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Oxford vaccine protection against severe COVID-19 wanes three months after second dose

22nd December 2021

The Oxford vaccine protection against severe COVID-19 wanes three months after the second dose highlighting the importance of a booster dose

Protection against severe COVID-19 from the Oxford vaccine (ChAdOx1) appears to wane as early as three months after the second dose according to a study by researchers from the MRC/CSO Social and Public Health Sciences Unit, University of Glasgow, UK.

Clinical trial data have shown that the currently available COVID-19 vaccines including ChAdOx1 provide a high level of protection against infection, however, evidence began to emerge of an increased incidence of infection among those who had been fully vaccinated. It was suggested that this might be due to either a waning of vaccine effectiveness or an increase in the dominance of new variants capable of immune escape. However, other data gathered over a 6 month period using the BNT162b2 vaccine found that its efficacy gradually reduced over this period of time.

For the present study, the UK team turned to data from Scotland and Brazil to examine the association between the time since the second vaccination with the Oxford vaccine (ChAdOx1) and the risk of severe COVID-19 outcomes. They chose Scotland and Brazil for comparative purposes because the Delta COVID-19 variant was the dominant strain in Scotland, whereas this was uncommon in Brazil. Consequently, if vaccine effectiveness reduced in both countries this would be unlikely to be because of the delta variant.

Using a retrospective design, the researchers linked data from the EAVE II study which provides COVID-19 data from 5.4 million people in Scotland and databases in Brazil to determine rates of infection, hospitalisation and deaths. They identified individuals from both countries aged 18 years and over who had received two doses of ChAdOx1 and set the primary outcome as the rate of severe COVID-19 outcomes, i.e., hospital admission or death, approximately two to three weeks after the second dose.


A total of 1972454 adults in Scotland and 42558839 in Brazil received two doses of ChAdOx1. However, an estimate of the vaccine effectiveness was based on a smaller cohort of 2534527 individuals with a mean age of 52 (49.9% male) in Scotland and 56013638 individuals in Brazil with a mean age of 48 years (47% male).

In Scotland the relative risk (RR) of severe COVID-19 was 2.01 (95% CI 1.54 – 2.62) 10 – 11 weeks after the second dose and increased further to 3.01 (after 14 – 15 weeks) and to 5.43 (after 18 – 19 weeks) compared with 2 – 3 weeks after the second dose. In Brazil there was a similar pattern at each time point, for example, the RR was 4.71 after 18 to 19 weeks.

The effectiveness of ChAdOx1 in Scotland decreased from 83.7% (95% CI 79.7 – 87) at weeks 2 – 3 to 63.7% (weeks 18 to 19). Similarly in Brazil vaccine efficacy reduced from 86.4% (2 – 3 weeks after second dose) to 42.2% (weeks 18 to 19).

Commenting on these findings, the authors noted how the similar drop in vaccine effectiveness was unlikely to be due to differences in circulating strains of the virus. They concluded on how these findings clearly illustrate how vaccine effectiveness reduces over time, highlighting the need for a booster vaccination dose.


Katikireddi SV et al. Two-dose ChAdOx1 nCoV-19 vaccine protection against COVID-19 hospital admissions and deaths over time: a retrospective, population-based cohort study in Scotland and Brazil. Lancet 2021