Hundreds of thousands of people are being urged to bring forward their second coronavirus vaccine dose.
It is part of an effort to get as many people fully-vaccinated as possible before restrictions are eased in England on 19 July.
Text messages will be sent to 650,000 people telling them they should make sure their second vaccine appointment is no more than eight weeks after their first.
Initially vaccine doses had been booked up to 12 weeks apart, with the aim of boosting immunity while spreading a restricted supply among more people.
But the Joint Committee on Vaccination and Immunisation (JCVI) has since changed its stance to say people should aim to have their second dose between eight and 12 weeks later.
Experts have said, however, that there is "no gain" in bringing the interval between doses closer than that.
On Wednesday, Professor Anthony Harnden, deputy chair of the JCVI, told Times Radio: "It’s quite clear from antibody T-cells studies that you get much lower response, and poorer quality memory response with the shorter interval – that’s a four-week interval compared to an eight-to-12-week interval.
"And the actual real data vaccine effectiveness studies show that there is a lower vaccine efficacy against symptomatic disease with shorter intervals compared to longer intervals.
"Then we got the modellers to look at this and actually, the number of infections will rise if we reduce the dose."
Sir Keith Willett, head of the COVID-19 vaccination programme in England, said: "Thanks to the hard work of NHS staff and volunteers up and down the country, almost two thirds of people in England have already received two doses of the jab.
"The crucial second dose gives 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 sites in the community to walk in centres, NHS staff are going to great lengths to ensure vaccines are as convenient as possible.
"So I urge everyone to bring their second-dose appointment forward to eight weeks as soon as they can."
Health Secretary Sajid Javid said: "Getting that vital second jab will also allow us to return to doing the things we missed, such as going on holiday. Everyone should take up the opportunity as soon as they can so we can all start to enjoy the freedoms we’ve missed."
Text invitations appear as an alert from "NHSvaccine" and include a web link to the NHS website to reserve an appointment.
People who cannot go online can call the service on 119 instead to book their jab.