According to the results of a preprint study, 4.6% of blood donors in the Milan metropolitan area already had antibodies against the coronavirus at the beginning of the epidemic. This number rose to 7.1% by the beginning of April. In addition, the social distancing put in place to contain the spread would seem to have helped especially to protect the youngest, reducing the number of new infections.
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The metropolitan area of Milan saw a flood of people infected weeks before the peak of the pandemic. What the experts already suspected has now been proven by a study on blood donors at the Policlinico di Milano, published in advance (pre-print) on medRxiv.
The researchers selected a random sample of about 800 healthy blood donors who routinely attend the Policlinico di Milano, where there is a major blood transfusion center attended by more than 40,000 donors each year from Milan and the Lombardy region. The donors who visited between February 24th and April 8th, 2020 were analyzed, following the trend of the epidemic from its official beginning until the culmination of the lockdown. A serological test for IgG and IgM antibodies against SARS-CoV-2 was also performed on each donor, along with routine testing. The test has a specificity of 98.3% against these antibodies and a sensitivity of 100%, producing very reliable results.
"The study - comments Dr. Luca Valenti, one of the research coordinators - was possible thanks to the blood samples stored in the polyclinic’s “biobank”, a systematic collection of biological materials that provides crucial resources to research amongst other medical activities. The biobank “keeps all these samples in safe conditions, so that they can be recovered or analyzed in case of need, without altering their characteristics", explained Dr. Valenti.
According to the study, at the beginning of the epidemic the seroprevalence of SARS-CoV-2 was 4.6%. This means that 1 in 20 people had already come into contact with the coronavirus and had also developed antibodies. The researchers indicated that during the phases of the study characterized by social distancing measures, there was a progressive increase of seroprevalence up to 7.1%", with confidence limits reaching 10.8%. This increase was found mainly in IgG, i.e. in older infections and therefore with an already developed immunity, rather than in IgM. In addition, “this progressive increase in the percentage of exposed subjects was mainly found in younger patients, while more recent infections were associated mainly with older donors" one of the researchers added.
"The purpose of this study - explained Daniele Prati, director of the Transfusion Centre at the Policlinico di Milano - was to examine the presence of SARS-CoV-2 infection in asymptomatic adults in one of the most affected areas in Italy, and at the same time to collect as many elements as possible to understand the risk factors and laboratory values associated with the disease. This is the first real scientific confirmation that there was a flood of infected people in the metropolitan area, even before the first cases of the disease were of widespread knowledge. Although this is a preprint article, it is the first serological study on asymptomatic people that tells us that we are far from herd immunity. Finally, the study reminds us that blood donor populations can help us a lot by studying diseases before they fully manifest themselves".
In conclusion, according to the study, the SARS-CoV-2 virus was already circulating in the population before the date when the epidemic officially began. The practice of social distancing seems to have particularly favored younger people, who have had time to develop long-term immunity. Finally, in all the donors who possessed the virus, there were alterations in the blood cell count and lipid profile: two clues that could help to better understand asymptomatic people, i.e. those who have the virus in circulation (and are therefore contagious) but do not manifest the disease.
Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico. #Covid-19, antibodies in 1 in 20 Milanese already weeks before the pandemic. Confirmed in a study by the Milan Polyclinic. May 20, 2020 SARS-CoV-2 seroprevalence trends in healthy blood donors during the COVID-19 Milan outbreak. Luca Valenti, Annalisa Bergna, Serena Pelusi, Federica Facciotti, Alessia Lai, Maciej Tarkowski, Alessandra Berzuini, Flavio Caprioli, Luigi Santoro, Guido Baselli, Carla Della Ventura, Elisa Erba, Silvano Bosari, Massimo Galli, Gianguglielmo Zehender, Daniele Prati. medRxiv 2020.05.11.20098442; doi: https://doi.org/10.1101/2020.05.11.20098442 - This article is a preprint and has not been certified by peer review.