Some scientists fear that an effective coronavirus vaccine may prove impossible to produce.

The UK's Chief Medical Officer warned on Friday that there is "concerning" evidence suggesting that people can be reinfected with the virus.

He said evidence from other forms of coronavirus also suggests that immunity quickly wanes.

No vaccine has ever been approved for use against previous forms of coronavirus.

David Nabarro, professor of global health at Imperial College, said the world may have to learn to live with the "constant threat" of COVID-19.

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Scientists fear that it may prove impossible to produce a working coronavirus vaccine and believe the world may have to simply learn to adapt to the permanent threat of COVID-19.

The UK's Chief Medical Officer, Christopher Whitty, told a Parliamentary committee on Friday that there was "concerning" evidence suggesting that it may not be possible to stimulate immunity to the virus.

"The first question we do not know is 'do you get natural immunity to this disease if you have had it, for a prolonged period of time?'" Whitty said.

"Now if we don't then it doesn't make a vaccine impossible but it makes it much less likely and we simply don't know yet.

He said there was "a little bit of evidence that some people have been reinfected with this having had a previous infection."

He added: "That's a slightly concerning situation."

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.

No coronavirus vaccine has ever been produced

Doubts about the possibility of a viable vaccine are based largely on the fact that no vaccine has ever been approved for use in the US or UK against other forms of coronavirus.

Whitty told the committee the evidence from other forms of coronavirus was that "immunity [to the virus] wanes relatively quickly."

He said that the world needs "to be careful that we don't assume that we are going to have a vaccine for this disease as we have had for, let's say measles, which once you have it you're protected for life."

"We cannot guarantee success," he added.

"Vaccines are looked for, for every infectious disease, they are not found for all of them."

Read more: The US is sprinting to develop a coronavirus vaccine or treatment. Here's how 19 top drugmakers are racing to tackle the pandemic.

The World Health Organisation on Saturday also threw doubt on the possibility that immunity to the virus could be induced.

In a statement about plans by some governments to introduce so-called "immunity passports," for those previously infected with the virus the organisation said in a statement that: "there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection."

Other scientists have also raised the possibility that a working virus may never emerge to deal with COVID-19.

In an interview with The Observer David Nabarro, professor of global health at Imperial College, London said the world had to realise that a vaccine may not be possible.

"You don't necessarily develop a vaccine that is safe and effective against every virus," he told the paper.

"Some viruses are very, very difficult when it comes to vaccine development - so for the foreseeable future, we are going to have to find ways to go about our lives with this virus as a constant threat."

Even if a fully effective vaccine proves impossible, Whitty believes that a partially effective vaccine would still be worth pursuing.

"You can have vaccines that are not capable of providing [high levels of] immunity, but they provide enough protection that people don't get severe disease.

"So we might get a vaccine that is rather less effective but is sufficiently effective, that if we vaccinated everyone at a high level of dying from this... we might well be able to massively reduce fatalities even if there was still natural infections."