This evidence review was compiled by members of the CITF Secretariat with the input from experts affiliated with the CITF and does not necessarily reflect the views of all CITF members.

By Varun C. Anipindi

As the world – and Canada – deals with increasing case numbers due to variants, scientists from the Imperial College London provide a snapshot of emerging data on real-life efficacy of COVID-19 vaccines against variants. In this Nature Reviews Immunology article, they also discuss future considerations needed to overcome this pandemic.  

While several vaccines have been authorized for use globally and have effectively curbed infection rates in many regions, the pandemic rages on: there are ongoing outbreaks due to SARS-CoV-2 variants of concern (VOCs) and differing levels of vaccine uptake threaten the real-world efficacy of these vaccines. Among the nearly 18 approved vaccine candidates, the Canadian government has approved the use of four vaccines (indicated below). Here, the authors provide a short summary collating efficacy data of these vaccines (Table 1).


Vaccine, Manufacturer Platform Trends of vaccine-induced protection against SARS-CoV-2 variants[1]
Wuhan strain Alpha








BNT162b2, Pfizer-BioNTech mRNA 95% 90% 75% No evidence of reduced protection Lower mean neutralization titers but effectiveness of 88%
mRNA-1273, Moderna mRNA 94.1% Observed reduction in neutralizing antibodies Observed reduction in neutralizing antibodies Observed reduction in neutralizing antibodies Vaccine sera capable of neutralizing virus
ChAdOx1-S[2], AstraZeneca-University of Oxford Viral vector 55-81% 75% 10% Unknown 92% effective OR one dose is 60-71% effective against hospitalization
Ad26.COV2-S, Johnson & Johnson Viral vector 66% 70% 72% in USA, 66% in Latin America, 57% in South Africa 68% Estimated at 60% effectiveness

Table 1: Impact of SARS-CoV-2 variants on vaccine effectiveness.

[1] The authors state that it is not possible to directly compare efficacy across studies due to methodological differences and variation in end points. Data must be interpreted with caution as different countries are also using different vaccination strategies such as intervals between doses and potent heterologous (mix/match dosing).

[2] Also distributed under the trade name COVISHIELD when manufactured by the Serum Institute of India.

Across all variants, a reduction in the activity of neutralized antibodies induced post-vaccination has been observed in vitro. While there is evidence of breakthrough infections, the authors state that severity of disease is nevertheless reduced, thereby suggesting that vaccines remain a highly effective tool against COVID-19.

Alongside the discussion of factors that can influence vaccine effectiveness, the authors also addressed topics related to observed adverse reactions post-vaccination, potential correlates of protection, challenges associated with booster shots, and worldwide vaccine deployment issues such as vaccine hesitancy and global vaccine equity.

As the pandemic unfolds, new studies and updated data will continue to impact the real-life effectiveness of the COVID-19 vaccines. This data may influence strategies around booster shots, public health measures, and other mitigation strategies required to address the next stage of the pandemic.

Tregoning JS, Flight KE, Higham SL, et al. Progress of the COVID-19 vaccine effort: viruses, vaccines and variants versus efficacy, effectiveness and escape. Nat Rev Immunol (2021).