While two doses are required to fully protect against COVID-19, both vaccines provide high levels of protection after the first dose, the committee said.
A batch of 50,000 doses has been allocated and those aged over 80 will be prioritised initially.
Britain is prioritising getting a first dose of a vaccine to as many people as possible over giving second doses, despite some doctors and scientists expressing concern.
In a shift from practices in the U.S. and elsewhere, the United Kingdom plans to give people second doses of both vaccines within 12 weeks of the first shot rather than within 21 days, to accelerate immunisations across populations as quickly as possible.
Northern Ireland is in the second week of a six-week lockdown in which non-essential retail is closed.
Brian Pinker, a retired maintenance manager from Oxford in south central England, received the jab at the city's Churchill Hospital, the National Health Service said.
Johnson said on Sunday that tougher restrictions were likely to be introduced, even with millions of citizens already living under the strictest tier of rules.
The latest virus moves are aimed at containing a severe wave of infections with a new coronavirus strain believed to spread faster.
Pinker said he was looking forward to celebrating his 48th wedding anniversary with wife Shirley in February.
India, with the world's second-largest number of known cases, approved the emergency use of two Covid-19 vaccines on Sunday, paving the way for one of the world's biggest inoculation drives.
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Health Secretary Matt Hancock said the Oxford-AstraZeneca jab is a "testament to British science" and "a pivotal moment in our fight against this terrible virus".
"With most of the country already under extreme measures, it's clear that we need to do more together to bring this new variant under control", he said.
The new lockdown announced by Johnson emulates the first national curbs in place from March to June, and goes further than another instituted in November, when schools were allowed to stay open.
Elsewhere, France's cautious approach appears to have backfired, leaving just a few hundred people vaccinated after the first week and rekindling anger over the government's handling of the pandemic.
As of Monday, the Centers for Disease Control and Prevention reported that almost 4.6 million shots have been dispensed in the USA, after a slow and uneven start to the campaign, marked by a confusion, a multitude of logistical hurdle and a patchwork of approaches by state and local governments. In particular, it doesn't require the super-cold storage needed for the Pfizer vaccine.
Johnson's government earlier touted a scientific "triumph" as Britain became the first country in the world to start inoculating people with shots of the Oxford/AstraZeneca vaccine.
Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said policymakers are being forced to balance the potential risks of this change against the benefits in the middle of a deadly pandemic. "When resources of doses and people to vaccinate are limited, then vaccinating more people with potentially less efficacy is demonstrably better than a fuller efficacy in only half".
"People should be in no doubt that the government will do everything that's necessary", he said, "but I must stress at this critical moment it is so vital that people keep disciplined".
Unions representing teachers have called for schools to turn to remote learning for at least a couple of weeks more due to the variant, which officials have said is up to 70 per cent more contagious.
Andrew Pollard, the director of the Oxford Vaccine Group that developed the vaccine, was among the first to receive it on Monday.