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Doctors and NHS leaders have warned that a no-deal Brexit would make it "likely" there will be a delay in flu vaccine supply this year.

They say the EU departure date coincides with the winter season in a way that creates "a perfect storm for the NHS".

Major flu vaccine manufacturer Sanofi said delays in deciding what strains of flu to use in the vaccine mean over a million doses will have to be imported after 31 October.

Vaccines are usually available for delivery by the end of August and throughout September, but the delay has had a knock on effect on the production schedule.

These doses - which represent about 20% of imported flu vaccines - are specifically intended for under-65s in at risk groups (such as disabled people and their carers) as well as healthcare professionals, who are routinely exposed to the virus.

The government said it was "working closely with vaccine suppliers to ensure they have robust contingencies in place".

"What we can see is we're likely to not have enough flu vaccine, we are likely not to have the flu vaccine coverage that we've had in previous years, and that is likely to have an impact on the NHS," Prof Andrew Goddard, president of the Royal College of Physicians, exclusively told Newsnight.

In an open letter last week, Prof Goddard and other doctors' leaders warned the prime minister about the risks of a no-deal Brexit.

They said that the government's "EU exit strategy must include provision for different flu scenarios". The government is yet to respond to the letter.

He said NHS no-deal planners have "done everything in their power to get ready. But there's only so much preparation you can do."

The government said: "We want to reassure patients that our plans should ensure that supplies of vaccines remain uninterrupted when we leave the EU on 31 October, whatever the circumstances."

'Worst case' planning

Different strains of the flu are used in the vaccine each year.

Normally, the make-up of the vaccine in the northern hemisphere are decided in February by the World Health Organization (WHO).

But this year, it took longer to identify which strains of the flu virus were to be included - and this had a knock-on effect on the production schedule.

Moreover, the nature of the vaccine means doses cannot be stockpiled in advance of an October no-deal. This issue is thought to affect all of the UK's suppliers, including Sanofi.

A spokesperson for the company, Hugo Fry, said that in case all the ports were booked after 31st October, the firm has doubled its efforts to make sure it has booked space on aircraft as well as ferries.

Even with these contingencies in place, the logistical process would have to run smoothly to ensure clinics have their vaccine supplies in time he said. Sanofi says it is confident that it has done all it can to ensure its flu vaccines arrive as scheduled.

"Unfortunately I didn't bring my crystal ball with me, but what we're doing is planning for what we think will be the worst case scenario."

Earlier this month, Newsnight reported that delays occurred during a "dry run" of emergency aircraft deliveries of radioisotopes used in cancer diagnosis and treatment. They are normally driven in on trucks.

Lessons from Australia

Vaccines shortages occur more often than they do for other medicines, because vaccines are biological (rather than chemical) and complicated to manufacture, Mr Fry said.

When vaccine shortages occur health services might try to rush more doses into the country. But this "might be difficult" during the "period post no-deal Brexit," he added.

Prof Helen Stokes-Lampard, chair of the Royal College of GPs, said she understands measures have been put in place to minimise the risk of supplies being disrupted but that "ultimately, we just don't know how things will pan out in the event of a no-deal.

Saffron Cordery, deputy chief executive of NHS Providers, said that there were already several signs of a tough winter ahead for the NHS and trust leaders were "very concerned".

"We are coming out of summer with record levels of demand for urgent and emergency care, leaving trusts struggling to catch up.

"We are seeing a severe level of flu in Australia and, on top of this, health services may be grappling with the 'new normal' of a no-deal Brexit."

What happens in Australia is taken as an indicator to the UK about what to expect, but it is not always reliable.

Official data from Australia show that there were a higher number of cases of flu this year than in most of the previous five years. However, a similar number of people were hospitalised.

Yet this year's flu has caused more deaths than strains in any of the past five years other than 2017, which was considered to be particularly virulent.

Prof Goddard said: "If there is a significant outbreak, or even an epidemic, it won't just bring more patients to hospitals, GPs and health clinics - especially older people and other vulnerable groups - but could also keep the very staff needed to treat them at home, with sickness levels rocketing at exactly the wrong time,"