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Study shows third COVID-19 vaccine dose 37% effective against Omicron after 7 days

A third COVID-19 vaccine dose appears to offer only limited protection against Omicron compared with infection with the Delta variant

A third COVID-19 vaccine dose appears, in practice, to be unlikely to offer much protection against the Omicron compared with the Delta variant. This was the finding of a preprint from a large Canadian study by a team led by Public Health Ontario, Toronto, Canada.

The Omicron COVID-19 variant was first reported to the World Health Organization (WHO) from South Africa on 24 November 2021 and the WHO has since designed it as a variant of concern due to an increased transmissible nature and the potential for immune evasion.

Moreover, early data suggests that while a third COVID-19 vaccine dose offers higher protection against Omicron, the authors 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.’

In trying to provide some much needed data on the real-world effectiveness of vaccine effectiveness (VE) against the Omicron or Delta variant, the Canadian team 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.


The team identified 3,442 Omicron positive, 9,201 Delta positive and 471,545 negative controls. Omicron infected individuals were generally younger, mean age 34.8 years (49.2% male) compared to 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 was 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.