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COVID-19 vaccine boosters improve protection to as much as 75% against a rapidly spreading omicron variant that’s much more likely to bypass two doses than earlier strains, preliminary U.K. data show.

The basic course of shots from AstraZeneca PLC and the Pfizer Inc.-BioNTech SE partnership provided much lower defenses against symptomatic infection with omicron, compared with the delta strain, the initial study showed. A booster lifted protection to 70% to 75% in the early days after the shot.

“These early estimates should be treated with caution, but they indicate that a few months after the second jab, there is a greater risk of catching the omicron variant,” Mary Ramsay, head of immunization at the U.K. Health Security Agency, said in a statement.

The findings come as the U.K. accelerates its booster campaign in the face of the heavily mutated variant. New evidence shows the strain is growing much faster than delta, and U.K. health officials expect omicron to become the dominant variant by the middle of December, accounting for more than half of new cases.

Effectiveness against severe disease is still unknown but expected to be higher than against symptomatic illness alone, the government said Friday.

The U.K. has relied largely on the Pfizer vaccine for boosters, complemented by Moderna Inc.’s shot when needed. Many people in Britain got the AstraZeneca vaccine for their first two doses.

The analysis looked at 581 people with confirmed omicron, and health authorities said the figures should be interpreted with caution until more cases have been studied.

The U.K. has moved to reimpose some measures, including indoor mask-wearing and work-from-home guidance as omicron proliferates. The new strain may be spreading faster in England than in South Africa and U.K. cases of the variant could top 60,000 a day by Christmas, according to epidemiologist John Edmunds.

Other early indications of vaccine effectiveness against omicron have given a mixed picture, with Pfizer and BioNTech saying initial lab studies show a third dose may be needed to neutralize it. Researchers in South Africa have also found a drop-off in the level of antibody protection from that vaccine versus the new strain, though so-called T-cells may still offer an immune defense against severe disease.

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