Sonabend R, Whittles LK, Imai N, et al. - As vaccination roll-out continued in England, the timeline and conditions for the stepwise lifting of non-pharmaceutical interventions (NPIs) were proposed in COVID-19 roadmap out of lockdown policy, with step one initiating on March 8, 2021. Researchers conducted this mathematical modelling study with the aim to assess the roadmap, the influence of the Delta (B.1.617.2) variant of SARS-CoV-2, and potential future epidemic trajectories.

• Vaccination and multi-strain dynamics were incorporated in a previously described model of SARS-CoV-2 transmission to explicitly capture the emergence of the Delta variant.

• A Bayesian evidence synthesis framework was used to calibrate the model to English surveillance data, including hospital admissions, hospital occupancy, seroprevalence data, and population-level PCR testing data, then the potential trajectory of the epidemic was modelled for a range of different schedules for relaxing NPIs.

• With increasing population immunity through vaccination, the increased transmission resulting from lifting NPIs starting on March 8, 2021 was successfully offset by the roadmap policy.

• However, with an estimated transmission advantage of 76% over alpha, the emergence of Delta variant might have resulted in 3,900 peak daily hospital admissions under the central parameter scenario, if NPIs were completely lifted up on June 21, 2021, as originally planned.

• Peak hospital admissions lowered by three fold to 1,400 (95% CrI 700–1700) per day by delaying until July 19, 2021.

• Findings overall suggest that even if vaccination coverage is high, complete lifting of NPIs might not be possible with the Delta variant, without a third wave of hospital admissions and deaths.

• The epidemic trajectory remained substantially uncertain, with particular sensitivity to the transmissibility of Delta, level of mixing, and estimates of vaccine effectiveness.

• Researchers emphasize the relevance of providing careful consideration to variants of concern, their transmissibility, vaccine uptake, and vaccine effectiveness when relaxing pandemic control measures in countries.