More than a million people in England will be sent invitations this week to book their coronavirus booster jab.
Texts will be received from Monday, while letters will be sent to those who are eligible later in the week, NHS England said.
Some 1.5 million people will be contacted and encouraged to use the National Booking Service.
Those eligible for boosters include anyone aged 50 and over, people living and working in care homes for the elderly, and frontline health and social care workers.
All those who are clinically extremely vulnerable and anyone aged 16 to 65 in an at-risk group for Covid (who were included in priority groups one to nine during the initial vaccine rollout) will also be eligible for a jab.
Issuing guidance earlier this week, the Joint Committee on Vaccination and Immunisation (JCVI) said people should receive their booster dose at least six months after they received their second coronavirus jab.
While there is a preference that people should get the Pfizer jab as a third dose, regardless of which jab they were initially given, the JCVI said half doses of the Moderna jab could be used as an alternative.
Health and Social Care Secretary Sajid Javid said boosters will “strengthen the wall of defence” created by the vaccines.
He said: “It is excellent that getting your booster jab has now become even easier thanks to the opening of the National Booking Service to those eligible.
“Booster doses are an important way of keeping the virus under control for the long term and will protect the most vulnerable through the winter months.
“I urge everyone who receives a letter or text to get their jab as soon as possible so we can strengthen the wall of defence across the country that each vaccine brings.”
Dr Nikki Kanani, GP and deputy lead for the Covid vaccination programme, said: “The NHS Covid vaccination programme has already prevented 24 million cases and saved more than 112,000 lives.
“As we head into winter we should not drop our guard so I would urge everyone to come forward and get a booster vaccination when they are invited.
“Getting a top-up vaccine is the best way to protect yourself and your loved ones from this cruel virus.”
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People who receive an invitation can book their appointment at www.nhs.uk/covid-vaccination, and those who cannot go online can instead call 119.
Millions more invitations are to be issued in the coming days and weeks, NHS England said.
The NHS officially launched its coronavirus booster campaign on Thursday, with hospital hubs having begun jabbing frontline health and care workers.
NHS England said local health teams will prioritise care home residents and staff who are eligible and offer a booster jab by the beginning of November.
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