New research from McMaster University, aimed at understanding the immune response to COVID-19 vaccines, found some long-term care residents no longer had high antibody levels months after their second dose, directly supporting government decisions to provide third doses.

 The study, supported by the Government of Canada through its COVID-19 Immunity Task Force (CITF), found over 97 per cent of residents produced an initial antibody response that is likely to provide protection against the virus.

 Three to five months after receiving their second dose, researchers found a drop in the antibody level in about 20 per cent of residents to the point where the antibody response may not be strong enough to provide effective protection against the virus.

“The vaccines have worked well in our nursing homes, but we need constant vigilance regarding how well vaccines and other measures are protecting vulnerable residents,” said Andrew Costa, co-lead of the Canada’s Global Nexus task force on Long-Term Care Crisis Management and Renewal and Schlegel Chair in Clinical Epidemiology and Aging at McMaster. “The study continues to shed light on risks that might lie ahead.”

About 70 per cent of Canada’s COVID-19 deaths have occurred in long-term care or nursing homes.

“Although it is clear antibody levels wane after a second dose of vaccine, it does not necessarily mean one is vulnerable to disease as there are other factors contributing to one’s immunity. That said, the antibody wane in long-term care residents was so significant compared to younger, healthier Canadians, that a third dose is certainly a prudent approach to avoid more deaths and sickness,” said Costa.

Researchers also looked at immune responses to Moderna’s SpikeVax and Pfizer-BioNTech’s Cominarty vaccines. While both elicited good antibody levels, most residents had a stronger response to Moderna, which contains a higher amount of mRNA.

“There are a few reasons why it might make sense to use the SpikeVax/Moderna vaccine for third doses and for older and frailer people moving forward. It has a higher amount of the active ingredient and sometimes this helps give the aging immune system a boost, just like the high dose influenza vaccine we give to older adults,” said immunologist Dawn Bowdish, co-principal investigator and a professor in the Department of Medicine at McMaster. “The longer interval between first and second doses may also have given the immune response more time to mature.”

The study, which was conducted in partnership with Schlegel Villages, St. Joseph’s Health System and Health Sciences North Research Institute, was published this week  in The Journal of the American Medical Directors Association and was based on blood samples taken from 138 residents in eight long-term care homes throughout Ontario between March and July 2021.

“Our team members, residents and the loved ones who volunteered to support this study were proud to do so and we’re grateful for their commitment to this important research,” said James Schlegel, President and CEO of Schlegel Villages. “Likewise, the team of researchers at McMaster is to be commended for getting these critically important results to decision-makers quickly so we can do all we can to ensure those who are most vulnerable to the effects of COVID-19 are as protected as possible.”

“We are proud to be part of this important research with McMaster University that aims to understand the effectiveness of the COVID-19 vaccines in long-term care residents,” said David Wormald, Vice President, Elder Care, St. Joseph’s Health System. “The pandemic has been especially difficult for residents and gaining a better understanding of residents’ immune response will enable homes to be safer as we continue to confront the challenges posed by COVID-19.”

“The resident population in long-term care homes, due to their advanced age and poorer overall health, experience faster waning of antibodies following vaccination compared to younger, healthier populations,” states Tim Evans, CITF Executive Director. “While the findings from CITF-funded studies like this one support the need for a third vaccine dose for this population, it remains to be determined if this will confer an adequate long-term antibody response and therefore other infection prevention and protective measures in seniors’ homes continue to be of the utmost importance.”

The research team is also working with scientific partners at the Ontario Ministry of Health, Public Health Ontario, University of Toronto, St. Mary’s General Hospital and the University of Waterloo. PointClickCare Technologies and the Lung Health Foundation are also supporting this research.

Breznik JA, Zhang A, Huynh A, Miller MS, Nazy I, Bowdish DM, Costa AP. Antibody Responses 3-5 Months Post-Vaccination with mRNA-1273 or BNT163b2 in Nursing Home Residents. Journal of the American Medical Directors Association. 2021 Nov 1. Doi: 10.1016/j.jamda.2021.10.001

About the COVID-19 Immunity Task Force

The Government of Canada established the COVID-19 Immunity Task Force (CITF) in late April 2020 to catalyze, support, fund, and harmonize research on SARS-CoV-2 immunity for federal, provincial, and territorial decision-makers in their efforts to protect Canadians and minimize the impact of the COVID-19. To date, the CITF has supported over 100 studies across Canada that are generating critical insights on the levels, trends, nature, and duration of immunity arising from SARS-CoV-2 infection and COVID-19 vaccination. The CITF is overseen by a Leadership Group of volunteers that includes leading scientists and policymakers from across Canada. The Task Force and its Secretariat work closely with a range of partners, including governments, public health agencies, institutions, health organizations, research teams, other task forces, and engages communities and stakeholders. For more information visit:

The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada. 

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