Around a million text messages are being sent to 38 and 39-year-olds in England, inviting them to book an appointment for a Covid vaccine.
Booking will open from 07:00 on Thursday on the NHS website.
Pregnant women will be directed to centres offering Pfizer and Moderna vaccines, NHS England says.
Nearly three-quarters of people in their 40s have had one dose, while a third of all adults have had two doses.
In line with guidance from the UK’s vaccines committee last week, NHS England said people 39 and under without an underlying health condition would be offered the Pfizer or Moderna vaccine, as an alternative to AstraZeneca.
The decision came after a review found younger people had a slightly higher risk of an extremely rare blood clot after receiving a first dose of the AZ vaccine.
The Joint Committee on Vaccination and Immunisation (JCVI) said adults aged 18-39 could be offered an alternative “if available, and if it does not cause delays in having the vaccine”.
The committee advised that pregnant women should be offered the Pfizer or Moderna vaccines where available, at the same time as other people their age get one, because data on these jabs had not raised any safety concerns.
“Matt Hancock urge everybody to get the vaccine as soon as they are eligible to protect yourself and your loved ones.”
But pregnant women have complained that booking an appointment to get those vaccines was a struggle.
There is currently a lack of data on the AstraZeneca vaccine in pregnancy – but this could change in the future.
Health Secretary Matt Hancock said: “I urge everybody to get the vaccine as soon as they are eligible to protect yourself and your loved ones.”
People can book their appointment at a vaccination centre, pharmacy or GP site available through the national booking service.
Text invitations will appear as an alert from ‘NHSvaccine’ and include a link to the NHS website.
Those who cannot go online can call the service on 119 to book a jab.
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