The use of a different covid-19 vaccine for the third “booster” dose than that used for the initial two doses provided a higher level of protection, a study found.1
Researchers in Chile reviewed the country’s national covid-19 vaccination program, including more than four million people who received two doses of CoronaVac (Sinovac), followed by a booster dose between February and November 2021. About 47% received Oxford AstraZeneca as a booster, while about 49% received the Pfizer BioNTech vaccine and just under 5% received the CoronaVac booster.
The team found that the corrected efficacy of the symptomatic covid-19 vaccine was highest for the Pfizer booster at 96.5% (95% confidence interval 96.2 to 96.7), followed by the AstraZeneca booster at 93.2%. 95% CI 92.9 to 93.9) CoronaVac 78.8% (95% CI 76.8 to 80.6).
The paper, presented to the European Congress of Clinical Microbiology and Infectious Diseases in 2022 in Lisbon and published in The Lancet Global Health, states that when it comes to effectiveness against serious illness and death, however, the AstraZeneca booster provides the greatest protection.
For covid-19-related hospital admissions, intensive care, and death, the study found that the adjusted efficacy of the AstraZeneca booster vaccine was 97.7% (95% CI 97.3 to 98.0), 98.9% (95%). CI 98.5 to 99.2) and 98.1% (95% CI 97.3 to 98.6), respectively. For Pfizer, the efficiency was 96.1% (95% CI 95.3 to 96.9), 96.2% (95% CI 94.6 to 97.3) and 96.8% (95% CI 93.9 to 98.3), and for the CoronaVac booster it was 956.% CI 83.7 to 88.5), 92.2% (95% CI 88.7 to 94.6) and 86.7% (95% CI 80, 5 to 91.0).
In their analysis, the researchers adjusted estimates for known confusing factors that could affect performance, such as age, gender, region of residence, income, nationality, and whether the patient had underlying conditions associated with severe covid-19.
The authors said their findings showed that all booster doses used were effective. They wrote: “Our results suggest that a homologous or heterologous booster dose for people with a full schedule of primary vaccination with CoronaVac provides a high level of protection against covid-19, including severe illness and death. Heterologous boosters showed higher vaccine efficacy than homologous boosters for all outcomes, providing additional support for the mixing and matching approach.
Many countries have already introduced third and even fourth doses of the covid-19 vaccine, with some, including the United Kingdom, opting for a mixed approach.
The researchers noted several limitations of the study, including the fact that they did not consider the impact of SARS-CoV-2 variants. During the study period, it was known that four variants were widespread in Chile – alpha, beta, gamma and delta – which could affect the effectiveness of the vaccine.
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