Real-world data from the U.K. showed that efficacy of a third dose of a COVID-19 vaccine waned significantly at the 10-week mark, leaving boosted individuals at risk of infection with Omicron.
In a December 23 U.K. Health Security Agency (HSA) technical briefing, vaccine efficacy against symptomatic infection generally fell to around 40% some 2.5 months after a booster dose.
Meaghan Kall, a PhD candidate and an epidemiologist at the HSA, tweeted that this effect “is not seen with Delta.”
For its analysis, the agency assessed 147,597 Delta cases and 68,489 Omicron cases from November 27 to December 17.
It found that for those with a Pfizer primary series and booster, efficacy against symptomatic disease was 70% immediately after the third dose but fell to 45% at 10 weeks or longer.
Those who had a Pfizer primary series but a Moderna booster saw protection against symptomatic disease hover around 70% to 75% for up to 9 weeks post-boost.