Dozens of companies, from biotech start-ups to Big Pharma, are racing to develop a safe and effective coronavirus vaccine, both to meet urgent medical need and for the potential payday.
Positive results from final stage trials of several vaccine candidates have raised hopes that there is light at the end of the tunnel in the quest to curb a pandemic that has already killed nearly 1.4 million people worldwide:
How many in the pipeline?
The World Health Organization (WHO) has identified 48 "candidate vaccines" at the stage of clinical trials in humans by mid-November, up from 11 in mid-June.
Eleven of them are at the most advanced "Phase 3" stage, in which a vaccine's effectiveness is tested on a large scale, generally on tens of thousands of people across several continents.
A European project led by the University of Oxford and AstraZeneca on Monday said an interim analysis of its phase 3 trials showed their vaccine had an average 70 percent efficacy in trials involved 23,000 people.
The statement said the drug showed a 62 percent efficacy when people were given two full doses, but this rose to 90 percent in those given only a half dose initially followed by a full dose.
It said the vaccine could be stored, transported and handled "at normal refrigerated conditions" for at least six months and announced plans to develop up to three billion doses in 2021, if it passes the remaining regulatory hurdles.
The announcement comes after a US-German collaboration between Pfizer and BioNTech reported early in November that phase 3 trials for its mRNA vaccine showed 90 percent efficacy in preventing Covid-19 symptoms and did not produce adverse side effects among thousands of volunteers.
"Efficacy was consistent across age, gender, race and ethnicity demographics," the company said.
Pfizer is currently seeking Emergency Use Authorization from the US Food and Drug Administration (FDA) and expects to roll out 1.3 billion doses by the end of next year.
Also this month, US biotech firm Moderna said its vaccine also showed almost 95 percent efficacy.
Moderna plans to submit applications for emergency approval in the US and around the world within weeks, and says it expects to have approximately 20 million doses ready to ship in the US by the end of the year.
The company is on track to manufacture between 500 million to a billion doses globally in 2021, it said.
In addition, several state-run Chinese labs are also thought to have produced among the more promising candidate vaccines.
Russia, meanwhile, has already registered two Covid-19 vaccines, even before clinical trials were completed.
Some methods for making a vaccine are tried-and-tested, while others remain experimental.
Inactivated traditional vaccines use a virus germ that has been killed, while others use a weakened or "attenuated" strain.
These vaccines work when the body treats the deactivated pathogen as if it were a real threat, producing antibodies to kill it without endangering the patient with full infection.
So-called "sub-unit" vaccines contain a fragment of the virus or bacteria they are derived from to produce a similar immune response.
"Viral vector" varieties deliver fragments of viral DNA into cells, often hitching a lift off of other virus molecules.
For example, a measles virus modified with a coronavirus protein -- the apparatus SARS-CoV-2 uses to latch on to human cells -- can be deployed to provide immunity to Covid-19.
Both Pfizer's and Moderna's vaccines are based on cutting-edge technology that uses synthetic versions of molecules called messenger RNA to hack into human cells, and effectively turn them into vaccine-making factories.
Trials of two candidate vaccines -- made by Johnson & Johnson and Eli Lilly -- were "paused" recently over safety concerns.
But that is not necessarily bad news, said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine.
"The fact that trials are paused should indicate that there should be confidence that the whole process of monitoring the safety of trial participants is working well," he said.
Recent cases in which recovered Covid patients were infected a second time with a new strain also raise the question of how long vaccines might last.
In October, the US FDA said it would need to see two months of follow-up data after vaccination before giving emergency authorisation for any vaccine use.
"What is different for Covid-19 vaccines is that speed of development and potential approval is much faster due to the public health emergency," noted the European Medicines Agency (EMA).
"Before approval, all vaccines in the European Union are evaluated against the same high standards as any other medicine," the EMA said in a statement.
