UHN study shows extending vaccine interval enhances COVID-19 immunity

UHN researchers find that extending the interval between Pfizer-BioNTech vaccine doses enhances COVID-19 immunity.

The vaccines developed for COVID-19 have been immensely successful in preventing infection and symptomatic disease. However, their distribution and administration has been – and continues to be – limited by supply. 

In March 2021, Canada’s advisory committee on vaccines recommended that the interval between first and second doses be extended from three weeks to up to four months. This strategic recommendation aimed to accelerate the proportion of Canadians receiving their first dose in the face of vaccine shortages.

At the time, the effects of changing the dosing interval were not well understood. To answer that question, researchers from the Ajmera Transplant Centre and Toronto General Hospital Research Institute performed a novel study. Published in the high-impact scientific journal Nature Immunology, this study shows that extending the dosing interval for the Pfizer-BioNTech vaccine not only maintains but actually enhances the vaccine’s ability to trigger immune responses.

Led by Dr. Deepali Kumar, Director of Transplant Infectious Diseases, Ajmera Transplant Centre and Dr. Atul Humar, Director, Ajmera Transplant Centre and the R. Fraser Elliott Chair in Transplantation Research, the study examined the effects of the interval between doses in 93 healthcare workers who received the Pfizer-BioNTech vaccine. Participants received their second dose after either a short interval (three to six weeks) or a longer interval (eight to 16 weeks).

Dr. Atul Humar (L), Director, Ajmera Transplant Centre and the R. Fraser Elliott Chair in Transplantation Research and Dr. Deepali Kumar (R), Director of Transplant Infectious Diseases, Ajmera Transplant Centre.

“Because healthcare workers were prioritized for vaccination and many of them were able to receive their doses according to the original schedule, they were an ideal population for our study,” says Dr. Humar.

One month after the healthcare workers received their second shot, blood work was done to assess the concentration of neutralizing antibodies—molecules produced by the immune system that target the SARS-CoV-2 virus and prevent it from infecting cells.

While all participants had robust antibody levels, these levels were higher in the individuals who received their second vaccine after the longer interval.

To determine how effective these antibodies were at neutralizing the virus, the researchers mixed the participants’ antibody samples with live SARS-CoV-2 virus. They found that samples from the group with the longer vaccination interval could neutralize more virus versus individuals with the short vaccination interval. These findings were consistent for the original virus as well as its Alpha, Beta and Delta variants.

“The findings show that Canada’s strategy for delaying the second dose was likely quite helpful in fighting against COVID-19 variants. This will inform ongoing vaccination efforts, especially within the many regions around the world that are still waiting for access to vaccines,” says Dr. Kumar. “We are also continuing to track our study participants to investigate long-term differences in immunity.”

This work was supported in part by generous donors to UHN Foundation.

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