A multidisciplinary team of professionals from Vall d'Hebron, coordinated by Dr. Magda Campins, head of the Preventive Medicine and Epidemiology Department and by Dr. Benito Almirante, head of the Infectious Diseases Department, in close collaboration with primary care centers, implemented an intervention to control SARS-CoV-2 transmission in nursing homes between April 10 and 24, 2020. The results, that are now published in https://wwwnc.cdc.gov/eid/article/26/9/20-2603_article Emerging Infectious Diseases, show that about 24% of residents and 15% of staff members were positive for SARS-CoV-2. More than half of them were asymptomatic, specifically 69.7% of residents and 55.8% of staff.This study included around 6,000 people, including residents and staff, in 69 nursing homes, both public and private, in Vall d'Hebron area of influence. "This is the first study that has been published about the impact of COVID-19 in residences with such a large sample of people. In addition, the homogenous action in all the residences provides great validity to the results", highlights Dr. Xavier Martínez, doctor of the Preventive Medicine and Epidemiology Department and Epidemiology and Public Health Research Group at Vall d'Hebron Institut de Recerca (VHIR) and one of the authors of the publication.The researchers analyzed the existence of symptoms related to COVID-19 and the results in PCR tests of 3,214 residents and 2,655 staff members in nursing homes. 23.9% of residents and 15.2% of staff members were positive for the infection. These data demonstrate a very high transmission of the virus in these centers, especially favored by the characteristics of the residences, such as shared rooms and bathrooms, other common spaces, and the coexistence of dependent people who require a lot of attention.On the other hand, the presence of symptoms related to COVID-19 in the 14 days prior to the diagnostic test was analyzed. 69.7% of the residents and 55.8% of the infected staff were asymptomatic, that is, they had not had fever or respiratory problems in the previous days.There results suggest that the high percentage of asymptomatic cases plays a relevant role in the spread of the virus in this type of centers and, therefore, it is essential to detect all cases, both symptomatic and asymptomatic, in order to cut the transmission chain. It is considered that the symptoms-based screening in this fragile group would be insufficient to detect all incident cases. "When there is a suspicion or case of COVID-19 in nursing homes, infection control actions should be done as quickly as possible. These measures should include tests on both asymptomatic and symptomatic people, as they can transmit the virus if they are infected", explains Dr. Blanca Borrás, physician in the Preventive Medicine and Epidemiology Department and Epidemiology and Public Health Research Group at VHIR and first author of the study. Active teams to control COVID-19Based on the results of the initial screening study in the nursing homes, in less than 72 hours from Vall d'Hebron, recommendations were made with infection control and prevention measures adapted to the characteristics of each center. These actions were implemented in coordination with the participation of the Departments of Preventive Medicine and Epidemiology, Microbiology and Infectious Diseases, the Hospital Direction and the primary care services SAP Muntanya and CAP Sant Andreu.Vall d'Hebron organized a team of 8 to 12 nurses, coordinated by Maria Gutiérrez - San Miguel, a professional from the Vall d'Hebron Process and Quality Direction, who traveled to the residences to take, together with Primary Care nurses, about 500 samples a day on average, both from residents and staff members. In less than 24 hours, the PCR results were available, which represented a very important workload for the Microbiology Department, which was key to implement the necessary isolation and control measures early in each case.The work that was done together with Primary Care was essential to carry out the actions in each of the 69 nursing homes, to monitor patients who didn't require hospital admission and to sectorize the nursing homes and avoid new infections. Some of the most important measures were isolating infected residents, training staff on prevention measures and designating specific areas for residents based on the results.The researchers recommend the implementation of a centralized prevention and control strategy for COVID-19 that include screening residents and staff, regardless of the presence of symptoms, to prevent and control outbreaks of COVID-19 in this highly vulnerable group of people.