Early efficacy of first Pfizer COVID-19 shot in new study doesn't disprove Canada's dosing strategy: author
Jules Mccusker receives a BioNTech Pfizer COVID-19 vaccine from Dr. Shauna Martiniuk, right, an emergency doctor with Mount Sinai Hospital, as Jowite Bydlowska, left, sits alongside him at a pop-up vaccination clinic run by the Waakebiness-Bryce Institute for Indigenous Health and the University of Toronto Dalla Lana School of Health, in Toronto, Saturday, April 10, 2021. The vaccination centre serves Toronto's Indigenous community. THE CANADIAN PRESS/Chris Young
Published Thursday, May 6, 2021 9:38AM EDT
The author of a new study that cast doubt on the early efficacy of the Pfizer-BioNech coronavirus vaccine against variants of concerns says he believes the data is not applicable to the situation in Canada where doses are being spaced out by up to four months.
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 evening 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, which is now pervasive in Ontario.
But author Dr. Laith Jamal Abu-Raddad told CP24 on Thursday that Qatari health officials have followed Pfizer’s dosing direction, giving out second doses no more than 21 days after the first in most instances.
“The one-dose estimate we provided is strictly the efficacy in the first three weeks immediately after the first dose and it should not be interpreted to mean the eventual efficacy after three or more weeks of the first dose,” he said. “We could not determine the latter exactly, as in our population everyone got the second dose at three weeks after the first dose.”
Following the monograph means many of the infections found in the study occurred shortly after subjects received their first doses, and not two weeks later and beyond when immunity is thought to increase as a result of the injection.
“The one-dose figures in our paper show that immunity was building up after the first dose, but it has not yet reached its eventual level.”
Among those who received both doses, the study found that 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 6,689 persons who had received one dose of the vaccine and in 1,616 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.”
It is presumed that many of those infections occurred in the early days after subjects received their first injection, meaning antibody levels did not have time to build within the body.
The Qatar study appeared to find 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 World Health Organization (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.
And a cabinet minister in the Ford government cited the study as a reason to question the NACI dosing schedule and the continued transmission of COVID-19 variant cases through points of entry.
“One dose of Pfizer is potentially like 30 per cent effective against the variants from the UK, this is a study out of Qatar so what we're asking about the federal government to do here is support us,” Associate Minister for Small Business Prabmeet Sakaria told reporters on Thursday.
Speaking about the study on Thursday, Associate Chief Medical Officer of Health Dr. Barbara Yaffe said Ontario has seen great results from the one-dose only regimen, with 70 per cent efficacy against infection overall, and more than 90 per cent efficacy against severe outcomes.
"We know that it takes time to respond to the vaccine, 14 days and longer in older people," she said.
"The experience in Ontario has been very good."
Chief Coroner Dr. Dirk Huyer added that the province has seen good levels of immunity starting at 14 days after injection, steadily increasing up to 28 days after the jab.
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 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.