Two-Dose ChAdOx1 nCoV-19 Vaccine Protection Over Time

Quick Takes

  • This is a retrospective population-based cohort study in Brazil and Scotland assessing changes in effectiveness of the ChAdOx1 nCoV-19 vaccine at preventing severe COVID-19 over time.
  • Vaccine effectiveness in preventing severe COVID-19 of ChAdOx1 decreased to 42.2-63.7% at 4-5 months post-second dose.
  • A booster vaccine should be considered in individuals who received the ChAdOx1 vaccine.

Study Questions:

Is the protection of the ChAdOx1 nCoV-19 vaccine against severe coronavirus disease 2019 (COVID-19) waning?


This retrospective, population-based cohort study in Brazil and Scotland, used national databases from the EAVE II study in Scotland; and the COVID-19 Vaccination Campaign, Acute Respiratory Infection Suspected Cases, and Severe Acute Respiratory Infection/Illness datasets in Brazil for vaccination, laboratory testing, clinical, and mortality data. The population studied was limited to adults (aged ≥18 years) who received two doses of ChAdOx1 nCoV-19 (Oxford–AstraZeneca in Scotland; Vaxzevria/Fiocruz in Brazil). The primary outcome studied was the incidence of severe COVID-19 (i.e., COVID-19 hospital admission or death) at least 3 weeks after the second dose. Entry to the Scotland cohort started from May 19, 2021, and entry to the Brazil cohort started from January 18, 2021. Follow-up in both cohorts was until October 25, 2021.


A total of 1,972,454 adults received two doses of ChAdOx1 nCoV-19 in Scotland and 42,558,839 in Brazil. In Scotland, the relative risk for severe COVID-19 increased to 2.01 (95% confidence interval, 1.54–2.62) at 10–11 weeks, 3.01 (2.26–3.99) at 14–15 weeks, and 5.43 (4.00–7.38) at 18–19 weeks after the second dose, translating to a decrease in effectiveness from 83.7% at 2-3 weeks to 75.9% at 14-15 weeks, and 63.7% at 18-19 weeks after the second dose. The pattern of results was similar in Brazil, with a decrease in vaccine effectiveness from 86.4% at 2–3 weeks, to 59.7% at 14–15 weeks, and 42.2% at 18–19 weeks.


Vaccine protection of ChAdOx1 nCOv-19 wanes within 3 months of the second vaccine dose.


Findings from this large study including populations from two countries (Scotland and Brazil) represent strong evidence of the waning vaccine protection of ChAdOx1 nCoV-19 against severe COVID-19, which seems to occur regardless of the dominant strain (gamma variant in Brazil, delta variant in Scotland). Effectiveness of the ChAdOx1 vaccine in preventing severe COVID-19 was 42.2-63.7% at 18-19 weeks; the effectiveness of the BNT162b2 vaccine reported in other studies for a similar time frame was 83.7%. A booster dose may be even more important in individuals who received the ChAdOx1 vaccine. The study does not address subpopulation vaccine effectiveness differences related to age, sex, or comorbidities, which could identify specific groups who would benefit from earlier boosting.

Clinical Topics: COVID-19 Hub, Prevention

Keywords: COVID-19, COVID-19 Vaccines, Critical Illness, Immunization Programs, Mass Vaccination, Primary Prevention, Vaccination

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