About a dozen different vaccines of Coronavirus are in various stages of testing worldwide, including in Britain, China and the US.
Dr Anthony Fauci, the top US infectious disease expert, this week said he is cautiously optimistic there will be a Covid- 19 vaccine by the end of the year or early 2021.
Several wealthy countries have already ordered millions of doses of those experimental vaccines.
Britain and the US, for example, have invested in a vaccine candidate being developed by Oxford University and produced by AstraZeneca. If it works, UK politicians have said Britons will be vaccinated with it. The US expects to start stockpiling it this fall and also has invested in other vaccine candidates.
Groups including the vaccine alliance GAVI are also working to buy doses for poor countries and AstraZeneca has agreed to license its vaccine to India's Serum Institute for the production of 1 billion doses.
The World Health Organization (WHO) is drafting guidelines for the ethical distribution of Covid- 19 vaccines.
How vaccines are distributed within a country will vary. Last week, US officials said they were developing a tiered system that would likely prioritize groups at greatest risk of severe complications from Covid- 19 and key workers.