Americans often think of vaccine hesitancy as primarily ideological, but demographic, socioeconomic and educational drivers may be even more powerful, and as best we can measure them, the ideological drivers are also moving relatively slowly, too. The percentage of Americans saying that vaccination should be required for all children in school fell from 82 percent in 2019 to 71 percent in 2022, according to the Kaiser Family Foundation, but the number of children claiming an exemption to existing mandates has risen much more slowly, to just 2.6 percent that same year.
In 2016, according to Pew, 88 percent of Americans said that the benefits of the M.M.R. vaccine outweighed the risk, against 10 percent who said it didn’t. The numbers were exactly the same in 2019, on the eve of the pandemic, and exactly the same again, after it, in 2023. (My colleague Jessica Grose wrote about these numbers and the stories we tell ourselves about them last summer.)
In Europe, the W.H.O. attributed the recent outbreak mostly to falling vaccination rates, too, though there the declines were of roughly the same scale: for the first dose of the M.M.R. vaccine, from 96 percent to 93 percent between 2019 and 2022, and for second doses, from 92 percent to 91 percent. In Russia and Kazakhstan, official coverage rates were even higher. True rates may be lower, and in certain demographic pockets perhaps much lower — a factor that determines spread risk much more than overall coverage numbers. Nevertheless, in all of these countries, including ours, a vast majority of people are vaccinated against diseases like measles and a vast majority of them continue to vaccinate their children against them, too.
The problem is, it doesn’t require vaccination levels to fall by half, or even by a quarter, to produce outbreaks of a disease like measles, which in populations that have never been exposed to it before is many times more infectious than Covid was at first. (Early alarmist estimates of its “reproduction number” suggested that every new case of SARS-CoV-2 might lead to 3.8 new cases; each new case of measles leads to 12 to 18.) Even somewhat imperceptible declines in vaccination coverage can open up once-unthinkable vulnerabilities — and not just for measles. Experts have long called the disease a “tracer” virus that, by virtue of its incredible infectiousness, exposes gaps in vaccination coverage and in the health system that other diseases may soon exploit, too.
This is the big lesson of the European outbreak. Measles is a well studied and analyzed virus, to which we owe much of foundational wisdom about infectious disease. But we don’t really know the exact level of protection that’s needed to prevent transmission in the real world or just keep it at bay, only a sense of the ballpark range — and that even very small drops in coverage from even very high levels of protection can produce quite gnarly out-of-control transmission.