A global screening programme to detect Covid-19 variants could help release international travel, with borders having to open “at some point”, the Government’s chief scientific adviser has said.

Sir Patrick Vallance suggested to MPs that worldwide progress in vaccinating against Covid-19 could also be key to unlocking international travel.

He said that “inevitably at some point travel will reopen” and “as more countries become vaccinated and as we get the ability to understand how much transmission is reduced by vaccination, it makes freer movement across countries much more possible again”.

This might involve “certification or whatever” but travel has to reopen in a safe way, possibly with a global screening programme to help detect variants, he told the Commons Science and Technology Committee.

May 17 has been pencilled in by the Government as the earliest date for the resumption of international travel.

Committee member Carol Monaghan pointed out that “other countries are way behind us in terms of vaccination” so questioned whether May 17 was a “sensible date”.

Sir Patrick responded: “We are going to have to measure as we go along and we don’t know.”

But he told MPs that concerning coronavirus variants “are likely to arise everywhere” and suggested that keeping borders shut was not a long-term strategy.

“Largely they have been detected in countries which have got good sequencing capabilities, so there will definitely be other variants that simply haven’t been detected because they will be in other countries that aren’t sequencing,” he said.

“I would expect to see more variants emerge.”

Sir Patrick said the process of “convergent evolution” meant similar variants were arising separately in “different parts of the world”.

He added: “This is something that means we will see those being acquired in this country and is again a reason to keep rates down as low as possible.”

Sir Patrick said that in order to prevent any “importation you have to have completely rigorous border controls which would stop everything coming in and that is an extremely onerous requirement”.

Even then “you won’t stop them (variants) completely”, he added.

Other measures – such as self-quarantine – aimed at reducing the chance of cases or variants coming in “will delay, rather than stop” that happening.

In New Zealand there are “no cases or very few cases and it maintains that through a very, very rigorous border system”, he continued.

But “clearly at some point they are going to have to open up their border and at that point they will see an influx of infections”.

Asked about the Government’s maintenance of a “red list” of countries with concerning variants, Sir Patrick told MPs: “There is some logic in thinking about where you have got the highest prevalence of either the virus overall or a particular variant.

“But I don’t think we should dream that you can stop these things coming in, or indeed evolving within domestic virus transmission.”

The Government’s chief scientific adviser also said a “zero Covid” strategy was not possible.

“Our focus needs to be on reducing the levels we have here. That is the key point, to keep things under control,” he said.

“As levels come down, test, trace and isolate becomes increasingly important, cluster identification – making sure we understand where there are outbreaks and how to deal with them – and of course the vaccine is going to make a huge difference to all of this.

“I do not think that zero Covid is possible. I think there’s nothing to suggest that this virus will go away, at least any time soon.

“It’s going to be there, circulating. It may be a winter virus that comes back over winters with increasing infection rates during that period.”

He said it would be very difficult for anywhere to keep cases out in the long term “because at some point countries have to open up borders”.