The UK’s announcement on Wednesday that it was opening its doors again to Americans was naturally welcomed on this side of the pond. If there had been hopes that Washington would reciprocate in kind – however – they were sorely dashed.

A combination of the rise in cases of the delta variant in the UK, the complexities of the US political system and uncertainty over the status of AstraZeneca’s vaccine has made the Biden administration reluctant to make any such sudden move.

Sources told the Telegraph that the Government has been working for months to iron out the rather technical problems that had been preventing quarantine-free travel for the fully vaccinated, and it has been making headway this week after figuring out a way for the NHS to acknowledge jabs administered overseas.

“We were working off travel restrictions set right at the beginning of the pandemic, with no acknowledgment that the situation had changed,” one source said, referring to the UK’s high rate of vaccination. “It’s just a little frustrating that our counterparts haven’t moved at the same pace.”

This week, Transport Secretary Grant Shapps cautioned that there are a “whole series of complexities to resolve” before an agreement can be reached.

For one, the US does not currently recognise the AstraZeneca vaccine – which has been approved by the World Health Organisation – because the pharmaceutical company hasn’t applied for the licence from the US Food and Drug Administration, Shapps told the Commons.

Even once it does, that process would likely take months.

This issue was tested the other week when the Telegraph and a host of other foreign journalists boarded a US Department of Defence-arranged flight from Washington DC. A Sky News reporter was initially turned away because she had had the AstraZeneca vaccine through the NHS. 

Some quiet backstage diplomacy followed and she was eventually allowed on board, but it showed what a headache it’s going to be establishing any kind of air corridor.

How many people have been vaccinated in the USA?

Joe Biden is in a bit of a bind. Keep the border closed to Britons and it will seem like a serious snub to an old ally. Open it in the midst of a resurgence of Covid-19 and he risks the wrath of his own public health experts.

Senior Biden officials are carefully monitoring the impact of delta on the UK, as concerns intensify within the administration about the potential economic damage of the virus to the US.

They see the UK s the canary in the coal mine. Boris Johnson has pushed towards full reopening, even as cases rose above 50,000 a day.

Washington watches in trepidation, as the US has tended to feel the ripple effects of various variants some time after the UK.

Health experts warn that America will almost certainly face the same steep rise in caseload facing Britain – just one or two months later. Complicating matters is evidence suggesting immunity from Covid begins waning six months after the shot was initially given. Most American seniors first received the vaccine in January, meaning their immunity could diminish as the variant ramps up.

“The data from both Israel and the UK – which both started seeing delta before we did – suggests the next month or two will be very difficult in the US, with a lot more infections and a lot more hospitalisations,” said Ashish Jha, dean of the Brown University School of Public Health, “and real potential for disruption of things we have just started getting back after the pandemic.”

A huge number of different US government departments have a say on coronavirus-related travel rules, from the Department of Department to the Centers for Disease Control and Prevention, the State Department and the White House.

The left hand doesn’t know what the right is doing.

Sources say the complications will likely delay any decision to August, possibly even as late as September.