A new COVID variant dubbed Eris could be aiding the spread of the disease across the UK.
Experts have said the apparent recent increase in COVID cases is no immediate cause for alarm because the number of total cases remains small - but that people should remain vigilant about the virus.
Bad weather encouraging people to spend more time indoors and waning immunity have been blamed for the rise - though wider immunity to severe disease remains strong.
However, any specific attribution to the so-called Barbieheimer phenomenon - which has sparked a rise in cinema attendance - is likely to be "pretty far-fetched", one expert said.
Eris - scientifically called EG.5.1 - has been declared a variant by the UK Health Security Agency (UKHSA), which has found that 5.4% of tests on people with a respiratory illness in the last week were found to be COVID - up from 3.7% the week before.
The overall COVID-19 hospital admission rate has risen to 1.97 per 100,000 population, up from 1.17 in the previous week.
But despite that, ICU admission rates have decreased to 0.05 compared to 0.07, the UKHSA said.
The increase in COVID cases is likely to be at least in part be down to EG.5.1 - a relative of the omicron strain, which currently accounts for 12% of sequences in the UK.
Prof. Christina Pagel, of University College London, tweeted that while the UK was now "definitely" in a new wave, it was unlikely to be as large as previous ones. She added that Eris would likely be dominant by September, when the UK may see a bigger surge as people return from holidays, schools reopen, and workers head back to the office.
Prof Pagel added: "We could of course spend the summer improving indoor air quality in schools, public buildings, workplaces, restaurants, bars, etc which has all sorts of benefits beyond Covid + plan a broader winter booster campaign, but sadly the govt seems to have given up."
Professor Lawrence Young, a virologist at Warwick University, told MailOnline: "Poor weather over the last month has meant more indoor mixing including during various events such as university degree congregations and increased cinema attendance."
Professor Hunter said that EG.5.1 was 'likely to become dominant at some point and push up total infections but probably not that dramatically'.
One in seven coronavirus cases in the country is now down to Eris, which was first raised as a concern in July by the UKHSA and is one of the variants being monitored by the World Health Organization (WHO).
The elderly continue to have the highest hospital admission rates, UKHSA data showed, with admission rates for those 85 and over increasing to 20.49 per 100,000 from 9.8 per 100,000, while rates among those aged 75 to 84 years increased to 9.45 per 100,000 from 5.54 in the previous report.
Prof Francois Balloux, Professor of Computational Systems Biology and Director at UCL, said people should not be alarmed by the increase.
“We are likely facing a small summer COVID surge, with the recent poor weather in the UK probably playing a role," he said. "Speculations that have been aired that the current increases in case numbers may be fuelled by the box-office success of the Oppenheimer and Barbie movies strike me as pretty farfetched.
“The EG.5.1 variant plays only a minor role in the current increase in cases, which is driven primarily by the weather and constantly waning immunity. There is nothing that feels particularly concerning about EG.5.1, relative to the many other Omicron sub-variants in circulation."
Dr Mary Ramsay, Head of Immunisation at the UK Health Security Agency (UKHSA), said: "We continue to see a rise in COVID-19 cases in this week’s report. We have also seen a small rise in hospital admission rates in most age groups, particularly among the elderly.
"Overall levels of admission still remain extremely low and we are not currently seeing a similar increase in ICU admissions. We will continue to monitor these rates closely."
She added: "The NHS will be in contact in autumn 2023 when the seasonal vaccine is available for those who are eligible due to health conditions or age, and we urge everyone who is offered to take up the vaccine when offered," she added.
Are Covid vaccines currently available and who is entitled to them?
According to the NHS website, it is not currently possible to book a COVID-19 vaccination online or over the phone.
A message on the site says: "For most people, vaccinations are only available seasonally. This service will open when seasonal vaccinations are available."
Information on the "Seasonal COVID-19 vaccine" says: "You may be able to get a seasonal COVID-19 vaccine in autumn 2023 if you’re at increased risk of getting seriously ill from COVID-19. For example, this may be due to a health condition or your age.
It says the NHS will contact people whose records suggest they may be eligible.
For those who are eligible, when the seasonal vaccine is available they can get it by booking online, by going to a walk-in COVID-19 vaccination site, by going to their GP, or through their care home.
In January, the Joint Committee on Vaccination and Immunisation (JCVI) said its interim advice to the government was that plans should be made for those at higher risk of catching severe Covid-19 to be offered a booster vaccination in the autumn.
It said "emergency surge vaccine responses" may be needed if a "novel variant of concern" emerged with "clinically significant biological differences compared to the Omicron variant".
At the time, Professor Wei Shen Lim, Chair of COVID-19 vaccination on the JCVI, said: "The COVID-19 vaccination programme continues to reduce severe disease across the population, while helping to protect the NHS.
"That is why we have advised planning for further booster vaccines for persons at higher risk of serious illness through an autumn booster programme later this year."