December 27, 2021
Booster shot protection against symptomatic COVID-19 caused by the Omicron variant appears to fade in about 10 weeks, according to new data from Britain.
U.K. health officials shared the data just before Christmas and noted that there haven't been enough severe cases of the Omicron variant to calculate how well boosters protect against severe disease. But they believe the extra shots provide significant protection against hospitalization and death.
"It will be a few weeks before effectiveness against severe disease with Omicron can be estimated," U.K. Health Security Agency officials wrote in the report. "However, based on experience with previous variants, this is likely to be substantially higher than the estimates against symptomatic disease."
Since countries began reporting Omicron cases in November, multiple studies have suggested the variant is better at escaping antibodies from vaccination and previous infection, according to The New York Times. The U.K. report adds to that, noting that both the initial vaccine series and booster doses were less effective and faded faster against the Omicron variant than the Delta variant.
Among those who received two doses of the AstraZeneca vaccine, a booster of the Pfizer or Moderna vaccine was 60% effective at preventing symptomatic disease 2 to 4 weeks after the shot. But after 10 weeks, the Pfizer booster was 35% effective, and the Moderna booster was 45% effective. (The AstraZeneca vaccine is not authorized in the U.S., but the Johnson & Johnson shot uses a similar technology, The New York Times reported.)
Among those who received three Pfizer doses, vaccine effectiveness was 70% about a week after the booster but dropped to 45% after 10 weeks. At the same time, those who received an initial two-dose series of the Pfizer vaccine and then a Moderna booster seemed to have 75% effectiveness up to 9 weeks.
The report was based on an analysis of 148,000 Delta cases and 68,000 Omicron cases in the U.K. through Dec. 20. So far, the U.K. health officials wrote, Omicron infections appear to be less severe and less likely to lead to hospitalization than Delta infections. At that time, 132 people with lab-confirmed Omicron had been admitted to hospitals, and 14 deaths had been reported among ages 52-96.
"This analysis is preliminary because of the small numbers of Omicron cases currently in hospital and the limited spread of Omicron into older age groups as yet," the report said.
The reinfection rate has also increased for the Omicron variant, the report found. Among the 116,000 people who had an Omicron infection, about 11,000 -- or 9.5% -- were linked to a previous confirmed infection, which is likely an undercount of reinfections. In the data analyzed, 69 Omicron cases were a third episode of COVID-19 infection, and 290 cases occurred 60-89 days after a first infection.