In the spring of 2021, as the mass vaccination campaign took off in Quebec, the question of the delay between the two doses of vaccine tore specialists apart. Should the vaccine be administered as the pharmaceutical companies recommended, with a delay of 21 to 28 days between the two doses? Or should we trust the conclusions of Public Health, which considered that it was better to distribute the first doses to as many people as possible, even if it meant extending the period of several weeks between the two doses?
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Struggling with an irregular and insufficient supply, Quebec had adopted the second strategy, at one point extending the interval to 16 weeks (4 months). The researchers took the opportunity to measure vaccine effectiveness according to the length of the interval. The starting hypothesis, based on knowledge of the functioning of the immune system, was that by leaving more time between two stimulations, certain types of antibodies would gain in maturity and efficiency.
And indeed, observed over the months the researcher and professor Andrés Finzi, of the department of microbiology, infectiology and immunology of the University of Montreal, the vaccinated who had waited at least 16 weeks between the first two doses benefited for months of slightly better vaccine protection than vaccinees whose interval between doses had been shorter. Memory B and T cell vigor was found to be effective against the Delta variant, in particular, which was circulating in the summer and fall of 2021.
Similar observations were reported last fall by public health authorities in Quebec and British Columbia. According to data published by the Institut national de santé publique du Québec (INSPQ) last October, the effectiveness of the Pfizer vaccine against COVID-19 infections increased from 75% (short interval of three to four weeks) to 88 % with an interval of seven to eight weeks. The effectiveness in preventing hospitalizations was even better: it increased from 92% (short interval) to 98% (long interval).
Similar improvements have also been reported for other vaccines.
This observation, says Professor Finzi, had caused some concern in the cohort of the short interval. “People who had had a short interval told me they were disappointed not to be as well protected as the others,” he recalls.
But since last fall, time has passed and new doses of vaccine have been added. In its latest results, released Tuesday on the scientific publication platform medRxiv, Andrés Finzi’s team delivers “good” and “very good” news.
The “very good” news is for the double vaccinated at the short interval: the booster dose allowed them to catch up with the level of protection of the cohorts of the long interval. “After the third dose, the difference in the advantage of the long interval over the short disappeared,” says Finzi.
The “good” news is for long-interval double vaccinates.
The remarkable protection they rode on for at least four months after the second dose eventually crumbled. But the booster dose, which the study participants received seven months after the last dose, allowed them to regain the high level of protection they had achieved after the second dose.
“Another proof that the booster dose is important for everyone,” says Andrés Finzi.
The data from the study continue to show that one cohort remains even better protected than the others: that of the “vaccinated infected”. Why do vaccinees who have been exposed to the SARS-CoV-2 virus remain better protected than everyone else? Is it because their immune system has learned to recognize this entire coronavirus, while the vaccine teaches them to spot only its characteristic spicule? Or is it due to the mode of transmission of SARS-Cov-2, which infects mucous membranes and stimulates different immune cells than the vaccine? These questions, like many others, still remain unanswered.