The criteria for eligibility has shifted this year to targeting those at the highest risk of serious disease, according to the UK Health Security Agency (UKHSA).
This means that the Covid-19 vaccine will be offered on the NHS to:
People outside these groups can pay for the vaccine privately if they choose.
Last year, the criteria also included adults aged 65 and over. The eligibility is the same across England, Scotland, Wales and Northern Ireland.
However, confusion has rippled through groups of patients who have managed to book an appointment online on the NHS booking system, despite not being eligible.
It is understood that the option to book both Covid and flu vaccines at the same time via the online system may have caused uncertainty.
There is a wider group of people available for the flu jab on the NHS, including:
Frontline health and social care workers can also get a flu vaccine through their employer.
The National Pharmacy Association (NPA) described the situation as "deeply frustrating" for both patients and pharmacists.
Henry Gregg, chief executive of the NPA, said: "We are aware of reports from across the country of patients who are not eligible on the NHS for this year's Covid vaccine arriving at pharmacies, having been able to book an appointment via the online system.
"This should have been avoidable and it is deeply frustrating for both pharmacies and patients. We urge any patient in this situation to treat pharmacy teams with respect as pharmacies try to manage the new NHS clinical criteria.
"We've urgently raised this issue with NHS England and the Department of Health and have asked for it be resolved as soon as possible."
An NHS spokesperson said: "Covid-19 vaccine eligibility changed for this autumn and winter following the JCVI recommendation that it should be offered to people aged 75 or over, anyone with a weakened immune system, or those living in older adult care homes.
"Please double check you are still eligible to receive a Covid-19 vaccine before booking - the NHS website sets out whether your age, health condition or medication means you are eligible, and your GP practice or pharmacist will confirm this before giving you the vaccine."