CME INDIA Presentation by Dr. Rajeev Jayadevan, MD DNB MRCP ABIM (Med/GE) Co-Chairman National IMA COVID Task Force.

Chadox1-S (Covishield) As A Booster Works Very Well
Chadox1-S (Covishield) As A Booster Works Very Well

This is good news for India.

  • AZ boosting was about the same as mRNA vaccine.
  • Both for symptomatic & hospitalisation, including Omicron.
  • Also: 2 dose protection lasts among older people, that’s very good news.
  • Actual AZ results may be even better than the study results suggest. There were more immunosuppressed & vulnerable groups in the AZ group, while Pfizer was a healthier subset.
  • This means that AZ performed well even when the recipients were of worse health status to start with.
Chadox1-S (Covishield) As A Booster Works Very Well
Chadox1-S (Covishield) As A Booster Works Very Well

Study Highlights

  • The ChAdOx1-S booster is being used as part of both homologous and heterologous vaccine programmes in many parts of the world.
  • It has been relied upon particularly in low- and middle-income countries due to the lower cost and the relative logistical ease of use as the cold-chain requirements.
  • Effectiveness:
    • Real-world VE (vaccine effectiveness) studies have found one and two doses of the ChAdOx1-S vaccine to be moderately effective against mild disease and highly effective against severe COVID-19, with VE against symptomatic disease peaking around 50% whilst VE against hospitalisation peaks at 85-90%.
  • Protection against more severe outcomes (hospitalisation and death) is maintained for longer
  • Waning against symptomatic disease caused by the Omicron variant occurs from a month after the second dose, but by 25 or more weeks after a second dose protection against hospitalisation with Omicron is estimated at around 30%.
  • Clinical trial data have indicated that ChAdOx1-S is highly immunogenic when given as a booster following primary vaccination.
  • Protection against more severe outcomes (hospitalisation and death) is maintained for longer.
  • In this new study authors estimate VE against symptomatic disease and hospitalisation following Delta or Omicron infection after booster vaccination in adults in England who received a ChAdOx1-S booster vaccine. It was   compared to those who received a BNT162b2 booster following a ChAdOx1-S primary course.
  • Illuminating Findings:
    • Protection against symptomatic disease in those aged 65 years and older peaked at 66.1% (16.6 to 86.3%) and 68.5% (65.7 to 71.2%) amongst those who received the ChAdOx1-S and BNT162b2 booster vaccines, respectively.
    • Protection waned to 44.5% (22.4 to 60.2%) and 54.1% (50.5 to 57.5%) after 5-9 weeks.
    • Protection against hospitalisation following Omicron infection peaked at 82.3% (64.2 to 91.3%) after receiving a ChAdOx1-S booster, as compared to 90.9% (88.7 to 92.7%) for those who received a BNT162b2 booster.
Chadox1-S (Covishield) As A Booster Works Very Well

Effectiveness of the ChAdOx1-S and BNT162b2 booster vaccines against hospitalisation as defined by linkage to the secondary care (SUS) data following infection with Delta or Omicron variants in those aged 65 years and older in England

Credit: Reference -1

How to Interpret?

This study supports the use of the ChAdOx1-S booster for protection against severe disease with COVID-19 in settings that have not yet offered booster doses and suggests that those who received ChAdOx1-S as a booster do not require re-vaccination ahead of others.
This is a reassuring real-world evidence.
Booster vaccination with ChAdOx1-S following a primary ChAdOx1-S course provides increased protection against mild and severe disease with Delta and Omicron infection.
Protection against symptomatic disease with Omicron has been found broadly similar to that seen with BNT162b2, with substantial waning by 10 or more weeks.
Vaccine effectiveness against hospitalisation has been found high after a booster dose of ChAdOx1-S at over 80%, though lower than that observed following a BNT162b2 booster.


1. Effectiveness of ChAdOx1-S COVID-19 Booster Vaccination against the Omicron and Delta variants in England.Freja Kirsebom, Nick Andrews, Ruchira Sachdeva, Julia Stowe, Mary Ramsay, Jamie Lopez BernalmedRxiv 2022.04.29.22274483; doi:

2. Thiruvengadam R, Awasthi A, Medigeshi G, Bhattacharya S, Mani S, Sivasubbu S, et al. Effectiveness of ChAdOx1 nCoV-19 vaccine against SARS-CoV-2 infection during the delta (B.1.617.2) variant surge in India: a test-negative, case-control study and a mechanistic study of post-vaccination immune responses. (1474–4457

3. UK Health Security Agency. COVID-19 vaccine surveillance report: Week 11 2022 [Available from:

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