The NHS has been given the green light to plan a coronavirus booster jab programme.
The third vaccine will be administered ahead of the UK's winter, with the coronavirus expected to be a seasonal infection, alongside flu.
More than 30 million of the most vulnerable individuals – all adults over 50 and anyone who qualifies for a free flu vaccine – should receive a third coronavirus jab dose from September, according to the Joint Committee on Vaccination and Immunisation (JCVI).
An autumn vaccine roll-out should help to maintain protection against COVID-19 – the disease caused by the coronavirus – and new variants, the JCVI states in its interim advice.
The jabs are thought to be effective for at least six months, however, the extent of protection beyond that is unclear. No decision has been made on which of the UK's four approved vaccines will be used as a booster.
Watch: Over 50s to get coronavirus booster jab in autumn
"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," said Professor Jonathan Van-Tam, England's deputy chief medical officer.
Lockdown meant the last flu season was unusually mild, however, Professor Van-Tam expects the infection will "make a comeback".
In September 2020, a Public Health England analysis revealed people who caught both flu and the coronavirus were more than twice as likely to die as someone with just the latter infection.
Read more: Man dies from blood clot after Moderna jab
"We will need to ensure protection against flu, as well as maintaining protection against COVID-19," said Professor Van-Tam.
The JCVI has stressed its advice is interim and could change before September, when the final recommendation will be published.
Better data on how long vaccine-induced protection lasts are expected in the next few months, along with the latest information on coronavirus hospitalisations and the impact of new variants.
A recent study suggests the Pfizer-BioNTech and Moderna vaccines could protect against the coronavirus for years.
The results are a "good sign for how durable our immunity is" post-jab, said lead author Dr Ali Ellebedy, from Washington University in St. Louis.
A separate study found a third Oxford-AstraZeneca dose leads to a "strong" immune boost against the coronavirus and its variants.
Nevertheless, study author Professor Teresa Lambe – from the University of Oxford – said "it is not known if booster jabs will be needed".
Speaking of the JCVI's interim recommendation, Dr David Elliman – a consultant in community child health – said: "The advice seems very sensible if we are going to offer boosters to all adults, however, that begs the question as to whether they are needed and, if so, should they be given."
Younger adults will not be given a third dose due to them having had their second vaccine relatively recently. The JCVI has stressed this decision will be revisited nearer the time, however.
"At the moment there is little clinical or laboratory evidence that boosters are needed and JCVI has understandably not committed itself yet," said Dr Elliman.
"If a booster is needed, exactly how much benefit would it give and would it justify the resources?"
A booster programme may also have moral implications.
“When, in large parts of the world, there is not enough vaccine to give one dose to the adult population, is it right we should be using up vaccine in this way?," said Dr Elliman.
Nevertheless, he added: "There are some people who are at particular risk, where the addition of a booster may make a substantial difference, particularly those who have major problems with their immune system and some occupations."
Watch: Do coronavirus vaccines affect fertility?