Deep immunity and the progress towards the endemic equilibrium of the Sars-CoV2

The path to endemicity is characterised by the progression of deep immunity, that is, the decrease in the severity of the virus. Deep immunity cannot be reliably derived from measures of cases, whether these come PCR or serological surveys, notably because the first are subject to participation bias and the second are not continuously collected. We rely on immunological theory to devise a novel indicator of the progression of deep immunity in the active population. This indicator, based on the changing age structure of COVID-related hospitalisations, is more robust than any measures base on serological surveys or PCR tests. We find that at least two-thirds of the path to endemicity has been covered, with a reduction of the severity of the virus of at least two-thirds of the 30-60 years-old population. Measuring the progression of the deep immunity in other age groups is less robust and relevant: - Virtually the whole under-30 population had acquired deep cross-immunity by exposure to endemic coronaviruses prior to the first wave, so that neither their exposure not the progression of their immunity can be robustly measured - The immunity of the elderly is not robust, and must be complemented by vaccines, so measuring their exposure is not as relevant).