UK Prime Minister Boris Johnson has said the world may have to live with the COVID-19 for years to come and admitted he can't guarantee a successful vaccine will be developed to combat it. He warned of the need to adapt and be flexible in the years ahead.

Johnson has been criticised over the past 24 hours for failing to provide enough clarity after announcing the decision to ease lockdown measures in the UK on Sunday.  

In an attempt to clarify the UK's stance Johnson answered questions from the general public and journalists at a press conference on Monday evening. 

He was asked about the progress the country has made in trying to develop a vaccine and whether the UK could be forced to live with the virus and lockdown measures for months or even years to come.

He said: "I hope, I hope, I hope we achieve a vaccine, and I'm hearing very encouraging things from Oxford and noises about developing a vaccine. But this is by no means guaranteed. I believe in 18 years we still don't have a vaccine for SARS."

"If you ask me am I absolutely certain that we won't be living with this for a long time, I can't say that. We may have to become ever more flexible with how we deal with this and other infections."

Johnson added that the UK government are putting vast sums of money towards the effort to develop a vaccine and says he's desperate to work with other countries on a solution.

"The UK is at the forefront of international activity to develop a vaccine. Countries were trying to do things individually but we want to bring countries together and develop strategies together."

The government's chief scientific advisor, Patrick Vallance, echoed Johnson's realism but said he's confident a drug will be developed, though it may not be a cure.

"There are therapeutics and drug developments on the way," he added. "I would be surprised if we don't end up with something. You can't guarantee that you'll develop a vaccine, we may get a vaccine or we may get a therapeutic but probably not both."

"Most people won't get this virus at all. Of those who do get symptoms, 80% will get cold symptoms and may have to go to bed. A smaller minority will need hospital treatment and oxygen, 1% will need serious treatment and will hopefully leave hospital afterwards."