A research letter, published by the Lancet on Monday, looked at 19,543 COVID-19 cases and 377 hospitalizations among 5.4 million people, including 7,723 cases and 134 hospitalizations in patients with the Delta variant, who tended to be younger and more affluent.
According to the data, a “strong vaccine effect” didn’t take hold until 28 days after receiving a first dose.
Fourteen days after receiving a second dose, researchers found that Pfizer was 92 per cent effective against the Alpha variant first identified in the U.K. and 79 per cent effective against the Delta variant, first identified in India. Meanwhile, the AstraZeneca vaccine provided 73 per cent protection against the Alpha variant and 60 per cent against the Delta variant.
Experts noted both vaccines are still effective against each variant, but said the discrepancies between the two cannot be ignored.
“That’s quite a significant difference,” said Dr. Eric Arts, a professor in Western University’s department of microbiology and immunology.
After 28 days post-vaccination, he said there was “only really” a difference in protection “of about 18 per cent” for the AstraZeneca vaccine, “which is not great.”
The Delta variant has been rapidly expanding throughout Canada.
According to Health Canada, it accounted for a majority of COVID-19 cases identified in March and April. Global News reached out to Health Canada to determine the total number of Delta variant cases in the country to date, but did not immediately hear back.
The variant has also threatened to delay reopening plans in the country and much of the world.
In the U.K., Prime Minister Boris Johnson has extended the lockdown by four weeks in an attempt to curb its spread.
Risk of hospitalization doubled
The data also suggests that the risk of hospitalization among those infected with the Delta variant was double that of the Alpha variant, especially in respondents with five or more severe medical comorbidities.
Researchers found that two doses of both the AstraZeneca and Pfizer vaccines were effective in reducing hospitalizations among people who had contracted the Delta strain, but the effectiveness of the shots was “diminished” when to compared to those with the Alpha variant.
Dr. Gerald Evans, chair of infectious diseases at Queen’s University, said there could be a variety of reasons for this.
“(Delta) seems to be a variant that may be causing more serious form of disease, but it gets very complex, and that’s chiefly because a lot of the people who are getting infected with Delta in the U.K., in particular, are unvaccinated,” he told Global News.
“They have no protection. They are younger. And in some younger people, we worry about an over-exuberant sort of immune response, which might make them sicker. Yes, that could be the variant that’s causing it, but it may just be also related to their age.”
Despite their findings, researchers urged against using the data to compare the vaccines against each other due to differences in the respondents who received each type of shot, and differences in how quickly immunity is developed with each shot.
“The point the study ended up showing is that you really need two doses,” said Omar Khan, an assistant professor of biomedical engineering at the University of Toronto. “One dose isn’t enough.”
Another study out of Public Health England found that two doses of either vaccine were more than 90 per cent effective at keeping people out of the hospital.
The study, published on Monday, analyzed the medical records of 14,000 people who had become infected with the Delta variant between April 12 and June 4.
— with files from Reuters and The Canadian Press