European Union gave approval on Dec. 21 for the Covid-19 vaccine developed by Pfizer Inc and BioNTech SE, the latest regulatory go-ahead for the shot, while the United States authorised Moderna Inc’s vaccine on Dec. 19, the second for the country and the first for the company worldwide.

The following is what we know about the race to deliver vaccines to help end the coronavirus pandemic that has killed more than 1.7 million people worldwide:

WHO IS FURTHEST ALONG?

US drugmaker Pfizer and German partner BioNTech are the Covid-19 vaccine trailblazers.

On Nov. 18, they became the first in the world to release full late-stage trial data. Britain was the first to approve the shot for emergency use on Dec. 3, followed by Canada on Dec. 9 and the US Food and Drug Administration (FDA) on Dec. 11. Several other countries including Saudi Arabia and Mexico have also approved it.

The European Medicines Agency (EMA) approved the shot on Dec. 21 and India is accelerating its review.

The World Health Organization could decide whether to give its emergency use approval for the Pfizer candidate by the end of the year as part of its COVAX programme aimed at providing shots for poor- and middle-income countries.

WHO WILL APPROVE MODERNA NEXT?

Moderna became a close second to Pfizer in many countries after it released a full data analysis for a late-stage trial on Nov. 30 showing a 94.1% efficacy rate for its vaccine. Canada approved the shot on Dec. 23 and the EMA will do so on Jan. 6.

WHO ELSE IS IN THE RUNNING?

Britain’s AstraZeneca is seeking approval for its vaccine in Britain after announcing interim late-stage trial data on Nov. 23. It had an average efficacy rate of 70% and as much as 90% for a subgroup of trial participants who got a half dose first, followed by a full dose.

However, it is not clear how the regulator will deal with the different dosages in the efficacy data in its assessment. While India is conducting an accelerated review, it has asked for more data. AstraZeneca is also in discussions with the EMA, which is conducting a rolling review of the vaccine.

India is expected to make a decision on whether to approve for the two full-dose regimen of the shot, which was shown to tbe 62% effective in late-stage trials, soon. Its review does not include the more effective dosage, with 90% efficacy which was given to a small subgroup of volunteers in the trials.

US drugmaker Johnson & Johnson plans to deliver trial data in January 2021, teeing it up for US authorization in February if its shot is effective. It reduced the enrolment target for its clinical trial to 40,000 volunteers from 60,000 on Dec. 9, potentially speeding results which are tied to how quickly participants become infected.

US firm Novavax is running a late-stage trial in Britain with data due in the first quarter of 2021. It expects to start a large-scale trial in the United States this month.

France’s Sanofi and Britain’s GlaxoSmithKline, however, announced a setback on Dec. 11 in their attempts to develop a vaccine. The drugmakers said it showed an insufficient immune response in older people in mid-stage trials and that they would start a new study in February.

WHAT HAPPENS IN THE TRIALS?

The companies typically test their vaccines against a placebo - typically saline solution - in healthy volunteers to see if the rate of Covid-19 infection among those who got the vaccine is significantly lower than in those who received the dummy shot.

HOW ARE VOLUNTEERS INFECTED?

The trials rely on subjects becoming naturally infected with Covid-19, so how long it takes to generate results largely depends on how pervasive the virus is where trials are being conducted. Each drugmaker has targeted a specific number of infections to trigger a first analysis of their data.

HOW WELL ARE THE VACCINES SUPPOSED TO WORK?

The World Health Organization ideally wants to see at least 70% efficacy. The FDA wants at least 50% - which means there must be at least twice as many infections among volunteers who received a placebo as among those in the vaccine group. The EMA has said it may accept a lower efficacy level.

WHAT ABOUT RUSSIA AND CHINA?

While Pfizer’s shot was the first to be rolled out following the publication of full Phase III trial data, Russia and China have been inoculating their citizens for months with several different vaccines still undergoing late-stage trials.

Russia said on Nov. 24 its Sputnik V vaccine, developed by the Gamaleya Institute, was 91.4% effective based on interim late-stage trial results. It started vaccinations in August and has inoculated more than 100,000 people so far.

India plans to make 300 million of the shots next year and Argentina has given the greenlight for emergency use of the shot, with some 300,000 doses arriving in the country on Dec. 24.

China launched an emergency use programme in July aimed at essential workers and others at high risk of infection. It has vaccinated about one million people as of mid-November using at least three shots - two developed by the state-backed China National Biotec Group (CNBG) and one by Sinovac Biotech.

Trial data on a Covid-19 vaccine developed by China’s Sinovac Biotech has varied: interim data from a late-stage trial in Turkey showed its CoronaVac shot is 91.25% effective, while researchers in Brazil say the shot was more than 50% effective.

The United Arab Emirates, meanwhile, said on Dec. 9 that one of the CNBG vaccines was 86% effective based on interim results from a late-stage trial in the Gulf Arab state.