Soumya Swaminathan, chief scientist for the World Health Organization, believes an optimistic scenario is a vaccine produced in the “tens of millions” next year, which would be mainly distributed to healthcare workers, and far larger volumes in 2022. To inoculate the world and defeat Covid-19 could take four to five years, she says.

We have no “crystal ball” to tell the future, she told the Financial Times. “It depends how the virus behaves: whether it mutates, whether it becomes more or less virulent, more or less transmittable.”

Peter Hotez, a professor at the Baylor College of Medicine in Houston who is developing a vaccine, says the US president sees vaccines as a “manufacturing problem,” like making enough ventilators or tests.

“Manufacturing is not the hurdle. It’s taking the time to collect enough efficacy and safety data,” he says. “The Operation Warp Speed language coming out of the White House and biotechs and pharma companies [saying] that they will have a vaccine by the fall—or in weeks or days—does so much damage.”

Vaccines are usually developed over many years and even decades. A 2013 paper from Dutch scientists says the average vaccine took 10.71 years and had only a 6 per cent success rate from start to finish. Each stage is an experiment: from the small phase one trials happening now to the large phase three trials needed for regulatory approval.

There are good reasons to believe this time will be quicker. The Covid-19 vaccines benefit from groundwork done for the Sars and Mers coronaviruses even if they were never approved, says Walter Orenstein, a professor at the Emory Vaccine Center in Atlanta.

New technologies are fueling the hope for a faster process. Analysts from Morgan Stanley estimate that Moderna’s vaccine has a 65 percent chance of success. They believe that before the end of the year we could see vaccines from Pfizer and their German partner BioNTech, and AstraZeneca and Oxford university.

But advances like the messengerRNA programming used by Moderna, BioNTech and another German company, CureVac, have never been used to create products approved by a regulator. The technique translates a protein from the virus into human cells and shows it to the B cells that secrete antibodies.

The pandemic has pushed governments and companies to pour money into Covid-19 vaccines, even if there has been a lack of global cooperation. Peter Bach, director of the Center for Health Policy and Outcomes at Memorial Sloan Kettering, says it helps that there are so many horses in this race.

If a vaccine proves safe, there will still be questions about its effectiveness. Paul Stoffels, chief scientific officer at Johnson & Johnson, says one of the biggest questions is whether it will stop infection or just the disease—or even, like the flu shot, only prevent the most severe symptoms of the disease.

“It would be good if it protects against both,” he says. “If you can prevent infection, it prevents spread going forward in the community.”

Before a vaccine is produced, developments in treatments—such as antivirals and antibodies—may help improve outcomes for Covid-19 patients. A vaccine that is only 60 or 70 per cent effective could still be approved by the regulator and have a significant impact on the spread of the disease, says Stéphane Bancel, Moderna chief executive.

He is “cautiously optimistic” he will see efficacy in the large and final phase three trial—but does not know if it will be 70 or 95 per cent. “Even if it was 70 per cent effective it would reduce tremendously the problem, which is that the virus is so contagious,” he says.

Political leaders will declare victory if a vaccine maker manages to move safely at speeds more suited to science fiction. However, the mass inoculation that could speed up the return to normal life is further away. The first vaccines will probably be given to healthcare workers who will be studied closely, as if they were still part of a trial.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, says we must remember there is still a chance we do not get a vaccine at all. It is not a “slam dunk,” he says.

He is concerned that people are not taking other public health measures to stop the spread because of the “optimism and enthusiasm” about a vaccine.

“People will just assume it’s like a Hollywood movie and at the very last minute, someone will swoop in their helicopter with a new vaccine that was only made a day ago. And the whole world is saved,” Mr. Osterholm says. “It’s human behavior. When you’re faced with such a serious challenge, you want any good news you can get.”