Flu could be a "bigger problem" than coronavirus this winter, one of the government's senior vaccine advisers has warned.
Professor Anthony Harnden, deputy chair of the Joint Committee on Vaccination and Immunisation, said data is expected soon on whether vaccines protecting people against flu and COVID can be administered at the same time.
He said a lack of immunity to flu across the population in the past year could come "back to bite us".
He said: “I will emphasise that actually flu could be potentially a bigger problem this winter than COVID.
"Reactogenicity, or how [the vaccines] react with each other, and what sort of side-effect profile that they give when given together, is really important.
“We’ve had a very, very low prevalence of flu for the last few years, particularly virtually nil during lockdown, and we do know that when flu has been circulating in very low numbers, immunity drops in the population, and it comes back to bite us.
"So, flu is going be really, really important this winter.
"Flu is going to be really important to get vaccinated against.”
Last winter saw the lowest levels of flu in the UK in 130 years.
It is thought that flu infections were drastically reduced by lockdown combined with an expanded flu vaccine rollout that offered free jabs to all over-50s.
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Harnden's fear is that flu will return, and emphasised the need for the vulnerable to be protected against both flu and coronavirus.
The warning comes as experts told the government that the NHS needs time to plan for a simultaneous rollout of a flu vaccine and COVID booster jab.
Professor Martin Marshall, chairman of the Royal College of GPs, and Chris Hopson, chief executive of NHS Providers, said unanswered questions about the booster jab programme could make planning it more complex.
"Any changes – a mix and match approach to vaccines, or adding more vaccines to our supply roster, or combining flu and COVID jabs, or vaccinating children – will add significant complexity to the frontline delivery task," Hopson said.
"We wouldn’t be able to start a booster or re-vaccination campaign until these questions have been answered."
Asked about giving children vaccines against COVID-19, Harnden said: “We need to think very carefully what the benefits are to children themselves, and those would be the deliberations we’ll be making in the next few years.”
The government has said it is planning for a booster vaccination programme later this year, but added that final decisions on what the rollout will be like depend on the data from ongoing trials.
Matt Hancock told BBC Breakfast he is expecting the clinical data on what vaccine combinations are most effective to come "in the next few weeks".
He said: "When we know the results of that then we will set out the full plans of the booster jabs over the autumn."
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