Current trajectory of the HIV epidemic in the MSM population

In the base case scenario, representing the current state of the HIV epidemic in the UK and using model inputs representing current prevention, screening and treatment policies (Table 3), zero transmission was not reached in the MSM population within the model time horizon of 50 years (2020–2070), using the incidence reduction definition.

The rate of PrEP uptake in the base case scenario was based on the rate of uptake to the Impact trial [21] and represents a conservative assumption given the lack of available real-world data on PrEP uptake in the UK. As a result, the proportion of MSM using PrEP in the model increased by a small percentage over time; in 2020, 4.90% (27,974 people) of MSM were using PrEP, compared with a predicted 5.04% (30,696 people) in 2070 (Supplementary Fig. 1).

These base case scenario results demonstrated that improvements in prevention, screening and treatment are required for zero transmission to be reached in the UK MSM population within an ambitious timeframe.

Reaching zero transmission in the MSM population

Both single- and multi-parameter scenarios were explored to investigate how zero transmission could be reached in the UK MSM population. For all parameters, changes were implemented linearly between 2020 and 2024, except for the rate of PrEP uptake, for which changes were implemented linearly between 2020 and 2022 (see supplementary appendix).

Single parameter changes and zero transmission

First, the ‘single parameter estimation’ method was used to explore the changes to each of the six key parameters required for zero transmission to be reached by 2030. Only two parameters, rate of PrEP uptake and probability of screening, could be increased sufficiently for zero transmission to be reached by 2030, in the absence of changes to any other parameters. Two parameters, annual rate of PrEP uptake and annual probability of screening for HIV-negative and undiagnosed individuals, could be individually increased to allow zero transmission to be reached by 2030. An increase in the annual rate of PrEP uptake of 9%, from 0.08% to 9.08% (resulting in 52.16% of MSM [323,923 individuals] using PrEP by the end of 2030; Supplementary Fig. 2) was required to reach the target. A similarly substantial increase of 74.5% in the annual probability of screening, from 22.35% in the base case to 96.85%, was required for the transmission target to be reached. It was not possible for zero transmission to be reached by 2030 if the percentage of individuals diagnosed within three months of transmission, the probability of starting treatment within three or six months of diagnosis, or the use of TasP, were increased alone. These results demonstrated that PrEP use and screening rates are significant drivers of model results, but that extremely large and likely unfeasible increases in these parameters alone are required for zero transmission to be reached by 2030.

A combination approach to zero transmission

A combination prevention scenario was developed to investigate using a combination of changes to PrEP uptake, screening rates, time to diagnosis and time to treatment parameters, but without adjusting the use of TasP (Table 3). The aim of this scenario was to investigate whether a combination approach could be used to reach zero transmission without increasing the UK’s current high rate of ART use and virological suppression (97%). In the combination prevention scenario, the annual rate of PrEP uptake was increased to 0.25%, resulting in the number of MSM using PrEP rising from 4.90% of MSM (27,974 individuals) in 2020 to 5.87% of MSM (36,456 individuals) in 2030 (Supplementary Fig. 3).

Under the combination prevention scenario, zero transmission was not reached within the model time horizon (Table 4). The lifetime risk for HIV-negative MSM not using PrEP to acquire HIV fell from 13.65% in the base case scenario to 9.55% in this future scenario (Table 4).

Table 4 Results for the base case scenario and combination prevention scenarios with three different rates of TasPThe impact of TasP

The impact of increasing TasP, representing the proportion of MSMLWH on ART who are virologically suppressed and so cannot transmit HIV, was investigated in two additional combination prevention scenarios (combination prevention, 98% TasP and combination prevention, 99% TasP; Tables 3 and 4). Increasing TasP from 97 to 98% did not allow zero transmission to be reached within the time horizon. Increasing TasP to 99% allowed zero transmission to be reached by 2030 resulting in 6,394 additional HIV cases avoided by 2030 and 48,969 additional HIV cases avoided by 2070, compared with the base case scenario (Fig. 2). In addition, the lifetime risk of acquiring HIV for HIV-negative MSM not using PrEP decreased from 13.65% in the base case scenario to 7.53%.

Fig. 2

Impact of a combination approach to prevention on HIV incidence (A) and prevalence (B). The incidence reduction target of a 60% decrease in incidence compared with 2010 is marked with a grey dashed line in panel A. TasP: treatment as prevention

Achieving zero transmission before 2030

An aspirational combination prevention scenario was developed to examine when zero transmission could be reached in the model with even greater increases to key parameters. The rate of PrEP uptake, screening rate and probability of starting treatment within three months of diagnosis were set at a higher level than in the combination prevention scenario and TasP was set to 99% (Table 3, Supplementary Fig. 3). These parameters were selected as they are important aspects of the ‘Towards Zero’ action plan [28]. In the aspirational combination prevention scenario, zero transmission was reached by 2026 (Supplementary Fig. 4), resulting in an additional 2,619 HIV cases avoided by 2030 over the combination prevention scenario.

Impact of PrEP commissioning

In the second half of 2020, PrEP began to be available to individuals at risk of HIV acquisition through the National Health Service (NHS) in England. Data for the number of MSM accessing PrEP through specialist sexual health services (SHS) in 2021 are now available; [20] these data reveal that the number of MSM accessing PrEP through specialist SHSs is substantially higher than the number that were able to enrol on the Impact trial [20, 21]. To investigate the potential impact of this change in PrEP access through NHS commissioning, the number of MSM using PrEP in the first year of the model was increased to match the number of MSM accessing PrEP through specialist SHSs in 2021 (50,152 MSM; Supplementary Fig. 3) [20] to create the NHS-commissioned PrEP base case scenario; all other inputs remained at their base case values (Table 3). In addition, an NHS-commissioned PrEP combination prevention scenario was developed, bringing together the increased number of MSM on PrEP in the first year of the model and increases to each of the six key parameters in line with the combination prevention scenario (Table 3). The rate of PrEP uptake, i.e. the proportion of HIV-negative MSM initiating PrEP per cycle, in the NHS-commissioned PrEP base case and NHS-commissioned PrEP combination prevention scenarios was not increased from the rates used in the base case and combination prevention scenarios, respectively, as it is not anticipated that the overall rate at which MSM will initiate PrEP in any given year has significantly changed as a result of NHS commissioning. The dramatic increase in the number of MSM using PrEP between 2019 and 2021 can be explained by the change in the availability of PrEP; during this period, free-at-the-point-of-access PrEP became available to all eligible MSM through NHS SHSs, rather than just those eligible MSM who were able to enrol on the Impact trial. Following this change, MSM who were eligible for and interested in PrEP, but not enrolled on the Impact trial, initiated PrEP through SHSs. PrEP uptake is hereafter expected to stabilise, now that the majority of MSM who are considered eligible for the intervention are accessing it [20].

Increasing the number of MSM on PrEP in the first year of the model in the NHS-commissioned PrEP base case scenario resulted in small improvements in the trajectory of the HIV epidemic but did not result in large changes to the number of additional HIV transmissions across the time horizon, or the lifetime risk of acquiring HIV for MSM not on PrEP, compared to the original base case scenario (Table 5).

In the NHS-commissioned PrEP future scenario, 55,558 MSM were using PrEP by 2030 and zero transmission was reached by 2028 (Supplementary Fig. 5), two years earlier than in the original future scenario. In total, 47,009 additional HIV transmissions were avoided over the course of the time horizon in the NHS-commissioned PrEP future scenario compared to the equivalent base case scenario and the lifetime risk of acquiring HIV for MSM not on PrEP fell from 13.20% to 7.28% (Table 5).

Table 5 Results for the NHS-commissioned PrEP base case scenario and future scenario