Delay in second COVID-19 vaccine dose does not reduce protection: study

The Quebec Institute of Public Health (INSPQ) says in a report released Monday that vaccine efficiency to prevent a COVID-19 infection was around 75 per cent two weeks or more after a single dose and that a single dose reduced the risk of hospitalization due to the novel coronavirus by about 95 per cent.

The efficacy rates hold, the study says, after four months.

Dr. Gaston DeSerres said the study confirms what the government said when it delayed second doses, so health-care professionals could vaccinate as many people as possible. 

"The main results of this study are showing that a single dose of vaccine is protecting nearly 75 per cent against COVID-19 - so symptomatic infection - and this protection is maintained for the interval of two weeks post-vaccination to 16 weeks, which  is the currently used interval," he said. 

"People had questions regarding will the protection induced by a first dose wane during this interval and the study shows that it doesn't."

The report is based on a study that took data from 280,000 private and public health-care workers between Jan. 17 and May 22 that looked at how efficient messenger RNA vaccines were in preventing SARS-CoV-2 infections, symptoms and hospitalizations.

The study found vaccine efficacy (VE) against symptomatic COVID-19 increased to 73.4 per cent after one dose and 94.2 per cent after two doses.

Efficacy against COVID-19 hospitalizations was 97.9 per cent after a single dose, and no hospitalization cases were reported after two doses. 

"So one does is really efficacious to prevent hospitalizations due to COVID-19," said DeSerres. 

DeSerres admitted that the study focussed on a younger population and that the vast majority received the Pfizer dose (about 90 per cent). 

"Still for the health-care workers, who represented a priority group, I think this study shows how protective a single dose was, and that a longer interval was not putting them at greater risk because of a waning protection," he said.