The gap between the first and second dose of the Oxford-AstraZeneca coronavirus vaccine will be increased from six weeks to 10 for people in Guernsey.
It means anyone who has an appointment to receive their second dose of the vaccine will need to reschedule.
The States said lengthening the gap between doses provided more protection.
President of health and social care, Deputy Al Brouard, said the evidence behind the decision was clear.
"New scientific evidence has emerged in respect of how we can ensure that the vaccines administered are as effective as possible," he said.
"Significantly, there is now clear evidence that by leaving a longer gap between doses of the AstraZeneca vaccine, the second dose is more effective, providing more protection to the individual."
In the UK, both parts of the Oxford and Pfizer vaccines are being given 12 weeks apart.
In Guernsey the increased gap only applies to the Oxford vaccine and the States said anyone in the 70-79 age bracket who had been booked in for a vaccine would be written to.