The effectiveness of the first two doses of the coronavirus vaccine is waning, and it's killing elderly and vulnerable people who are double-vaccinated, a chief medical adviser has said.
Dr Susan Hopkins at the UK Health Security Agency echoed Health Secretary Sajid Javid and urged more people to come forward for their booster shot.
The doctor's comments come as the Health Secretary said that "if we all come together and play our part" then the country can "avoid a return to restrictions" this Christmas.
Speaking about the mortality rates relating to Covid-19 on the BBC's Andrew Marr Show, Dr Hopkins said: "The people who are dying are the same people who have died all the way through.
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"It is particularly the older age groups, so the over-70s in particular, but also those who are clinically vulnerable, extremely vulnerable, and have underlying medical conditions."
She added: "As we've mentioned, the immune effects wane and what we see is, especially in the older or the vulnerable groups, those are the people whose immunity will wane the most.
"So, if you're a healthy 30-year-old, then two doses will protect you for a longer period. That's why those people need to come forward for their third dose as soon as possible."
She echoed what Mr Javid said: "We know immunity begins to wane after six months, especially for the elderly and the vulnerable, and booster vaccines will top-up their protection to keep people safe over the winter."
The doctor said this waning protection from the jabs is potentially lethal, especially over the winter months.
She added: "We're still seeing deaths in mainly the unvaccinated population ... but increasingly, because of immune waning effects, there are deaths in the vaccinated group as well."
Outlining the uptake of boosters so far, Dr Hopkins said: "It's been quite good. There's over 60 per cent of the population that are being offered boosters [who] are taking it up.
"I think it's slower than we saw in the first round.
"I think that may be due to people thinking they're already protected, which is why we're giving a lot of public health messages about why it's so important for them to come forward for that third dose."
So far, almost 10 million people in the UK have received a top-up jab, but the Department of Health and Social Care said that around 30 per cent of over-80s and 40 per cent of over 50s in England are still yet to receive the booster shot.
People over 50 and those most at risk from Covid-19 are eligible for a booster six months after their second jab, with 3 million more people in England being invited to have their booster jabs next week.
She added: 'We know that the virus is circulating at very high levels in our community. So unless people get vaccinated, we will have a long and difficult winter.'
