The UKHSA said its analysis is not an assessment of hospital severity, as that will require more time.
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Unvaccinated people with Omicron are up to eight times more likely to end up in hospital than those who are boosted, data suggests, while 20 weeks after a second dose of AstraZeneca there is no effect against symptomatic infection with the variant.
The latest research shows the importance of coming forward for a third dose of vaccine, the UK Health Security Agency’s chief medical adviser Dr Susan Hopkins said.
The findings confirm vaccine effectiveness also wanes over the same period of time in Pfizer and Moderna jabs, down to 10%.
The UKHSA, which published the latest data on Friday, said their research shows that among people who had two doses of AstraZeneca, there was no effect against symptomatic infection with Omicron from 20 weeks after the second dose.
But the organisation said one dose of any vaccine was associated with a 35% reduced risk of hospital admission among symptomatic cases with Omicron.
Most people in the UK’s booster rollout have been offered either Pfizer or Moderna.
Around two to four weeks after a booster jab, vaccine effectiveness against symptomatic infection ranged from around 65-75%, down to 55-70% five to nine weeks on, and down to between 40 and 50% from 10 or more weeks after the third dose, the UKHSA said.
The organisation said the risk of hospital admission is lower for Omicron cases with symptomatic or asymptomatic infection after two and three doses of vaccine.
The body, working alongside Cambridge University MRC Biostatistics unit, said there was an 81% reduction in the risk of hospital admission after three doses compared to unvaccinated Omicron cases.
For the research, they analysed 528,176 Omicron cases and 573,012 Delta cases between November 22 and December 26 to assess the risk of hospital admission in England after people tested positive for Omicron.
They found that the risk of having to go to emergency care or be admitted to hospital with Omicron, for symptomatic or asymptomatic infection, was approximately half of that for Delta, and the risk of hospital admission alone with Omicron was around a third of that for Delta.
Health and Social Care Secretary Sajid Javid said: “This is more promising data which reinforces just how important vaccines are.
“They save lives and prevent serious illness.
“This analysis shows you are up to eight times more likely to end up in hospital as a result of Covid-19 if you are unvaccinated.
“It is never too late to come forward for your first dose and it’s vital that everyone comes forward to get boosted now as we head into the new year.”
When it comes to the risk of hospital admission among symptomatic cases with Omicron, the UKHSA said there was a 67% reduction up to 24 weeks after the second dose and a 51% reduced risk 25 or more weeks after the second dose.
A third dose was associated with a 68% reduced risk of hospital admission compared to similar unvaccinated people.
When the reduced risk of hospital admission was combined with vaccine effectiveness against symptomatic disease, the vaccine effectiveness against hospital admission was estimated as 52% after one dose, 72% between two and 24 weeks after the second dose, 52% 25 or more weeks after the second jab and 88% two weeks after a booster dose.
Dr Hopkins said: “The latest set of analysis is in keeping with the encouraging signs we have already seen.
“However, it remains too early to draw any definitive conclusions on hospital severity, and the increased transmissibility of Omicron and the rising cases in the over 60s population in England means it remains highly likely that there will be significant pressure on the NHS in coming weeks.
“The data once again shows that coming forward for your jab, particularly your third dose, is the best way of protecting yourself and others against infection and severe disease.”
The UKHSA said its analysis is not an assessment of hospital severity, as that will require more time.
