The most advanced candidate is being developed by Britain's Oxford University, the UN global health body said on Thursday at the end of a two-day coronavirus research conference.

The WHO said it remains optimistic, even if the current trials do not succeed.

"We have a broad pipeline with four different types of vaccine," said Ana Maria Henao Restrepo, who is responsible for research and development at WHO.

More than 150 active substances are currently being researched in the search for a coronavirus vaccine.

The AZD1222 vaccine from Oxford is the first to enter phase 3 of a clinical trial, meaning large-scale tests on many people to determine the drug's effectiveness and safety.

Five further potential vaccines are currently in phase 2 tests, where trials occur on a smaller number of patients whose health condition suggests the substance should show an effect.

In phase 1, active substances are typically tested on healthy people to check their tolerance.

Clinical trials of vaccines are also underway in Germany, including at the Mainz-based company Biontech and at CureVac in Tuebingen.

It remains uncertain whether a vaccine will be widely available by the end of the year, a hope expressed by WHO chief scientist Soumya Swaminathan in June.

It usually takes 10 to 15 years to develop a vaccine. However, the global spread of the new coronavirus has pushed countless researchers, pharmaceutical companies and regulators to unprecedented efforts to accelerate development.

Australian Associated Press