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Third COVID-19 vaccine dose only 37% effective against Omicron after 7 days

A third COVID-19 vaccine dose has been found to offer limited protection against infection with the Omicron compared with the Delta variant

A third COVID-19 vaccine dose seems to be unlikely to offer much protection against the Omicron compared with Delta variant according to the results of a real-world study by a team led by Public Health Ontario, Toronto, Canada.

The Omicron COVID-19 variant has been designated by the World Health Organization (WHO) as a variant of concern due to an increased transmissible nature and the potential for immune evasion. There is a flurry of on-going research to better understand the impact of this variant and fortunately, early data suggests that while a third COVID-19 vaccine dose offers higher protection against Omicron although the authors of the study added that ‘even with three vaccine doses, neutralisation against the Omicron variant was lower (by a factor of 4) than that against the Delta variant.’

Real-world evidence of vaccine effectiveness (VE) against the Omicron is urgently needed and was the purpose of the study by the Canadian team. They examined individuals 18 years and over, with a positive PCR test result between November 2021 and December 2021, excluding long-term residents and those who had received only a single COVID-19 vaccine dose or a second dose less than 7 days prior to testing.

The researchers identified confirmed cases of infection, irrespective of symptoms or severity using provincial reportable data. Any positive samples with an S-gene Target failure (SGTF), were considered to be due to infection with Omicron since this failure is absent in those with the Delta variant. They compared infection rates (compared to those who were unvaccinated and which served as the reference group) for individuals with either two or a third COVID-19 vaccine dose against both the Omicron and Delta variants.


The team identified 3442 Omicron positive, 9201 Delta positive and 471,545 negative controls. Omicron infected individuals were generally younger, mean age 34.8 years (49.2% male) compared with 43.7 years (same gender proportion) for Delta infections.

After two doses of a COVID-19 vaccine, VE efficacy against Delta was 71% (95% CI 66 – 75%) > 240 days after the second dose but this figure increased to 93% (95% CI 92 – 94%) > 7 days after a third dose. In contrast, two vaccine doses were not protective against Omicron at any point in time and the VE was – 38% (95% CI – 61 to – 18%) after the second dose. However, VE against Omicron was 37% (95% CI 19 – 50%) > 7 days after a third COVID-19 vaccine and these findings were consistent for all combinations of the vaccines used.

The authors discussed how their findings have potentially important implications in so far as proof of vaccination (defined by at least two doses) should no longer be considered as fully vaccinated. They concluded that protection from three vaccine doses offers some protection against the variant but the effectiveness against severe disease remains uncertain.


Buchan SA et al. Effectiveness of COVID-19 vaccines against Omicron or Delta infection MedRxiv 2021.

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