The world reached a vaccination milestone this week, with 2 billion total doses now administered, according to the tracker from Our World in Data.

Why it matters: The global rate is speeding up considerably. It took 30 days to go from 500 million doses to 1 billion, 24 days to go from 1 billion to 1.5 billion, and just 16 days to jump from 1.5 billion to 2 billion.

• But the vast majority of those vaccinations are still coming in the countries or regions that are either producing vaccines at scale (China, India, U.S., EU) or could afford to buy their way to the front of the line.

Breaking it down: High-income countries (as defined by the World Bank) are generally far ahead of the pack, with a combined 16% of the world's population but 38% of all vaccinations.

• But even among the wealthy, progress is uneven. 58% of Brits have had at least one dose, for example, versus just 13% of South Koreans and 9% in Japan.
• Meanwhile, low-income countries are almost all struggling, with 9% of the world's population but 0.3% of vaccinations. Sub-Saharan Africa is the furthest behind (see chart).

That gulf in supply has been exacerbated by the fact that the COVAX initiative — on which many low- and middle-income countries are relying — is running months behind schedule.

• Efforts to expand manufacturing capacity, particularly in the developing world, could help close the gap over time. But for now, attention is falling on the rich countries that have purchased far more than they need.
• President Biden will likely leverage the White House announcement today on dose sharing (see item 2) to lobby other rich countries to increase their pledges, including at the upcoming G7 summit.
• The EU, for example, has promised to donate 100 million doses by the end of the year, but has offered little in the short term.

What to watch: Some low-income countries like the Democratic Republic of the Congo — which recently returned 1.3 million doses that were at risk of expiring — have struggled to administer the few doses they have obtained.

• The challenges include training and deploying health workers, transporting vaccines where they're needed — door to door if necessary — and communicating effectively about the benefits of vaccination.
• As access to vaccines finally starts to increase in the developing world, more global attention and resources will have to be committed to the delivery phase. The World Bank has started funding some of those efforts.

Worth noting: China has nearly tripled its daily vaccination rate over the past month, according to Our World in Data, and it's now giving more shots per day (19.7 million) than all but 14 other countries have over the entirety of the pandemic.