TOKYO -- COVID-19 vaccine manufacturers will have produced a total of more than 12 billion doses by the end of 2021, enough to cover the portion of the world's population needed for herd immunity, a report shows.

But rich countries are on track to surpass 1 billion excess doses in their stockpiles by year-end, while access to vaccines remains limited in developing nations, even as output is accelerating.

Global vaccine production reached 6.1 billion doses at the end of August, a report by U.K.-based science data provider Airfinity finds. With manufacturers now producing 1.5 billion doses a month, total output is forecast to hit 12.2 billion by the end of December -- more than the 11.3 billion needed to inoculate 80% of the world's population older than 12.

Of the projected 12.2 billion shots, 6.5 billion will come from Europe and the U.S., with 5.7 billion from China. Monthly output is expected to increase to 2 billion doses next year.

Group of Seven countries already have surplus supplies totaling more than 300 million doses, the report said, and the excess stock is projected to reach 1.2 billion by year-end -- with 1 billion not yet set aside for donations to other nations.

The questions going forward are how wealthy countries will redirect vaccine surpluses to poorer counterparts and how recipient nations will build the infrastructure for administering shots.

Vaccine rollouts are picking up pace, especially among developed nations. So far, 5.52 billion shots have been administered worldwide, with 1 billion doses currently being given monthly, according to Our World in Data, a project whose members include researchers at the University of Oxford.

Airfinity's study covered vaccination data in the U.S., the U.K., European Union, Canada and Japan, assuming that these economies will continue vaccinating 80% of people 12 and older and administer booster shots as well.

Japan is expected to have a glut starting in October, even after giving booster shots.