A coronavirus vaccine will "most likely" not be ready before the end of 2021, according to the head of the world's second-largest pharmaceutical company.

"I'm afraid that the most likely scenario is that we will not have a vaccine before the end of next year," Severin Schwan, the CEO of Roche, said on a call with reporters on Wednesday.

Schwan said an antibody test — which can determine whether someone has had COVID-19 and could be launched as early as May — would instead be the key to allowing people to return to normal life.

Some leading researchers are more optimistic that a vaccine could be developed and rolled out this year.

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A coronavirus vaccine will probably not be ready for use before the end of 2021 at the earliest, according to the chief executive of the Swiss pharmaceutical giant Roche.

Dozens of teams of researchers across the world are racing to develop a vaccine to stem the spread of coronavirus infections, bring down death rates, and allow countries to reopen their economies.

However, Severin Schwan, Roche's chief executive, said he was skeptical that a vaccine could be fully tested, manufactured, and widely distributed within the next 12 to 18 months.

"I'm afraid that the most likely scenario is that we will not have a vaccine before the end of next year," Schwan said on a Wednesday conference call with reporters, adding that an 18-month timeline is "very ambitious."

He said that antibody tests, rather than a vaccine, would be key to allowing people to return to normal life before 2021.

Read more: Tests that can tell if you're immune to the coronavirus are on the way. Here are the companies racing to bring them to the US healthcare system.

The blood tests, which Roche plans to launch in May, can determine whether someone has had COVID-19, the illness caused by the coronavirus, and is likely immune to the virus. That could allow the population to gradually return to work.

Some researchers are more optimistic that a vaccine will be produced this year.

Read more: There are more than 70 potential coronavirus vaccines in the works. Here are the top efforts to watch, including the 16 vaccines set to be tested in people this year.

Professor Sarah Gilbert, who is leading a team of Oxford University developers working on one, said in the best-case scenario, her team could have a vaccine ready by September.

Gilbert told The Times of London earlier this month that she was "80% confident" the vaccine her team is developing would work.

"I think there's a high chance that it will work, based on other things that we have done with this type of vaccine," she said.

"It's not just a hunch, and as every week goes by we have more data to look at," she said, adding, "I would go for 80%, that's my personal view."

Human trials for Gilbert's team's vaccine are expected to start this week. It remains unclear how it could be produced widely enough to meet demand if it is successful in clinical trials.

This article was updated with new quotes from Roche's CEO.