Since late last year, there’s been a wide gulf between the willingness of Republicans and Democrats to be vaccinated against the coronavirus. That continues. In September, Kaiser Family Foundation polling found that Republicans were much more likely than Democrats to say that they intended to never receive a dose of a vaccine.

       The importance of this is obvious. The higher the density of unvaccinated people in an area, the more easily the virus can spread — and the more risk of serious illness people in the area face.

       And yet it also remains the case that there are more unvaccinated people in counties that voted for President Biden than ones that voted for former president Donald Trump.

       Combining the most recent vaccination percentages from the Centers for Disease Control and Prevention with county-level population data from the Census Bureau, we see that there are about 4 million more adults in blue counties who are unvaccinated than in red counties. When expanding that to the entire population, the spread is 12 million people. (The distinction between all residents and all adults is important because of who is eligible for the vaccine.)

       There are some obvious and important qualifiers here, of course. One is that there are a lot more people who live in blue counties than red ones. On average, 62 percent of adults in the most pro-Biden counties are vaccinated, compared to 41 percent of adults in the most pro-Trump ones. Another is that we don’t have full data; Texas, for one, doesn’t report vaccination rates by county. A third is that these figures don’t incorporate natural immunity acquired through having contracted the virus. That may limit the spread even in areas that have lower vaccination rates. And, finally, just because someone lives in a Biden-voting county doesn’t mean that they are a Democrat; about 45 percent of Trump voters live in counties that voted for Biden.

       If we map out the areas where unvaccinated adults live, it’s mostly — but not entirely — a map of American population centers. You’ll notice, though, that there are also a number of red dots smattered across the map, often in places where there are no well-known large cities.

       More important is the graph at the bottom. It shows the relationship between the total population in a set of counties and the number of unvaccinated people. The diagonal line that runs from bottom left to top right marks the line where half of the population is unvaccinated; dots that sit closer to that line have higher densities of unvaccinated residents than dots that sit further to the right from the line. The second diagonal line shows that only a quarter of the population is unvaccinated; the more-heavily Democratic counties sit closer to that line.

       That’s just adults, for whom the vaccine has been universally available for months. If we look at overall populations, the map doesn’t change much but the graph does.

       Now, we see that the most densely Republican counties have more than 50 percent of their population unvaccinated. The most densely Democratic ones are much further from that 50-percent line. Notice, too, that the counties Biden won by the smallest margin line up more with the heavily Republican counties than the heavily Democratic ones.

       The story of the vaccination effort is certainly that the United States is not where we might hope to be in order to stamp out the virus. But it is also that the people least likely to be vaccinated are Republicans and the areas most likely to have more unvaccinated people are ones that voted for the Democrat in 2020. The good news for those areas, though, is that the gap continues to close.