Two COVID vaccine studies on single dose efficacy show vastly different results

Thomas Catenacci, DCNF

Two recently-released studies examine the efficacy of single coronavirus vaccine doses against the delta variant showed vastly disparate outcomes.

The first, a Canadian pre-print study posted on medRxiv earlier this month, concluded that a single dose of the Pfizer, Moderna and AstraZeneca vaccines provide “good to excellent protection” against the delta variant of coronavirus. But a single dose of the Pfizer and AstraZeneca vaccines was shown to have relatively low efficacy combating the variant, according to a U.K. study published this week in The New England Journal of Medicine.

“Effectiveness was notably lower after the first vaccine dose among persons with the delta variant,” the U.K. study stated.

The study found that a single dose of the Pfizer vaccine was 36% effective against the variant while a single dose of AstraZeneca’s jab was 30% effective. The efficacy climbed to 88% and 67% respectively following a second dose of the two vaccines, according to the study.

Meanwhile, the Canadian study, which has yet to undergo a peer-review, found that a single dose of the Moderna, Pfizer and AstraZeneca coronavirus vaccines was 83%, 66% and 64% effective against the variant respectively.

“Our real-world vaccine effectiveness estimates suggest that even a single dose of these three vaccine products provide good to excellent protection against symptomatic infection and severe outcomes caused by the four currently circulating [variants of concern],” the authors of the Canadian study wrote.

Their study additionally found the vaccines were nearly 90% effective against the delta variant after two doses.

“With different variants threatening to disrupt our route out of the pandemic, this real-world evidence shows that [the AstraZeneca vaccine], along with other vaccines used in Canada, provides a high level of protection against the most serious forms of the disease, even after just one shot,” Mene Pangalos, an executive at AstraZeneca, said in a statement Friday touting the study showing high efficacy.

However, the pharmaceutical company hasn’t addressed the study showing lower efficacy.

The Food and Drug Administration has approved the vaccines produced by Moderna and Pfizer for emergency use authorization, but has yet to give the green light to the AstraZeneca vaccine. Both the Moderna and Pfizer vaccines have proven to be highly effective at preventing spread in real world conditions since their approval several months ago.

Authors of the two studies and AstraZeneca didn’t immediately respond to a request for comment.

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