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A living evidence approach to variants of concern and COVID-19 vaccine effectiveness

A new look at immune responses and long COVID

Why is this important? 

  • Since April 2021, the KSAU and McMaster University have produced 41 editions of a living review on COVID-19 vaccine effectiveness against variants of concern, which examines how well vaccines prevent infection, severe disease, death, and transmission in the general population

    • Variant of concern: a virus variant that increases disease spread or severity, or decreases the effectiveness of diagnostic tests, vaccines, and therapeutics, compared to previous variants

  • We are now extending this review to also look at how well vaccines produce an immune response against variants of concern, and whether vaccines prevent the development of long COVID among people who have already been infected

    • Long COVID: new or persistent symptoms more than 12 weeks after COVID-19 infection

  • The new review will be a living evidence synthesis, which means that we will use systematic methods to continuously identify new evidence and update the review on a regular basis

  • Results from this living evidence synthesis will be used to inform models of COVID-19 disease and transmission

    • Modelling results will then be provided to CoVaRR-Net and COVID-END to guide public health policy development and COVID-19 research priorities in Canada

What questions did we want to answer?

  • How effective are COVID-19 vaccines in preventing infection, disease (including long COVID), death, and transmission caused by variants of concern? How does this vary by the number of doses received, vaccine brand, and previous infection?

  • How well do COVID-19 vaccines produce an immune response against variants of concern depending on the number of doses received, vaccine brand, and previous infection?

How have we done this living evidence synthesis?

  • We identified preprints through COVID-19 Evidence Alerts from McMaster PLUSTM and identified published literature by searching various electronic databases (e.g. MEDLINE, Cochrane)

  • We examined studies that report on how well vaccines prevent infection, disease, death, and transmission caused by the two most recent variants of concern, Delta and Omicron

    • We compared the immune response against Delta and Omicron for various groups, including those who received 4 doses vs. 3 doses, and those with hybrid immunity (vaccination + infection) vs. only vaccination

    • We compared the likelihood of developing long COVID among vaccinated people who have been infected vs. unvaccinated people who have been infected 

How up to date is this living evidence synthesis?


Will be updated when the first results are available.


What are the main results of our living evidence synthesis?


The first two reports for the long COVID outcomes are linked below. Our findings for the immunological outcomes can be found here. 


How can I find more information about this project?


View protocol on the Open Science Framework:

PROSPERO registration:

Open Science Framework registration:

Report #1 

Report #2

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