Literature Review: effectiveness of the COVID-19 vaccines approved for use in Canada against circulating variants of concern

By Mariana Bego

Variants of concern (VOCs) can potentially make COVID-19 vaccines less effective, both in terms of the immediate immune response and the durability of immunity. Over 20 manuscripts studying the effectiveness of the four vaccines approved for use in Canada against the circulating SARS-CoV-2 VOCs in this country were reviewed and summarized. Neutralization titers from VOCs were compiled and compared to the titers from the virus’ original strain (used to engineer the COVID-19 vaccines). Overall, neutralization was observed for all vaccines and variants combinations, albeit at different levels.

The four major priorities for the global response to VOCs are stated as:

1) To evaluate existing vaccines for efficacy against variants;

2) To assess the effectiveness of new vaccines or modified vaccines against variants if current vaccines are deemed inadequate;

3) To reduce the probability of additional VOCs emerging;

4) To coordinate the worldwide response to the pandemic.¹

Key points:

• Variants may cause a few breakthrough infections in individuals who are vaccinated, but researchers have not found evidence of widespread virus escape.
• mRNA vaccines seem very effective against Alpha (B.1.1.7) and Gamma (P.1), and are still effective, but to a lesser extent, against Beta (B.1.351) and Delta (B.1.617.2).
• The Johnson & Johnson vaccine seems very effective against Alpha and Gamma, with reduced efficacy against Beta. Results regarding the vaccine efficacy of J&J against Delta have not yet been made public.
• Although results for AstraZeneca/Covishield showed the least efficacy at preventing a SARS-CoV-2 infection due to a variant, the vaccine proved nonetheless effective against all circulating variants at preventing hospitalizations or deaths.

Although no biomarker was designated as a gold standard “correlate of protection,” most studies used levels of viral neutralization as a predictor of vaccine effectiveness.^2-3 In Table 1, neutralization titers from VOCs were compiled and compared to the titers from the virus’ original strain (used to engineer the COVID-19 vaccines). Titers were considered similar if they were up to two times lower than the titers against the original strain, and significantly lower if they were more than four times lower (Table 1, red text). Overall, neutralization was observed for all vaccines and variants combinations, albeit at different levels.

One of the vaccines – AstraZeneca –  failed to raise neutralizing antibodies in one small trial in South Africa, where Beta was dominant (Table 1, light blue shaded box).⁴ In this small study, seven of twelve vaccine recipients (46%) without evidence of previous SARS-CoV-2 infection, showed no neutralization activity; five showed between a 4.1 to 31.5-fold reduction in neutralization.⁴ However, experts have warned about overinterpreting these results because of the small number of study participants and the fact that it only focused on mild COVID-19 cases.⁵ Meantime, the World Health Organization (WHO) published a statement highlighting how the AstraZeneca vaccine has been proven effective at preventing severe COVID-19 with the original strain, although its effectiveness at preventing severe illness caused by the Beta variant remains unknown. The WHO still recommends the use of this vaccine, but highlights the overall need for increased surveillance on vaccine effectiveness in the presence of new variants.⁶

Table 1.

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