Vaxzevria 73% effective at preventing Omicron-related infections after fourth dose, according to real-world evidence study in Thailand
Vaccine effectiveness similar across all vaccines studied
AstraZeneca’s Vaxzevria (ChAdOx1-S [Recombinant]) was effective at preventing COVID-19 infections due to Omicron when used as a fourth dose booster, according to new data from Faculty of Public Health, Chiang Mai University in collaboration with Chiang Mai Provincial Public Health Office.
In the real-world evidence study, Vaxzevria showed vaccine effectiveness (VE) of 73% (95% Confidence Interval [CI] 48-89%) against the highly transmissible Omicron variant when a fourth dose was given on top of any previous primary or booster vaccine.1 According to the study authors, these are the first known data assessing the effectiveness of a mixed (heterologous) four-dose COVID-19 vaccine schedule.
The findings were published as a preprint in Research Square.
The data showed that a fourth dose of any of the COVID-19 vaccines studied, including Vaxzevria, were 75% effective in preventing Omicron infection (VE 75%, 95% CI 71-80%). The VE of 73% for Vaxzevria was similar to that seen with mRNA vaccines which showed VE of 71% (VE 71%, 95% CI 59-79%). VE was adjusted for age, gender, calendar time and preceding vaccine series type.
Lead author Emeritus Professor Suwat Chariyalertsak, MD, Dr.PH, Faculty of Public Health, Chiang Mai University, Thailand, said: “This study provides much-needed data showing a fourth dose of any COVID-19 vaccine can help prevent infection due to the highly contagious Omicron variant. Providing continuous protection with boosting is especially important for at-risk groups such as the elderly and those living with chronic health conditions. The data also support the effectiveness of heterologous, or ‘mix and match’, vaccination schedules which may help ongoing efforts to increase population coverage of booster doses.”
John Perez, Senior Vice President, Head of Late Development, Vaccines & Immune Therapies, AstraZeneca, added: “These new data further add to our understanding of the importance of booster doses to protect against COVID-19 infection in an evolving variant landscape. Building on Vaxzevria’s effectiveness in preventing severe disease and death, we now know that Vaxzevria can help prevent Omicron-related infections when given as a fourth dose, with greater protection against infection than was seen following a third dose.”
Additionally, a preliminary analysis of hospital data reports VE against severe COVID-19 (requiring invasive mechanical ventilation), and death during the Omicron wave in February and March of 2022. Across all age groups studied, a three-dose mixed schedule provided 98% protection against severe infection or COVID-19-related death. Following a fourth dose booster, the authors observed only a single death, in a person with comorbidities, suggesting very high effectiveness. Analysis of this dataset is ongoing and will be reported at a later date.
The study reported real-world VE of Vaxzevria, CoronaVac and mRNA vaccines utilising an active surveillance network which enabled comparisons of the same patient profiles across both Delta-predominant and Omicron-predominant periods.
To date, over 3 billion doses of Vaxzevria have been released, which based on model outcomes is estimated to have helped save over 6 million lives between 08 December 2020 and 08 December 2021.
