Video report by ITV News Science Editor Tom Clarke
Over-45s are now being offered the Covid vaccine after the government announced that all over-50s in the UK have been offered the first dose.
The NHS Digital website - which is taking bookings for over-45s - crashed on Tuesday morning after more people were told they can book their vaccines. Users were met with an error message stating the site was experiencing "technical difficulties". It was back online a short while later.
It comes after Prime Minister Boris Johnson hailed a “hugely significant milestone” in the vaccination rollout. The UK met its target of offering the jab to all of its top nine priority groups - those aged over-50, the clinically vulnerable and frontline health and social care workers - three days ahead of its target date of April 15.
The government will now begin phase two of the programme, which targets adults aged 40-49.
The next group after over-40s will be adults aged 30-39, and then all adults aged 18-29.
The government has pledged that all UK adults will be offered their first dose by the end of July.
Almost 40 million doses of the Pfizer and AstraZeneca vaccines have been delivered since the rollout began in the UK in December - with 32 million first jabs and more than seven million second doses.
The introduction of the Moderna jab will be key in phase two of the vaccination programme.
Under 30s will not be offered the AstraZeneca vaccine and will likely be offered the Pfizer or Moderna vaccine.
It comes after the government announced earlier in April that the AstraZeneca vaccine should not be administered to those under 30 amid concerns over rare blood clotting events.
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The government has also ordered 40 million doses of the Pfizer vaccine, which is still being rolled out. The number of doses is enough to fully vaccinate nearly one-third of the UK population.
Stephen Powis, national medical director at NHS England, said the NHS will be using the Moderna vaccine in 21 of its larger vaccine centres, but as more supply comes, it will be rolled out to more centres.
The vaccine can be easily used in many more sites than the Pfizer one as it can be stored in a normal freezer, Prof Powis said.
He said: "Now that we have three jabs, that means a futher big milestone in the programme, that means we are on track to be able to offer the vaccine to all adults by the summer."
He praised the NHS' "largest ever and most successful ever" vaccination programme, saying 95% of all over-50s took the jab.
Addressing concerns about the AstraZeneca vaccine, he said: "Our regulators take safety very seriously, and they have looked carefully at this and of course recommended that the benefit of the vaccine far far outweighs any side effects.
"These are very very rare effects."
Coronavirus restrictions in England also eased further on Monday, with outdoor restaurants and pubs, non-essential shops, gyms, hairdressers among the businesses allowed to reopen.
Queues were seen outside high street shops and hordes of people were seen flocking to London's Soho to enjoy pints and meals outdoors.
Prof Powis said: "It’s good news that infection rates are low that means hospitalisation rates are low and we’re able to relax some of the lockdown measures. But we’re not out of the woods yet, and it’s really important that we do this carefully."
Three more vaccines have been ordered but await approval for use in the UK:
Novavax – 60 million doses ordered and approval could be confirmed in April. The jab is being manufactured in Barnard Castle.
Johnson & Johnson - 30 million doses ordered, this jab could be crucial in speeding up the rollout because it only requires one injection. The vaccine has been approved for use in the US but awaits the green light in the UK.
Valneva – 60 million doses ordered, and the jab is to be made in Livingston, Scotland. Early results have been encouraging as the vaccine begins phase 3 trials, which is the final stage.