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The World Health Organization on last Thursday added three more candidates to its running list of coronavirus vaccines in human trials, bringing the total to six. The public health arm of the United Nations last published its tally on April 14.

The global medical community is racing with unprecedented speed and cooperation to develop a vaccine for COVID-19, which has so far infected 2.9 million globally, killed 200,000, and triggered mass lockdowns that are causing economic catastrophes. Even with the rapid fight against the deadly pathogen, experts say a cure is at least 12 to 18 months away, and even that timeline is considered optimistic.

The half-dozen vaccine candidates that are farthest along are in either Phase I or Phase II of clinical trials, which are aimed at evaluating whether a vaccine is safe and effective in humans. The clinical trial process has three additional stages: Phase III; regulatory review; and Phase IV.

Three of the most promising vaccines are under development in China, the first country to confront the COVID-19 virus.

The vaccine developed by CanSino Biological, headquartered in Tianjin, and the Beijing Institute of Biotechnology is in Phases I and II.

That effort is based on the technology CanSino developed in producing a vaccine for Ebola. The candidate developed by Sinovac is building off the Beijing-based company's experience fighting China’s severe acute respiratory syndrome (SARS) epidemic in 2004; it's in Phase I.

The Beijing Institute of Biotechnology has partnered with the Wuhan Institute of Biological Products for China's third leading vaccine candidate; it's in Phase I as well.

A fourth promising vaccine, by Moderna and the National Institute of Allergy and Infectious Diseases in the U.S., was the first to be tested on humans in mid-March, earlier than expected. Moderna and NIAID were able to quickly develop the vaccine, now in Phase I, because of prior work on other coronaviruses like SARS and Middle East respiratory syndrome, known as MERS.

Inovio Pharmaceuticals started the first human trials of its vaccine in early April. The small Pennsylvania-based biotech company is funded in part by the Bill & Melinda Gates Foundation. Its work on COVID-19 is building on previous efforts to combat HIV, HPV, Zika, and hepatitis B.

The University of Oxford in the U.K. is developing the sixth vaccine in human trials; it too is building on prior work against other coronaviruses like MERS. Sarah Gilbert, a professor leading the study, attributes the speedy timeline to ongoing research into "Disease X," an unknown pathogen—with pandemic potential—her team had expected to pop up.

In addition to the six vaccines in human trials, the WHO identified 77 others that are in the earlier, preclinical evaluation stage—seven more than last time the agency updated its list.