Post-infection immunity provides excellent protection from COVID-19 ICU hospitalization during Delta and Omicron waves
Abstract
We used the official open data covering all COVID-19 cases in the Czech Republic (population of 10.7 mil.) between September 2021 and August 2022 classified according to the vaccination and post-infection statuses to demonstrate the protective effects of vaccination and previous infection on three COVID-19 related outcomes of different severity - PCR positivity, hospitalisations and ICU hospitalisations of COVID-positive patients. The fact that previously uninfected and unvaccinated individuals are much more prone to becoming infected, hospitalised, or admitted to ICU is not surprising; however, the rapid drop in incidences of hospitalised and ICU patients in previously infected cohorts (both vaccinated and unvaccinated) is striking.
While during the Delta variant predominance, the immunity conferred by previous infection protected relatively well even from becoming infected (but see below), this protective effect grew much smaller once the Omicron variant took hold. However, the protection from hospitalisations and, particularly, ICU admissions remained very high in the previously infected cohorts, regardless of the vaccination status.
This is, in our opinion, a crucial piece of information that remains largely neglected, although its potential implications on the vaccination policies for previously infected individuals are great. We do not discuss or dispute the usefulness of vaccination of previously uninfected individuals here - the fact that to get into the 'previously infected' group, one must first survive the original infection must be taken into account.
However, considering all the above, we propose that (i) vaccination strategies for previously infected and previously uninfected individuals should differ as previous infection alone provides strong protection from the severe course of the disease and that (ii) previous infection should be taken into account in the risk-benefit analysis and decision making about vaccination on an individual basis.