Eligible people in Norfolk and Waveney can get their Covid booster jabs sooner - but they will have to book as walk-in appointments won't be available.
The government announced last week that, by January, everyone over 18 would be offered a booster jab and that the six-month wait after second jabs would be cut to three months.
The government had said that age groups would be called in turn.
Health bosses had been waiting for the clinical guidance to allow that to happen and people over 40 and those at higher risk of Covid-19 will, from Saturday (December 11), be able to book their appointments three months after their second jab.
But health bosses have said they will have to use the national booking system and not head to Norfolk and Waveney's walk-in centres in the expectation of getting a jab.
The large scale vaccination centres across Norfolk and Waveney are run by Cambridgeshire Community Services NHS Trust.
The trust's medical director Dr David Vickers said: "People eligible for a booster - those aged over 40 and those at higher risk from Covid-19 - will be able to access their booster vaccination three months (91 days) after their second dose from Saturday, December 11, via our large scale vaccination centres, subject to the national protocol being available.
"This decision will enable thousands more people to get protected earlier.
“To ensure as many eligible people as possible can access these boosters in a planned and managed way, all boosters should be booked via the National Booking Service at www.nhs.uk/covidvaccine.
"Our walk-in services for boosters will close with effect from midnight on Friday, December 10 and until then will only deliver boosters on a walk-in basis for those eligible people who have had their second vaccination six months ago.
“All other eligible groups can continue to access their first, second and third vaccinations via our walk-in services.
“We are actively finalising arrangements to extend opening hours in some of our centres to deliver even more capacity and we will share this information in the coming days.”
Those aged 18 to 39 will also be eligible for a booster when the NHS calls them forward.
People will be notified by age group, with the older and more clinically vulnerable receiving their jabs first.
Dr Vickers added: "This is the biggest change in eligibility since the programme was launched.
"We strongly encourage people who are eligible to come forward for their booster or vaccination.
"The vast majority of those being hospitalised with serious illness from Covid-19 are those who have not been vaccinated.
“Vaccines are the best way to protect yourselves and your loved ones and reducing Covid-related hospital admissions will help the NHS cope better during the winter months and reduce disruption to routine and planned care.”
Where are the large scale vaccination centres?
Large scale vaccination centres are open seven days a week (other than Dereham) for walk-in clinics at the following locations.
Paddock Road Surgery, Paddock Road, Harleston
Connaught Hall, Station Road, Attleborough
King's Lynn Arts Centre, King Street, Kings Lynn
Castle Quarter Shopping Centre, Castle Meadow, Norwich
Beetley Ward, Dereham Hospital, Northgate, Dereham (open Thursdays, Fridays, Saturdays and Sundays only)
But, from Saturday (December 11) it will not be possible to access booster vaccinations on a walk-in basis.
Who is eligible to get a booster jab by appointment only from Saturday, December 11, three months after their second jab?
People aged 40 and over
People who live and work in care homes
Frontline health and social care workers
People aged 16 and over with a health condition that puts them at high risk of getting seriously ill from Covid-19
People aged 16 and over who are a main carer for someone at high risk from Covid-19
People aged 16 and over who live with someone who is more likely to get infections (such as someone who has HIV, has had a transplant or is having certain treatments for cancer, lupus or rheumatoid arthritis)
People who are pregnant and in one of the eligible groups can also get a booster dose.
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