While originally, clinical trials suggested that the Johnson & Johnson Jannsen COVID-19 vaccine was 85% effective at preventing severe illnesses, clinical trial data released in January 2021 showed that 4 weeks after the administration of the first dose, the single-shot vaccine was only 66.3% effective at preventing coronavirus infection.

Given that the original trials were conducted before the onset of the delta variant, experts believe that although the vaccine is effective against the new emerging variants, the efficacy is lower as compared to the protection it provided against the original strain.

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However, in respect to the latest findings, J&J announced that the single-dose COVID vaccine was 79% protective against COVID-19 infections and 81% protective for COVID-19-related hospitalizations. Considering the study collected data from March 1, 2020, through July 31, 2021, it was found that the effectiveness of the vaccine did not wane over the duration of the study, even after the delta variant had come into existence. This indicated that the one-shot vaccine was still effective against the Delta variant, which was termed as the variant of concern by the World Health Organization (WHO).