Estimation of COVID-19 outbreak size in Italy
Authors: Ashleigh R Tuite, Victoria Ng, Erin Rees, David Fisman
Journal: The Lancet Infectious Diseases (2020)

Italy is currently experiencing an epidemic of COVID-19 which emerged in the Lombardy region.1Day M Covid-19: Italy confirms 11 deaths as cases spread from north.BMJ. 2020; 368: m757Crossref PubMed Scopus (31) Google Scholar During the interval between Feb 25–29, 2020, we identified 46 cases of COVID-19 reported in 21 countries in Europe, Africa, North America, and South America that were either in individuals with recent travel from Italy, or who had presumed infection by a traveller from Italy.2News BNO Tracking coronavirus: map, data and timeline.https://bnonews.com/index.php/2020/02/the-latest-coronavirus-cases/Date: 2020Date accessed: February 29, 2020Google Scholar In six cases, in four of the affected countries (Switzerland, France, Austria, and Croatia), land travel was a likely route of introduction, or was documented to have been the route of introduction.2News BNO Tracking coronavirus: map, data and timeline.https://bnonews.com/index.php/2020/02/the-latest-coronavirus-cases/Date: 2020Date accessed: February 29, 2020Google Scholar We used air travel volume between Italian cities and cities in other countries as an index of connectedness, using data available from the International Air Transport Association for February, 2015, (2·61 million total departing international air passengers from Italy). We used the methods of Fraser and collegues3Fraser C Donnelly CA Cauchemez S et al.Pandemic potential of a strain of influenza A (H1N1): early findings.Science. 2009; 324: 1557-1561Crossref PubMed Scopus (1585) Google Scholar to estimate the size of the underlying epidemic in Italy necessary for these cases to be observed with a reasonable probability. To estimate the time at risk of COVID-19 exposure for travellers departing Italy, we obtained data from the United Nations World Tourism Organization for the proportion of international travelers that are non-residents of Italy (63%)4The World Tourism OrganizationItaly: country-specific: basic indicators (compendium) 2014–2018 (12.2019).https://www.e-unwto.org/doi/abs/10.5555/unwtotfb0380010020142018201912Date: 2020Date accessed: February 29, 2020Google Scholar and the average length of stay of tourists to Italy (3·4 days),5United NationsEncyclopedia of the nations: average length of stay of visitors - tourism indicators - UNCTAD handbook of statistics - country comparison.https://www.nationsencyclopedia.com/WorldStats/UNCTAD-average-length-stay-visitors.htmlDate accessed: February 29, 2020Google Scholar and assumed the Italian epidemic began 1 month preceding Feb 29, 2020.6Goes de Jesus J Sacchi C Claro I et al.First report of COVID-19 in South America.http://virological.org/t/first-report-of-covid-19-in-south-america/409Date accessed: February 29, 2020Google Scholar We also did sensitivity analyses in which we included outbound travel to all countries regardless of reported case importations, inflated travel volumes by 35%, to account for the relative increase in flight numbers from 2015–19, and excluded cases in bordering countries or which were documented to have been introduced by overland travel. When all cases were considered we estimated a true outbreak size of 3971 cases (95% CI 2907–5297), as compared with a reported case count of 1128 on Feb 29, 2020, suggesting non-identification of 72% (61–79%) of cases. In sensitivity analyses, outbreak sizes varied from 1552 to 4533 cases (implying non-identification of 27–75% of cases; table).TableEstimated COVID-19 outbreak size, Italy, Feb 29, 2020Estimate95% CI% Unreported*Based on reported case count of 1128.Unreported range (%)*Based on reported case count of 1128.All cases39712907–52977261–79All cases (adjusted to 2019)29372150–39176248–71Exclude bordering countries and overland travel45333238–61727565–82Exclude bordering countries and overland travel (adjusted to 2019)33522395–45646653–75Include travel to all countries20991500–28594625–60Include travel to all countries (adjusted to 2019)15521109–2114270–47* Based on reported case count of 1128. Open table in a new tab We have previously used similar methods to estimate a much larger epidemic size in Iran, with a far greater degree of under-reporting, based on many fewer exported cases. The reason for this difference relates to the relatively high volume of travel from Italy, relative to Iran.7Tuite AR Bogoch I Sherbo R Watts A Fisman DN Khan K Estimation of COVID-2019 burden and potential for international dissemination of infection from Iran.medRxiv. 2020; (published online Feb 25.) (preprint).https://doi.org/10.1101/2020.02.24.20027375Google Scholar In summary, we suggest that the numerous COVID-19 case exportations from Italy in recent days indicate an epidemic that is larger than official case counts suggest, and which is approximately on a par with that currently occurring in South Korea, which reported 3526 cases (and fewer deaths) as of Feb 29, 2020.2News BNO Tracking coronavirus: map, data and timeline.https://bnonews.com/index.php/2020/02/the-latest-coronavirus-cases/Date: 2020Date accessed: February 29, 2020Google Scholar Since initial submission of this letter, aggressive case finding efforts combined with ongoing epidemic growth have resulted in a dramatic increase in reported cases in Italy, which as of March 12, 2020, stand at 15 113.8Aggiornamento 12/3/2020.https://pbs.twimg.com/media/ES7HcmTXgAIGyHM?format=jpg&name=mediumDate accessed: March 12, 2020Google Scholar ART and DF report grants from Canadian Institutes for Health Research. VN and ER declare no competing interests.