Protection against symptomatic COVID-19 with the Omicron variant shot up after a booster dose among adults who received the two-dose Pfizer vaccine primary series (Comirnaty), though even that faded with time, British researchers found.

While protection against symptomatic disease dwindled to next to nothing (8.8%, 95% CI 7.0-10.5) at 25 or more weeks, vaccine effectiveness jumped to 67.2% (95% CI 66.5-67.8) at 2 to 4 weeks following a booster dose of Pfizer and 73.9% (95% CI 73.1-74.6) following a booster dose of Moderna, reported Jamie Lopez Bernal, PhD, MBBS, of the U.K. Health Security Agency in London, and colleagues.

But this protection declined as well, with vaccine effectiveness falling to 45.7% (95% CI 44.7-46.7) at 10 weeks or more after a Pfizer booster, and to 64.4% (95% CI 62.6-66.1) at 5 to 9 weeks after a Moderna booster, they noted in the New England Journal of Medicine.

Protection was even lower for those who received an initial regimen of AstraZeneca’s vaccine, with no detectable protection against Omicron at 20 to 24 weeks after the second dose of vaccine, they added.

“Waning of protection has been observed with time since vaccination, especially with the Delta variant, which is able to at least partially evade natural and vaccine-induced immunity,” Lopez Bernal’s group wrote.

This adds to the evidence for waning effectiveness of the two-dose vaccine series during the Omicron wave. Data published in an early edition of the Morbidity and Mortality Weekly Report on Tuesday found that vaccine effectiveness against laboratory-confirmed COVID-associated emergency department and urgent care visits among kids ages 5 to 11 was 46% at 14 to 67 days following the second dose.

Lopez Bernal’s team examined data from Nov. 27, 2021 to Jan. 12, 2022 using a test-negative design, including 886,774 eligible adults infected with the Omicron variant, 204,154 infected with the Delta variant, and 1,572,621 test-negative controls. They noted that since 90% of sequenced cases on November 27 were Omicron, they used cases tested on or after November 27 “when the positive predictive value was more than 80.”

Overall, infection with Omicron occurred a median 39 days after a booster dose. Of all participants, 58.4% were women, and 83.2% were white. Of those who previously tested positive, 11% were infected with Omicron versus 1.8% who were infected with Delta.

Interestingly, the study only measured symptomatic disease, not severe disease. “We are unable to determine protection against severe forms of disease … owing to the small number of Omicron cases resulting in hospitalization so far in our data set and the natural lag between infection and more severe outcomes,” Lopez Bernal and colleagues wrote.

A potential limitation to the study was unique to the U.K., where individuals ages 40 and younger were recommended to receive mRNA vaccines rather than AstraZeneca’s vaccine due to the risk of vaccine-induced thrombotic thrombocytopenia, meaning the primary population receiving the Pfizer vaccine was likely “young adults and teenagers.” Older populations with more pre-existing conditions were more likely to have received booster doses, the authors noted.

Other limitations included the large proportion of travel-associated cases in the early part of the U.K.’s Omicron wave, which were excluded from the analysis.

“Our findings indicate that two doses of vaccination with [Pfizer’s vaccine] are insufficient to give adequate levels of protection against infection with the Omicron variant and mild disease,” they concluded.

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