A new study involving tens of thousands of participants in Qatar found a single dose of the Pfizer-BioNech coronavirus vaccine was only 30 per cent effective at preventing infection by the B.1.1.7 variant now pervasive in Ontario, and only 55 per cent effective at preventing hospitalization or death.
Following the outcomes of nearly 40,000 people tested for COVID-19 in Qatar, the study published in the New England Journal of Medicine on Wednesday found one shot of the Pfizer jab was only 29.5 per cent effective in preventing infection, asymptomatic or otherwise.
One shot of Pfizer was found to be 54.5 per cent effective in preventing “severe, critical or fatal” outcomes due to infection by the B.1.1.7 variant.
Public Health Ontario said in recent weeks that more than 90 per cent of positive samples it screens appear to have the characteristics of the B.1.1.7 variant first discovered in England last year.
Among those who received both doses, the vaccine’s efficacy against infection by B.1.1.7, with or without symptoms, was 87 per cent, increasing to 90 per cent 14 days after the second dose.
It was 100 per cent effective against severe symptoms or death.
“In Qatar, as of March 31, breakthrough infections have been recorded in 6689 persons who had received one dose of the vaccine and in 1616 persons who had received two doses,” study authors wrote. “Seven deaths from Covid-19 have been also recorded among vaccinated persons: five after the first dose and two after the second dose.”
The Qatar study found poorer efficacy and used a larger sample size than many of the studies and datasets the National Advisory Committee on Immunization (NACI) used to justify its original recommendation to space out dosing by 16 weeks.
Other studies cited by NACI found a single dose of a coronavirus vaccine to be 65 to 80 per cent effective in preventing a severe outcome.
NACI cited the British example, but doses there were often only spaced out by 12 weeks. The WHO itself has recommended a maximum spacing of six weeks, but only in critical circumstances.
Ontario officials have suggested the gap between first and second doses may be shortened soon if increases in supply continue, but no firm policy change has been made.
Pfizer Corporate Affairs Director Christina Antoniou told CP24 that their position remains that the vaccine doses must be granted 21 days apart for maximum efficacy.
“We do not have any Pfizer-led data regarding a single-dose approach and our current research is specific to two doses 21-days apart. Our position has not changed and the statement we posted on March 23 remains valid.”
Though three other vaccines are approved for use in Canada, Pfizer-BioNTech remains the most heavily deployed vaccine in the Ontario.
Using data on the first 3.5 million doses administered in Ontario between Dec. 2020 and April 2021, Epidemiologist Dr. Zain Chagla said the current spaced-out dosing strategy has been overwhelmingly effective.
From when Canada’s immunization campaign began in mid-December 2020 to late April 2021, nearly 6,800 people have been infected with COVID-19 after receiving one dose of a vaccine.
“Among these, 4,515 cases were reported within 14 days of their first vaccine dose and 2,274 cases were reported at a minimum of 14 days,” a Health Canada spokesperson told CP24 last week.
UHN infectious diseases specialist Dr. Isaac Bogoch said a first dose is still better than no dose.
"You are afforded pretty significant protection after the first doses from getting infected – the other important point to is we just don’t see proportionately as many people who have gotten a first dose come in to hospital."
Qatar’s experience shows similarities to Ontario’s, even though the province dwarfs Qatar by population.
Qatar saw B.1.1.7 and B.1.351 variants become dominant over earlier so-called “wild” type COVID at about the same time in mid-March.
For the B.1.351 variant first discovered in South Africa, the study saw even poorer results from one dose.
The study found one dose of the Pfizer-BioNTech vaccine was only 17 per cent effective in preventing infection by B.1.351.
It was found to be zero per cent effective in preventing hospitalization or death due to B.1.351.
The B.1.351 variant is only sporadically detected in Ontario, with only 246 examples detected in the past month.
One dose of Pfizer coronavirus vaccine only 30 per cent effective at preventing infection by B.1.1.7 variant: study - CP24 Toronto's Breaking News
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