Born in Monmouth,[5] Harries studied medicine at the University of Birmingham gaining an intercalated BSc in pharmacology in 1981 and medical degrees, MB ChB, in 1984.

[11] Although the organisation was established on 1 April 2021, Harries assumed this role following a hand-over period which lasted until Dido Harding, the interim chief executive, departed on 7 May 2021.

[21] Cheltenham Festival, a four-day event started weeks later and attended by about 150,000 people, was referred to in the following month by Sir David King, the government's chief scientific adviser from 2000 to 2007, as "the best possible way to accelerate the spread of the virus".

[26] In December 2021, The Daily Telegraph reported that it was understood that Harries was the source of a contested figure that there was an average 17-day delay between infection and hospitalisation for COVID-19, used by Health Secretary Sajid Javid.

[27] In November 2023, the UK COVID-19 Inquiry heard that Harries had recommended in an email that, in the worst situation, elderly COVID-positive patients should be discharged into residential care homes to manage capacity problems in hospitals.