The emergence of Omicron during the winter of 2021-22 is visible as a sharp drop in immunity in multiple countries (since existing immunity was suddenly less effective against the new variant).
The proportion of unvaccinated people with past infections in a country is roughly correlated with its overall Covid-19 mortality, since unvaccinated but infected people have been at greatest risk of bad outcomes.
Even countries, such as Italy and Britain, with strong vaccination track records may be starting to lose immunity as the pace of booster uptake drops over the summer (although most countries are prioritising boosters for the highest-risk populations).
As Australia transitions its Covid-19 strategy, it has rapidly gained immunity over the past year through a combination of vaccination and infections.
Vaccination remains an essential element in building immunity. Some new studies suggest that infection confers more immunity and protection against severe disease than does vaccination alone.
Even countries, such as the US, with high McKinsey Covid-19 Immunity Index scores continue to experience many new Covid-19 cases as a result of the extremely high transmissibility of current Sars-CoV-2 variants.
A successful autumn booster campaign in Northern Hemisphere countries will be important to increasing immunity levels before winter comes.
From ‘When Will the Covid-19 Pandemic End?’, McKinsey & Co