The UK's chief medical officers have defended the Covid vaccination plan, after criticism from a doctors' union.
The UK will give both parts of the Oxford and Pfizer vaccines 12 weeks apart, having initially planned to leave 21 days between the Pfizer jabs.
The British Medical Association said cancelling patients booked in for their second doses was "grossly unfair".
But the chief medical officers said getting more people vaccinated with the first jab "is much more preferable".
Pfizer has said it has tested the vaccine's efficacy only when the two vaccines were given up to 21 days apart.
But the chief medical officers said the "great majority" of initial protection came from the first jab.
"The second vaccine dose is likely to be very important for duration of protection, and at an appropriate dose interval may further increase vaccine efficacy," they said.
"In the short term, the additional increase of vaccine efficacy from the second dose is likely to be modest; the great majority of the initial protection from clinical disease is after the first dose of vaccine."
Source: BBC News, 1 January 2021