Background Multimorbidity prevalence is projected to increase in many countries. Current policy focuses on better managing the health and social care needs of older adults living with multimorbidity. We aimed to explore the potential impact and equity of hypothetical prevention policies on the future burden of multimorbidity in England.

Methods We used a validated microsimulation model of multimorbidity accumulation in England, combining data from adults in the Clinical Practice Research Datalink Aurum database linked to the 2015 English Index of Multiple Deprivation (IMD) and Office for National Statistics (ONS) 2019 population estimates. We modelled two multimorbidity outcomes: basic multimorbidity (2+ chronic conditions), complex multimorbidity (3+ chronic conditions affecting 3+ body systems).

We compared three hypothetical health improvement scenarios to the baseline scenario of continuing current trends over a 30-year projection period (2019–2049).

1. targeted intervention on the worst-off: for the two most deprived IMD quintiles we reduced by 50% the gap in transition times to next state compared to the least deprived quintile.

2. proportionate universalism: we increased transition times for all IMD quintiles by 10%, and then for each quintile reduced by 50% the gap in transition times compared to the least deprived quintile.

3. no socioeconomic inequalities: all IMD quintiles had the same transition times as the least deprived quintile.

We projected cumulative incident cases and numbers living with multimorbidity by IMD quintile and age-group. Results are the median from 10 initial model runs, alongside 95% uncertainty intervals, scaled to the ONS population. Analyses were conducted using R v4.2.2.

Results Compared to the baseline scenario, the proportionate universalism scenario postponed/prevented the most cases between 2019 and 2049: approx. 1.2 m[1.2 m-1.2 m] basic and 1.9 m[1.8 m-1.9 m] complex multimorbidity cases. For basic multimorbidity, the no socioeconomic inequalities scenario postponed/prevented approx. 530,000[520,000–550,000] cases, whilst the targeted intervention scenario postponed/prevented approx. 110,000[100,000–110,000] cases.

The most equitable scenario was no socioeconomic inequalities, which reduced absolute inequalities by 230,000[220,000–240,000] basic, and 350,000[350,000–360,000] complex multimorbidity cases. For all scenarios, reduction in overall cases of multimorbidity and inequalities was concentrated in the working age population.

The projected improvements lead to greater absolute numbers of adults living with multimorbidity in all scenarios, particularly among the over 65s; for example, an extra 2.3 million[2.3 m-2.4 m] over 65s with multimorbidity in 2049 in the proportionate universalism scenario compared to the baseline.

Conclusion Equitable prevention policies could postpone a substantial number of multimorbidity cases, however this could lead to greater absolute numbers of multimorbidity cases as individuals live longer.