” he said, “think about how many exposures are going on that are actually being protected.”