UK Prime Minister Boris Johnson has warned the country to prepare for more COVID-19 infections and deaths as it reopens its economy.

"The bulk of the work in reducing the disease has been done by the lockdown," Johnson said. "As we unlock, the result will inevitably be that we will see more infection [and] sadly we will see more hospitalizations and deaths."

He added: "If we are to get there in the way that we all want, people continue to be cautious and they continue to exercise restraint and just do the basic things to stop the spread of the virus – washing your hands, giving people plenty of space, doing things in fresh air."

The UK has also started offering vaccinations to all those aged over 45, after it met its target to offer vaccines to all adults aged over 50 by mid-April.

"We are very confident about our vaccine supplies. This was always going to be the second-dose month and people should come forward for their second doses," Johnson added.

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Linda Bauld, professor of public health at the University of Edinburgh, said 99 percent of those invited for their second dose have accepted it.

"The big question is, what happens when you get into younger adults with busy lives, who are not directly at risk?" she warned.

From Monday, shops, outdoor dining and gyms opened in England.

Central London's Soho area was packed with people finally coming out of a lockdown that has lasted more than three months. Long queues of shoppers formed outside large retail stores, ready to spend their savings. Last month, the Bank of England said that $186 billion had been saved by households since the start of lockdown.

The big concern now is new COVID-19 variants. The B1.351 strain first identified in South Africa has been detected in South London, with dozens of cases confirmed and widespread "surge testing" to try to contain it.

Studies suggest the Pfizer-BioNTech vaccine is less effective against the variant.

"The biggest threat is a new variant that is an 'escape mutant,' so it challenges our vaccine program," said Bauld. "All the vaccines are being further developed now to contribute to booster campaigns for the older and more vulnerable in the autumn but I fully expect we will see cases again."

Until then, the aim is to get as many people vaccinated as possible.

This week, the Moderna vaccine will begin to be used in the UK, alongside AstraZeneca's and Pfizer's jabs.

Despite having the fifth highest death toll for COVID-19 globally, the UK has developed one of the fastest vaccine roll-outs in the world.

Video editor: Nuno Fernandes