UPDATE 1-Britons can be confident in vaccine monitoring, health secretary says

(Adds further quotes on rollout)

LONDON, April 8 (Reuters) - Britons should be confident that the system to monitor COVID-19 vaccines is working following the change in advice on giving young people the Oxford-AstraZeneca jab, Health Secretary Matt Hancock said on Thursday.

"People can be reassured that we have the high class safety system run by our world class regulator (...) and then we're totally transparent with all of the side effects, no matter how extremely rare they are like these ones," he told Sky News.

Britain's vaccine advisory committee said on Wednesday that an alternative to Oxford-AstraZeneca's vaccine should be given to under 30s where possible due to a "vanishingly" rare side effect of blood clots in the brain.

Hancock said new guidance would not delay Britain's vaccination programme because alternatives from Pfizer and Moderna would be available for the affected group.

Britain is aiming to give a first dose of COVID-19 vaccine all over-50s by mid-April and all adults by the end of July.

Hancock said there were 10.16 million people aged between 18 to 29, of whom 1.6 million had already had a first dose of vaccine.

"We have more than enough Pfizer and Moderna vaccines to cover all of the remaining eight and a half million people," he said.

"We are on track to hit the target that we've set that we will ensure every adult in the UK is offered the jab by the end of July." (Reporting by Paul Sandle and Sarah Young; editing by Costas Pitas and Alistair Smout)