The NHS has been given the green light to start planning a Covid vaccine booster programme in the UK ahead of this winter.
A bigger flu season than normal is expected, meaning extra protection against Covid is likely to be needed.
More than 30 million of the most vulnerable should receive a third dose, vaccine experts are advising.
They will include all adults aged 50 and over, and anyone over 16 who qualifies for a flu jab.
Health service bosses had previously said they needed lots of warning of an autumn Covid-19 booster rollout in order to plan the logistics alongside vaccinating millions of people against flu.
Interim advice from the Joint Committee on Vaccination and Immunisation (JCVI) is that boosters will help maintain protection against Covid-19 and new variants for those most at risk, before winter comes.
The vaccines are thought to protect most people against serious illness for at least six months, but a lack of data on exactly how long immunity lasts is prompting a safety-first approach.
No decisions have yet been made on which vaccines will be used.
Winter flu comeback
Prof Jonathan Van-Tam, deputy chief medical officer for England, said: "We want to be on the front foot for Covid-19 booster vaccination to keep the probability of loss of vaccine protection, due to waning immunity or variants, as low as possible - especially over the coming autumn and winter."
He said other respiratory viruses, particularly flu, "will make a comeback" and be an additional problem this winter.
"We will need to ensure protection against flu, as well as maintaining protection against Covid-19," Prof Van-Tam said.
Health and Social Care Secretary Sajid Javid said ministers were working with the NHS to rapidly deliver the programme from September.
"Our first Covid-19 vaccination programme is restoring freedom in this country, and our booster programme will protect this freedom," he said.
Scotland's health secretary Humza Yousaf said the government had been working closely with NHS boards to plan for the booster campaign, while the current vaccination programme "continues at pace and remains on schedule".
Wales' Health Minister Eluned Morgan said she was working with Welsh health boards to ensure they can deliver a booster programme "from the start of September".
"In line with the other nations of the UK, the Welsh government welcomes the JCVI advice," she said. "It very much aligns with our thinking and our planning assumptions to date."
The JCVI's final advice will be published before September, when better data will be available on how long protection from the first two doses of the vaccines lasts. The latest figures on hospitalisations, emerging variants and trials will also be taken into account at that point, and could change their advice.
Who could get a third dose?
In the meantime, the JCVI's advice is to offer a third Covid jab (and a flu jab) to the following people from September 2021:
- adults aged 16 and over who are immunosuppressed or clinically extremely vulnerable
- residents in care homes for older adults
- all adults aged 70 and over
- frontline health and social care workers
After those groups, it will be:
- all adults aged 50 and over
- adults aged 16-49 who are in a flu or Covid-19 at-risk group
- those living in the same house as people who are immunosuppressed
Prof Wei Shen Lim, Covid-19 chair for JCVI, said all these groups would also be eligible for the annual flu vaccine and were strongly advised to have it.
Younger adults will be not be given a third dose, because they will only have had their second dose in the summer, although this decision will be revisited at a later time, the JCVI said.