A total of 10,262 infections were reported in people completely vaccinated against Covid-19 in the US between January and April 2021. These are the data that emerge from a report by the Centers for Disease Control and Prevention (CDC), relating to an arc of time in which about 101 million people have been vaccinated in the States. The cases of post-injection-shield infection occurred in 46 states and territories of the United States. Experts point out that only “a small percentage of fully vaccinated people (that is, who have received all recommended doses of an authorized Covid vaccine) will develop symptomatic or asymptomatic infections with SARS-CoV-2”, and that it can happen despite the high level. of efficacy shown by these products which remain “fundamental tool to control the ongoing global pandemic”. The CDC “is working with state and territorial health departments to investigate Sars-CoV-2 infections in fully vaccinated and to monitor trends regarding the characteristics of these cases and the variants identified “. We look at what are called ‘breakthrough’ infections, that is, when the presence of the virus (Rna or antigen) is detected in respiratory samples taken from a person more than 14 days after the last dose of Covid vaccine. State health departments voluntarily report vaccine breakthrough infections to the CDC. Among the cases detected, 63% occurred in women. The mean age of patients is 58 years, 27% of infections were asymptomatic, 10% involved patients known to be hospitalized, 160 (2%) died. Among the 995 hospitalized patients, 29% were asymptomatic or hospitalized for reasons not related to Covid. The median age of deceased patients was 82 years and 18% were asymptomatic or died from a cause unrelated to Covid. Looking at viral sequencing data, available for 5% of reported cases, it emerges that 64% of these were identified as being related to variants of concern (English in 56% of cases, followed by Californian, Brazilian and African). The experts also recall that in the period considered, the transmission of Sars-CoV-2 continued at high levels in many parts of the country (there were about 355,000 cases reported nationally during the week from 24 to 30 April 2021). And that infections are expected among the vaccinated, especially before the immunity of the population reaches levels sufficient to further reduce transmission. However, the CDCs point out, these infections occur only in a small fraction of all vaccinated people and represent a small percentage of all Covid cases. The percentage of infections in vaccinated people attributed to variants of concern is in line with the percentage of these variants circulating in the United States. The report’s authors highlight some limitations of the analysis in question, including the fact that the number of reported cases is likely a substantial underestimate of all Sars-CoV-2 infections among fully vaccinated people. However, starting from May 1, 2021, the CDCs have switched from monitoring all reported ‘vaccine breakthrough’ infections to investigating only cases among patients who are hospitalized or dying, thus focusing “on those of maximum relevance from a point of clinical and public health view “. However, the centers will continue to conduct studies at multiple sites to evaluate the effectiveness of the vaccine.