//Natural immunity in Italy

Natural immunity in Italy

Clear graphics from Italy, The decline of COVID-19 severity and lethality over two years of pandemic


20 to 40-fold reductions during the period of dominance of Omicron compared to the initial acute phase.

Phase 1, Ancestral

Probability of hospitalization per infection of 5.4%

Probability of ICU admission of 0.65%

Probability of death of 2.2%

Phase 5, Omicron

Probability of hospitalization, 95.1% reduction in risk

Probability of ICU admission, 97.3% reduction in risk

Probability of death, 97.5% reduction in risk

Using epidemiological and genomic surveillance data

To estimate the number of daily infections in Italy in the first two years of pandemic.

Attack rate

Ascertainment of SARS-CoV-2 infections

Phase 1 (ancestral)


Attack rate 2.8%

Phase 2 (ancestral)

Less stringent NPIs

Attack rate 11.4%

Phase 3 Alpha variant

Mid -February 2021 to early July 2021

Alpha infected about 10.1% of the Italian population

Phase 4, Delta

Second half of 2021

Progressive relaxation of NPIs

Attack rate 17.3%

Phase 5, Omicron

End of December 2021

Attack rate, 51.1% of the Italian population became infected with Omicron

Evolution of population susceptibility

Percentage of the population susceptible to SARS-CoV-2

End of first phase, 97.5%

February 20, 2022, 13%

By February 20th, 2022

a marked proportion of individuals unprotected against SARS-CoV-2 infection can be found among vaccinated subjects,

due to the waning of vaccine protection

Evolution of COVID-19 severity and lethality

Probability of hospitalisation

Probability of ICU admission

Probability of death

Evolution of population susceptibility

Natural, post infection immunity is now the main factor reducing population sensitivity

Vaccination protection is seen to be declining

Omicron natural infection had the largest protective effect

Repeat exposure to omicron (and sub variants) is likely to have the same effect going forward.

NPIs will reduce repeat exposures

NPIs will reduce exposure to other respiratory viruses such as RSV