The gap in vaccination rates between counties that voted for Donald Trump and those that voted for President Biden in 2020 is only getting bigger with time, according to a new KFF analysis.

Why it matters: Vaccination rates provide the strongest indication of which communities are still vulnerable to outbreaks as the Delta variant rapidly spreads.

State of play: The virus is still killing hundreds of Americans every day, and experts fear that number will only rise with the spread of new variants.

• On the other hand, study after study has shown the effectiveness of coronavirus vaccines.

Driving the news: A study published yesterday in Nature found that antibodies in people who had recovered from COVID were significantly less potent against Delta than the Alpha variant that first detected in the U.K.

• One dose of a vaccine, however, significantly boosted their response.
• Further, antibodies from people who had received only one dose of the Pfizer or AstraZeneca vaccines "barely inhibited" Delta, but two doses generated an effective response in 95% of people.

Between the lines: Some public figures — like Sen. Rand Paul — have pushed the idea that people previously infected with the coronavirus don't need to be vaccinated.

• Others have made the case that we're closer to herd immunity than vaccination rates alone imply, because of the protection generated by prior infection.
• But this new data suggests that unvaccinated individuals and communities with low vaccination rates — which are more likely to be Republican — are still vulnerable to outbreaks regardless of how hard they've been hit before.

Yes, but: Political ideology isn't the only demographic fault line in vaccine rates.

• Hispanic and Black Americans still have lower vaccination rates than Asian and white Americans, and the rates for all four groups have plateaued, according to another KFF analysis.