The Covid-19 epidemic in the United States risks becoming a tale of “two Americas,” as Anthony Fauci warned in June: a nation where regions with higher vaccination rates are able to beat back the coronavirus, while those with lower vaccination rates continue to see cases and deaths.

At face value, it’s a division between those who are vaccinated and those who are unvaccinated. But, increasingly, it’s also a division between Democrats and Republicans — as vaccination has ended up on one of the biggest dividing lines in the US, political polarization.

Polarization, of course, is not a new force in American life. Growing polarization doesn’t just mean a Congress more starkly dividing between left and right; it means people’s political views now closely hew with views on seemingly unrelated issues, like which movies should win Oscars. But throughout the pandemic, polarization has manifested as stark differences in how Democrats and Republicans each approach Covid-19, from hand-washing to social distancing to masking.

That polarization has now opened political rifts in vaccination rates, with people’s decision to get a shot or not today a better predictor of states’ electoral outcomes than their votes in prior elections. It’s led the US’s vaccination campaign to hit a wall, missing President Joe Biden’s July 4 goal. Meanwhile, the more infectious delta variant is spreading, raising the risk of infections, hospitalizations, and deaths in unvaccinated — and often heavily Republican — areas.

To put it bluntly: Polarization is killing people.

“That’s a perfectly accurate interpretation,” Seth Masket, a political scientist at the University of Denver, told me. “We’re at the point where people are choosing riskier personal behavior due to following the lead of people in their party.”