Having two doses of a Covid vaccine offers less defense against symptomatic infection of the Omicron variant than with Delta, experts said, although a booster vaccine greatly increases protection.
The British Health Safety Agency (UKHSA) has said Omicron is expected to become the dominant variant of the coronavirus in the UK by mid-December, based on current trends.
He added that there could be more than a million coronavirus infections by the end of the month.
UKHSA Chief Medical Advisor Dr Susan Hopkins said: “I think what we are seeing is if you had two doses over three months ago that won’t stop you from contracting a symptomatic disease. ”
However, a Pfizer / BioNTech booster shot, administered after an initial set of Oxford / AstraZeneca or Pfizer, increased the level of protection, providing 70-75% protection against symptomatic infections.
The results 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 January 9 – and 120 deaths. A total of 448 Omicron cases have also been reported, up from 249 on Thursday, with the UK total so far now standing at 1,265.
The UKHSA report offers insight into the degree of protection that the two initial vaccinations may offer against Omicron. Data suggests that people who received two doses of the Oxford / AstraZeneca jab 25 weeks or more ago have much lower protection against symptomatic infections with Omicron than with Delta.
While the data suggests that these people have about 40% protection against Delta at this point, the 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 has been granted largely to older or more vulnerable people.
A similar trend was seen for those who received two doses of the Pfizer vaccine, with around 60% protection against Delta at 25 weeks or more since the second dose, and just under 40% protection against Omicron at the same time – although, again, there are uncertainties around the numbers.
About 44 million people received their second injection at least three months ago, but around 22 million have received a booster since then.
Experts have said Omicron infections are doubling every two to three days in the UK, raising fears the variant will overwhelm the NHS if it is as virulent as Delta. In the UK, around 39% of people over 12 received a booster dose, compared to 81% of people who received two doses.
“The booster really adds to the protection,” said Dr Mary Ramsay, UKHSA immunization manager.
While the effectiveness of jabs against serious illness is still unknown, the team says it is expected to be higher, drawing parallels to the vaccine’s declining effectiveness for Delta compared to previous variants.
“We saw this reduction in protection against milder illnesses with Delta in June. And what we haven’t seen is a reduction in protection against hospitalization, ”Ramsay said.
This is important because experts have already warned that a drop in effectiveness against serious infections of, say, 96% to 92% could lead to a doubling of the number of people who are not protected from hospitalization.
While some data from South Africa suggests Omicron infections may be mild, Hopkins said it is too early to know if the variant causes less severe disease than Delta.
The results match 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 don’t always protect against symptomatic illnesses.
The group of seven German tourists in their 20s and 30s had recently traveled to South Africa and were subsequently found to be infected with Omicron. All had mild to moderate symptoms and were not admitted to hospital, but the results contrast slightly with the more encouraging early lab results released by BioNTech and Pfizer this week.
Professor Wolfgang Preiser, Stellenbosch University in South Africa and lead author of the Omicron Case Group report, said: “We consider any claim that three doses protect against symptomatic infection is unsubstantiated. by the available evidence.
“Most importantly, the message isn’t that vaccination doesn’t work – it just doesn’t work as well as it did before against pre-Omicron viruses and an updated vaccine is desirable.”