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That's in stark contrast to the 93% effectiveness against the Delta variant they observed among vaccinated and boosted individuals seven or more days later. Researchers reviewed provincial data on 3,442 Omicron positives, 9,201 Delta positives and 471,545 "test-negative controls."

There were important differences between the populations testing positive for each variant. Omicron cases were 10 years younger on average, more likely to be male and two-dose vaccinated, and less likely to have "any comorbidities" or a booster, relative to controls. Delta cases were far less likely to be vaccinated at all, relative to controls.

Digging further into the preprint, which hasn't been peer-reviewed, California-based epidemiologist Tracy Hoeg was floored that two-dose vaccination actually reduced protection from reinfection by Omicron 38% four to six months after injection, and 42% by eight months.

"Why mandate?" Hoeg tweeted while acknowledging the Ontario Public Health study didn't look at vaccine effectiveness against severe COVID infections.

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The Danish-American citizen highlighted official Denmark data suggesting two-dose vaccine recipients are "equally" protected against ICU admissions as boosted individuals. Their reinfections have surpassed those of the unvaccinated, however, and the boosted are "catching up."

Denmark also got attention Wednesday from the anonymous COVID data analyst known as "ianmSC," whose Twitter account is known for its original charts. It claimed the country, widely praised for its high vaccination rate, had set a single-day internal record for COVID cases.

While fact-checker Snopes branded the account's November tweet about a Vermont COVID spike "misleading," the Denmark chart appears to match the country's chart on the Reuters COVID-19 tracker. The wire service said Denmark reported 26,200 cases Wednesday, its largest single-day total.