People who have received a Covid-19 vaccine could still pass the virus on to others and should continue following lockdown rules, England's deputy chief medical officer has warned.
Prof Jonathan Van-Tam stressed that scientists "do not yet know the impact of the vaccine on transmission", reports BBC.
Writing in the Sunday Telegraph, he noted that the vaccines offer "hope" but infection rates must come down quickly.
Prof Van-Tam said "no vaccine has ever been" 100% effective, so there is no guaranteed protection.
It is possible to contract the virus in the two- to three-week period after receiving a jab, he said - and it is "better" to allow "at least three weeks" for an immune response to fully develop in older people.
"Even after you have had both doses of the vaccine you may still give Covid-19 to someone else and the chains of transmission will then continue," Prof Van-Tam said.
"If you change your behaviour you could still be spreading the virus, keeping the number of cases high and putting others at risk who also need their vaccine but are further down the queue."
This week, senior doctors called on health officials in England to cut the gap between the first and second doses of the Pfizer-BioNTech vaccine.
The maximum wait was extended from three to 12 weeks in order to get the first jab to more people across the UK.
But the British Medical Association said the policy was "difficult to justify" and the gap should be reduced to six weeks.
Its chair, Dr Chaand Nagpaul, told the BBC there were "growing concerns" that the vaccine could become less effective with doses 12 weeks apart.
Responding to the criticism, Prof Van-Tam said: "What none of these (who ask reasonable questions) will tell me is: who on the at-risk list should suffer slower access to their first dose so that someone else who's already had one dose (and therefore most of the protection) can get a second?"
More than 5.8 million people in the UK have received their first dose of a vaccine, according to the government's coronavirus dashboard.
They will bring the number of mass vaccination sites across England to 49 - as well as 70 pharmacies, more than 1,000 GP surgeries and 250 hospitals offering the jab.
Prof Van-Tam stressed that the UK needs to "bring the number of cases down as soon as we can whilst we vaccinate our most vulnerable".