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. 2022 Nov;27(45):2200125.
doi: 10.2807/1560-7917.ES.2022.27.45.2200125.

Tracking the progressive spread of the SARS-CoV-2 Omicron variant in Italy, December 2021 to January 2022

Paola Stefanelli  1 , Filippo Trentini  2   3 , Daniele Petrone  1 , Alessia Mammone  4 , Luigina Ambrosio  1 , Mattia Manica  5   3 , Giorgio Guzzetta  5   3 , Valeria d'Andrea  3 , Valentina Marziano  3 , Agnese Zardini  3 , Carla Molina Grane'  6   3 , Marco Ajelli  7 , Angela Di Martino  1 , Flavia Riccardo  1 , Antonino Bella  1 , Monica Sane Schepisi  4 , Francesco Maraglino  4 , Piero Poletti  5   3 , Anna Teresa Palamara  1 , Silvio Brusaferro  1 , Giovanni Rezza  4 , Patrizio Pezzotti  1 , Stefano Merler  5   3 ; Genomic SARS–CoV–2 National Surveillance Working Group  8 ; Italian Integrated Surveillance of COVID–19 Study Group  9 ; Italian Integrated Surveillance of COVID-19 Study Group; Genomic SARS-CoV-2 National Surveillance Working Group
Collaborators, Affiliations

Tracking the progressive spread of the SARS-CoV-2 Omicron variant in Italy, December 2021 to January 2022

Paola Stefanelli et al. Euro Surveill. 2022 Nov.

Abstract

BackgroundThe SARS-CoV-2 variant of concern Omicron was first detected in Italy in November 2021.AimTo comprehensively describe Omicron spread in Italy in the 2 subsequent months and its impact on the overall SARS-CoV-2 circulation at population level.MethodsWe analyse data from four genomic surveys conducted across the country between December 2021 and January 2022. Combining genomic sequencing results with epidemiological records collated by the National Integrated Surveillance System, the Omicron reproductive number and exponential growth rate are estimated, as well as SARS-CoV-2 transmissibility.ResultsOmicron became dominant in Italy less than 1 month after its first detection, representing on 3 January 76.9-80.2% of notified SARS-CoV-2 infections, with a doubling time of 2.7-3.3 days. As of 17 January 2022, Delta variant represented < 6% of cases. During the Omicron expansion in December 2021, the estimated mean net reproduction numbers respectively rose from 1.15 to a maximum of 1.83 for symptomatic cases and from 1.14 to 1.36 for hospitalised cases, while remaining relatively stable, between 0.93 and 1.21, for cases needing intensive care. Despite a reduction in relative proportion, Delta infections increased in absolute terms throughout December contributing to an increase in hospitalisations. A significant reproduction numbers' decline was found after mid-January, with average estimates dropping below 1 between 10 and 16 January 2022.ConclusionEstimates suggest a marked growth advantage of Omicron compared with Delta variant, but lower disease severity at population level possibly due to residual immunity against severe outcomes acquired from vaccination and prior infection.

Keywords: COVID–19; SARS–CoV–2; doubling time; genomic survey; omicron; prevalence.

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Conflict of interest statement

Conflict of interest: MA has received research funding from Seqirus. The funding is not related to COVID-19. All other authors declare no competing interest.

Figures

Figure 1
Figure 1
Geographical distribution of SARS-CoV-2 variant Omicron, Italy, 6 December 2021–17 January 2022
Figure 2
Figure 2
Estimates of notified infections attributable to Omicron and Delta variants over time, Italy, 02 December 2021–22 January 2022
Figure 3
Figure 3
The impact of Omicron spread on SARS-CoV-2 circulation, Italy, December 2021–January 2022
See this image and copyright information in PMC

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