The UK's top scientific adviser says that enforcing a travel ban, as President Donald Trump announced this week, is not an effective way to stop the spread of the novel coronavirus.

"In the way the world works you can't stop that, unless everybody does it all it once, and it's certainly too late now to try and do that," Patrick Vallance told a press briefing Thursday.

Even screening people at the airports for the coronavirus is not a sure way to prevent those infected from entering, Vallance added.

Trump banned most travel to the US from 26 countries in Europe, notably excusing the UK and Ireland. Trump has also used the virus to reignite his calls for a US-Mexico border wall.

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The UK's top scientific adviser says travel bans — like the one President Donald Trump has imposed on most of Europe — are not an effective way of stopping the novel coronavirus from spreading.

Patrick Vallance, the UK government's chief scientific adviser, said on Thursday that Britain would not enforce a travel ban because evidence had showed the method to be ineffective.

"Quite early on we looked at the question of stopping flights, and the assessment was that if we stopped flights directly from China at the beginning, unless you got something like a 95% effect, in other words, if you could stop all of the routes from China to the UK by 95%, the effect on the delay of the epidemic was minimal," Vallance said.

"I think the evidence has borne this out. In the way the world works you can't stop that, unless everybody does it all it once, and it's certainly too late now to try and do that."

He made the comments at a press briefing alongside Prime Minister Boris Johnson, which you can watch here:

—UK Prime Minister (@10DowningStreet) March 12, 2020

Trump on Wednesday announced a ban on most travel to the US from 26 European countries for 30 days starting Friday morning, notably excluding the UK and Ireland.

Vallance on Thursday said that even screening people for the virus at airports was not a sure way to prevent those infected from entering.

"The screening measures and so on at airports sort of sounded sensible, but we know that, for example, the first case in the US went through a screening measure, and that was the first person that wasn't detected, and popped up," he said.

"So these things sound great but don't always work."

Trump delivering remarks to the news media during a meeting with bankers on the US coronavirus response. Reuters

Trump has also used the threat of coronavirus to support his calls for a US-Mexico border wall, despite no evidence that any of the 1,215 confirmed cases in the US were linked to someone illegally crossing the southern border.

The president has also referred to the outbreak as a "foreign virus" and sought to deflect attention to the rest of the world, despite facing criticism for dragging his feet over a meaningful US federal response.

Explaining the European travel ban, Trump has said Europe "failed to take the same precautions" as the US in largely opting not to restrict travel from China and other hot spots. "As a result," he has argued, "a large number of new clusters in the United States were seeded by travelers from Europe."

Earlier Thursday, UK Chancellor Rishi Sunak also said travel bans didn't help contain the virus.

"With regard to flight bans we are always guided by the science as we make our decisions here," Sunak told the BBC's "Today" radio program. "The advice we are getting is that there isn't evidence that interventions like closing borders or travel bans are going to have a material effect on the spread of the infections."