Boris Johnson has warned that it is “too early” to say if England’s coronavirus lockdown will end in the spring, as a leading scientific adviser said it would be unwise to consider reopening pubs and restaurants until May.

Amid signs that severe restrictions could drag into the summer, it was confirmed that June’s Glastonbury festival — the largest arts event of its kind in the world — would be cancelled for the second year in a row.

Although Gavin Williamson, education secretary, said he would “certainly hope” that schools in England would reopen before Easter, Downing Street declined to predict that this would definitely happen.

The prime minister said the government would look at “how we’re doing” when the four top priority groups had been vaccinated. Ministers believe they are on course to complete that task by mid-February.

Scientific advisers are warning Mr Johnson not to repeat the mistake of opening up too early and the prime minister has told colleagues he wants to make sure that this is “the last lockdown”.

Experts on the government’s Scientific Pandemic Influenza Group on Modelling (Spi-M) found that lifting lockdown measures entirely from April might lead to a “huge wave of infections” and between 3,000 and 6,000 deaths, depending on the level of protection the vaccine confers.

“The key here is a slow relaxation,” said Matt Keeling, professor of populations and disease at the University of Warwick, and a member of Spi-M. He added that many decisions would need to be reactive and would very much depend on substantially reducing cases and the pressure on hospitals over the coming months.

Marc Baguelin, a lecturer in infectious disease modelling at Imperial College London and also a member of Spi-M, said he thought it was unwise to consider opening bars and restaurants until May.

The modellers made the point that, under the assumption that the vaccine was only 90 per cent effective, at least 1m of the 15m most vulnerable people would still be at risk of being infected. And they added that an “optimistic” assessment was that take-up of the vaccine would be 85 per cent, leaving even more people unprotected.

Many Conservative MPs want the economy to reopen once priority groups have been vaccinated.

On Wednesday Patrick Vallance, chief scientific adviser, urged Mr Johnson not to lift the lockdown too early. “It’s worth remembering Albert Einstein’s definition of insanity — doing the same thing over and over again and expecting a different outcome,” he said.

Britain’s coronavirus vaccine programme continues to offer some hope to ministers; almost 5m people have now been given a first jab. A total of 363,508 doses were administered on Wednesday, the highest daily figure to date. However the UK recorded another 1,290 deaths.

There was also some more positive health data, with evidence that Covid-19 case rates had declined and there is “some indication” thathospital and intensive care unit admissions began to stabilise in the second week of January.

Case rates per 100,000 have fallen across all regions and age groups, according to the latest national surveillance data from Public Health England.

London continues to be worst-affected area, with a rate of 629.7 between January 11 and 17, but the capital has also seen the biggest fall in case rates: down from 935.1 per 100,000 the week before.

And amid concerns that many are failing to self isolate due to the potential financial impact of missing work, reports emerged on Thursday that the government is considering overhauling the support system.

According to a document seen by the Guardian, one proposal being discussed is a payment of £500 to anyone who tests positive for coronavirus regardless of their ability to work from home or employment status. Another option was a payment to anyone on less than £26,495.

Currently those eligible for discretionary payments or the test and trace support payment receive a £500 lump sum via their local town hall in addition to benefits and sick pay.

A health department spokesperson said local authority costs for administering test and trace support payments were covered by central government, and each council was empowered to make discretionary payments outside the scheme. “£50m was invested when the scheme launched, and we are providing a further £20m to help support people on low incomes who need to self-isolate.”