Janssen vaccine reports efficacy after single injection: FDA decision imminent

Simon Collins, HIV i-Base

On 29 January 2021, Johnson & Johnson reported 66% efficacy against moderate to severe COVID-19 after a single dose of the Ad26.COV2.S  adenovirus-based vaccine developed by Janssen. The vaccine reduced the risk of severe COVID-19 by 85% and results included efficacy against the B.1.351 variant in South Africa.  [1]

On 25 February, the results were published in an FDA briefing report with a planned FDA review the next day, which recommended authorisation in the US. [2, 3]

The phase 3 ENSEMBLE 1 study randomised almost 44,000 participants in eight countries and included 34% aged over 60 years old. Approximately 44% of participants were in the US, 41% in Central and South America (Argentina, Brazil, Chile, Colombia, Mexico, Peru) and 15% in South Africa. Approximately 40% had at least one comorbidities associated with an increased risk for severe COVID-19. This included obesity (28%), type 2 diabetes (7%), hypertension (10%) and HIV (3%).

Efficacy results were based on COVID-19 symptoms 28 days after the injection and were similar in different regions: 72% in the US, 66% in Latin America and 57% in South Africa. Efficacy increased over time with no cases of severe COVID-19 reported seven weeks after infection.

Overall tolerability was good, with grade 3 fever reported in 0.2% of participants and no cases of anaphylactic reactions.

A second study, ENSEMBLE 2, that includes sites in the UK, is also looking at a two-dose schedule with this vaccine.


On 26 February 2021, (just after the HTB mailing) the US FDA vaccine advisory panel recommended unanimously that the vaccine should be approved for emergency use authorisation (EUA). [3]

Although recommendations are non-binding, an EUA is expected shortly.


  1. Johnson & Johnson announces single-shot Janssen COVID-19 vaccine candidate met primary endpoints in interim analysis of its phase 3 ENSEMBLE trial. (29 January 2021).
  2. FDA Briefing Document. Janssen Ad26.COV2.S vaccine for the prevention of COVID-19. (28 February 2021).
  3. J&J press statement. Johnson & Johnson single-shot COVID-19 vaccine candidate unanimously recommended for emergency use authorization by U.S. FDA advisory committee. (26 February 2021).

This report was updated on 27 February 2021 to include the FDA approval.

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