Millions of vulnerable people with underlying health conditions, such as cancer, diabetes and heart disease, are being urged to book their Covid jabs.
Charities including Cancer Research UK, Mencap and the Terrence Higgins Trust have written an open letter to those in priority group six.
They are trying to reassure people that vaccines are safe and effective.
The Department of Health said more than half of the seven million people in that group have had their first dose.
Priority group six includes those aged 16 to 64 with an underlying health condition, their carers, and people with learning disabilities.
Underlying health conditions include chronic respiratory, heart, kidney and liver disease and neurological conditions, immunosuppression, asplenia, diabetes, morbid obesity and severe mental illness.
People with sickle cell disease, lupus and those on a GP learning disability register, as well as people who have vascular disease or have had a stroke are also included in group six.
Those with learning disabilities have been found to be up to six times more likely to die from Covid-19.
The NHS will send about two million text messages in England over the weekend asking those in group six to book an appointment at a vaccination centre or pharmacy.
The 18 charities said in the letter: "The good news is, even if you have not received your letter yet, you can now get an appointment at a vaccination centre or community pharmacy, quickly and easily, by using the national booking system or calling 119 in England."
The charities, which include the African Caribbean Leukaemia Trust, Carers UK and Diabetes UK, urged anyone with concerns to discuss them over the phone when booking or with a health professional at their appointment.
Prof Stephen Powis, national medical director of NHS England, urged people who were "hesitant" about getting the jab to access information and get vaccinated.
"By getting vaccinated you're not just protecting yourself, you'll also be able to protect your loved ones because there's evidence now that transmission may be reduced by getting a jab," he told BBC Breakfast.
Health Secretary Matt Hancock said the backing of the charities was "invaluable".
Vaccines minister Nadhim Zahawi said the campaign should help "reassure" those with any concerns about the vaccine.
According to latest government figures, more than 23.6 million people have received their first dose of a coronavirus vaccine in the UK.
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