Hundreds of thousands of people are encouraged to bring forward their second dose of coronavirus vaccine.
It is part of an effort to get as many people fully vaccinated as possible before the restrictions are eased in England on 19 July.
Text messages are sent to 650,000 people telling them to make sure their second vaccine appointment is no more than eight weeks after their first.
Initially, vaccine doses were ordered at intervals of up to 12 weeks in order to increase immunity, while spreading a limited supply among several people.
But the Joint Committee on Vaccination and Immunization (JCVI) has since changed its position to say that people should aim to get their second dose between eight and 12 weeks later.
However, experts have said that there is “no gain” in bringing the interval between doses closer than that.
On Wednesday, Professor Anthony Harnden, Vice President of JCVI, told Times Radio: “It is clear from antibody T-cell studies that you get much lower response and memory response of poorer quality with the shorter interval – it is an interval of four weeks compared to an interval of eight to 12 weeks.
“And the actual efficacy studies of real data show that there is a lower vaccine efficacy against symptomatic disease at shorter intervals compared to longer intervals.
“Then we got the models to look at this, and in fact, the number of infections will increase if we reduce the dose.”
Sir Keith Willett, head of the COVID-19 vaccination program in England, said: “Thanks to the hard work of NHS staff and volunteers working up and down the country, almost two thirds of the population of England have already received two doses of the jab.
“The crucial second dose provides maximum protection against the virus and it will protect you, your family and those around you.
“As we approach July 19, there has never been a more important time to get jabbed, and it has never been easier – from pop-up places in the community to going to centers, NHS staff go to great lengths to ensure that vaccines are as convenient as possible.
“So I urge everyone to bring their second dose appointment up to eight weeks as soon as they can.”
Health Secretary Sajid Javid said: “Getting the important second jab will also allow us to return to doing the things we missed, such as going on holiday. Everyone should take the opportunity as soon as they can so we can all start enjoying the freedoms we have missed. “
Text invitations appear as a warning from the “NHS Vaccine” and include a web link to the NHS website to reserve an appointment.
People who cannot go online can instead call the 119 service to order their jab.