Having two doses of a Covid vaccine offers less defence against symptomatic infection from the Omicron variant than with Delta, experts have said, although a booster jab raises protection considerably.

The UK Health Security Agency (UKHSA) said Omicron was projected to become the dominant variant of coronavirus in the UK by mid-December, based on current trends.

It added that there could be more than a million coronavirus infections by the end of the month.

Dr Susan Hopkins, the UKHSA’s chief medical adviser, said: “I think what we’re seeing is that if you’ve had two doses more than three months ago, then it’s not going to prevent you from getting symptomatic disease.”

However, a Pfizer/BioNTech booster jab, given after an initial round of either Oxford/AstraZeneca or Pfizer, raised the level of protection, offering 70-75% protection against symptomatic infection.

The findings came as the UK reported 58,194 new cases of Covid-19 on Friday – the highest number of positive cases in a 24-hour period since 9 January – and 120 deaths. A total of 448 Omicron cases were also reported, compared with 249 on Thursday, with the total across the UK to date now standing at 1,265.

The UKHSA report offers early insights into the degree of protection the initial two vaccinations may offer against Omicron. The data suggests that people who have had two doses of the Oxford/AstraZeneca jab 25 or more weeks ago have far lower protection against symptomatic infection with Omicron than with Delta.

While the data suggests such individuals have about 40% protection against Delta at this time point, protection against symptomatic infection with Omicron could be less than 10%. However, there is a great deal of uncertainty around that figure given the small number of people studied and the fact that the Oxford/AstraZeneca jab was largely given to older or more vulnerable people.

A similar trend was seen for those who have had two doses of the Pfizer jab, with about 60% protection against Delta at 25 or more weeks since the second dose, and just under 40% protection against Omicron at the same time point – although, again, there are uncertainties around the figures.

About 44 million people received their second jab at least three months ago, but about 22 million have received a booster since then.

Experts have said Omicron infections are doubling every two to three days in the UK, leading to concerns that the variant could overwhelm the NHS if it is as virulent as Delta. In the UK, about 39% of those over 12 years old have had a booster dose, compared with 81% of people having had two doses.

“The booster is really adding to protection,” said Dr Mary Ramsay, the head of immunisation at the UKHSA.

While the effectiveness of the jabs against severe disease is still unknown, the team says it is expected to be higher, drawing parallels to the drop in vaccine effectiveness for Delta when compared with earlier variants.

“We did see this reduction in protection against milder disease with Delta way back in June. And what we didn’t see was any reduction in protection against hospitalisation,” said Ramsay.

This is important because experts have previously warned that a drop in effectiveness against severe infections from, for example, 96% to 92% could lead to double the number of people who are not protected against hospitalisation.

While some data from South Africa has suggested that infections with Omicron may be mild, Hopkins said it was too early to know whether the variant causes less severe disease than Delta.

The findings chime with a report by scientists on Friday of the first cluster of Omicron cases in a group of people who had all received booster doses, suggesting that even three doses does not always protect against symptomatic illness.

The group of seven German tourists in their 20s and 30s had recently visited South Africa and were subsequently found to have been infected with Omicron. All had mild to moderate symptoms and were not admitted to hospital, but the findings contrast slightly with more encouraging early laboratory results released by BioNTech and Pfizer this week.

Prof Wolfgang Preiser, of Stellenbosch University in South Africa and senior author of the report on the cluster of Omicron cases, said: “We regard any claim that three doses would protect against symptomatic infection as not supported by available evidence.

“But importantly the message is not that vaccination does not work – it just does not work as well as it used to against pre-Omicron viruses and an updated vaccine is desirable.”