Two shots of Pfizer vaccine 70 per cent effective against Omicron hospitalization, research shows

New research out of the Gauteng province of South Africa is providing more data on how effective the Pfizer-BioNtech COVID-19 vaccine is against the Omicron variant.

Published in the New England Medical Journal, the study found that a two-dose regimen of the Pfizer vaccine is 70 per cent effective against hospitalization with the Omicron COVID-19 variant.

As of Nov. 15, the Omicron variant was being detected in more than 75 per cent of COVID-19 tests that were being sequenced in South Africa, and the variant has quickly spread to be the dominant strain in many countries.

Canada saw a surge of COVID-19 cases over the holidays, with both Quebec and Ontario breaking previous one-day records, which experts say estimate to be less than the true scope of national case counts due to testing delays and laboratory backlogs.

• Newsletter sign-up: Get The COVID-19 Brief sent to your inbox

In order to assess how effective two doses of the Pfizer vaccine are against the Omicron variant in preventing hospitalizations due to coronavirus, researchers used data from South African care organization Discovery Health to compare data sets of people diagnosed with the Delta variant with those who had the Omicron variant.

The study’s data included PCR test results, medical history, chronic illnesses and body mass index to calculate the number of COVID-19 risk factors per patient, according to the guidelines of the U.S. Centers for Disease Control and Prevention.

Among the fully-vaccinated study participants, researchers compared the Pfizer vaccine’s effectiveness against COVID-19 hospitalization associated with the Omicron variant from Nov. 15 to Dec. 7 in South Africa, which they referred to as the period of dominance, and comparedit with data from Sept. 1 to Oct. 30, when Delta was the dominant variant.

Researchers analyzed 133,437 PCR test results related to the Delta variant from Sept. 1 to Oct. 30, of which 29 per cent were obtained at least 14 days after the patients had received the second dose of the vaccine. They also analyzed 78,173 PCR test results related to the Omicron variant, in which 41 per cent were obtained at least 14 days after the second dose of the vaccine.

The overall test positivity results were 6.4 per cent during Delta’s dominant period compared with 24.4 per cent during Omicron’s dominant period.

However, the COVID-19 admission rate was 10.8 per cent during the Delta dominant period versus 2.2 per cent in the Omicron dominant period, as a percentageof positive PCR test results.

During the Omicron-dominant period, researchers found a vaccine effectiveness of 70 per cent, compared to 93 per cent associated with the Delta dominant period studied against hospitalization for COVID-19.

During the Delta-dominant period studied, 684 unvaccinated people were admitted to hospital with COVID-19, compared to 220 during the Omicron-dominant period.

Of those vaccinated with one dose of the Pfizer vaccine, 71 were admitted to hospital with COVID-19 during the Delta period, compared to 34 during the Omicron period.

Of the study participants who had two doses of the Pfizer vaccine and had a positive PCR test result less than 14 days after their second dose, 13 were admitted to hospital with COVID-19 during the Delta period, while zero were admitted during the Omicron period.

However, of those who had two doses of the Pfizer vaccine and had a positive PCR test 14 days or more after their second dose, 77 were admitted to hospital with COVID-19 during the Delta period, compared to the 121 admitted during the Omicron period.

The study states that during the Omicron-dominant period they “saw a maintenance of effectives of the Pfizer vaccine, albeit at a reduced level” against hospital admission for COVID-19.

“The addition of a booster dose of vaccine may mitigate this reduction in vaccine effectiveness,” the researchers wrote.