Vaccinating the world will be perhaps the single greatest global challenge of 2021, and that process is now beginning in earnest.

The big picture: If you're reading this in Europe, the U.S. or one of several other wealthy countries, you will probably have access to a vaccine in 2021. But if you're in a lower-income country, particularly in sub-Saharan Africa, you could be waiting until 2023.

The first vaccines to gain regulatory approval in the West — from Pfizer/BioNTech and Moderna — are bound almost exclusively for wealthy countries, at least in the near term.

• Many rich countries have hedged their bets by buying enough doses of several different vaccine candidates to cover their populations even if some candidates aren't approved.

By the numbers

At least 7.7 billion vaccine doses have already been purchased, with another 3.9 billion reserved should countries or blocs elect to expand their orders, according to Duke University's tracker.

• If you combine both categories, the U.S. has reserved nearly one-quarter of the global supply with 2.6 billion doses.
• Not a single country in sub-Saharan Africa has announced a deal to purchase vaccine doses, though Johnson & Johnson is aiming to produce 300 million doses in South Africa next year.

It's not just that access to doses varies wildly by income level — richer countries are also buying different vaccines than poorer ones.

• India is home to the world's largest vaccine manufacturer, and has sealed massive deals to begin to cover its population of nearly 1.4 billion.
• Those deals are with Novavax, which has yet to release efficacy data, and AstraZeneca, whose vaccine appears to be about 70% effective but is undergoing further evaluation and testing.
• The EU has purchased roughly the same number of doses but has a much more diversified portfolio. It includes 300 million doses of the Pfizer vaccine, the first shipments of which will begin on Thursday.
• Several developing countries have struck deals for just one vaccine candidate, often from Russia (as with Kazakhstan, Nepal and Venezuela) or China.

Breaking it down: A forecast from The Economist Intelligence Unit projects that the most fortunate countries — including the U.S., U.K., EU and Japan — will spend the first half of 2021 vaccinating priority groups and the second half of the year vaccinating the remainder of their populations.

• In other high-income countries — as well as in countries like China, Brazil, India and Russia that are producing or manufacturing vaccines at scale — mass vaccinations will begin next year but likely continue into early 2022.
• Other middle-income countries aren’t expected to be able to vaccinate all high-priority groups until late 2021, with mass vaccinations continuing throughout 2022.
• Low-income countries that can’t afford to purchase doses in bulk will still be vaccinating high-priority groups into early 2022, with mass vaccinations continuing throughout 2023 and perhaps into 2024.

What to watch

There are a number of variables that could shift that outlook significantly in either direction.

1. Further announcements on the safety, efficacy and scalability of additional vaccines, including those from Oxford/AstraZeneca, J&J, Novavax and Sanofi, which have together committed more than 2 billion doses to developing countries.

2. More news on Russia's Sputnik V vaccine and on the several vaccines in development in China.

3. The ability to increase manufacturing capacity in the developing world, and to build strong supply chains to ship and distribute doses globally.

4. The efficient distribution of unused doses by rich countries to poorer ones.

5. The engagement of the Bidenadministration. President-elect Biden has said he'll re-engage with the World Health Organization and restore America's global leadership, but he hasn't spoken about a U.S. role in global vaccine distribution.

State of play

The single biggest factor will be the success of the COVAX initiative, from the Gavi vaccine alliance, the Coalition for Epidemic Preparedness Innovations and the WHO.

Driving the news: Those groups told reporters in a briefing on Friday that they're on track to meet their ambitious goal of ensuring at least 20% of the population in virtually every country on earth has access to a vaccine by the end of 2021.

The big picture: COVAX is the only route to a vaccine for dozens of countries, and wealthier participants will effectively subsidize their access.

What to watch: The aim for the first half of 2021 is to provide enough doses to all participant countries to cover frontline health and social care workers.

• Then comes the 20% wave, which is intended to reach the most vulnerable people in each country.
• Dr. Krishna Udayakumar, director of Duke's Global Health Innovation Center, says the 20% target will be very difficult to hit next year, and 60%-70% vaccination rates — the target for herd immunity — are unlikely before the end of 2022 even in a best-case scenario.
• Yes, but: Vaccinating the most vulnerable and combining vaccination with other public health measures to slow the spread will allow "a real reduction in death and disability but also really an uptick in economic activity," he adds.