Background Spain is one of the European countries most affected by the COVID-19 pandemic. Serological surveys are a valuable tool to assess the extent of the epidemic, given the existence of asymptomatic cases and little access to diagnostic tests. This nationwide population-based study aims to estimate the seroprevalence of SARS-CoV-2 infection in Spain at national and regional level.Methods 35 883 households were selected from municipal rolls using two-stage random sampling stratified by province and municipality size, with all residents invited to participate. From April 27 to May 11, 2020, 61 075 participants (75•1% of all contacted individuals within selected households) answered a questionnaire on history of symptoms compatible with COVID-19 and risk factors, received a point-of-care antibody test, and, if agreed, donated a blood sample for additional testing with a chemiluminescent microparticle immunoassay. Prevalences of IgG antibodies were adjusted using sampling weights and post-stratification to allow for differences in non-response rates based on age group, sex, and census-tract income. Using results for both tests, we calculated a seroprevalence range maximising either specificity (positive for both tests) or sensitivity (positive for either test). FindingsSeroprevalence was 5•0% (95% CI 4•7-5•4) by the point-of-care test and 4•6% (4•3-5•0) by immunoassay, with a specificity-sensitivity range of 3•7% (3•3-4•0; both tests positive) to 6•2% (5•8-6•6; either test positive), with no differences by sex and lower seroprevalence in children younger than 10 years (<3•1% by the point-of-care test). There was substantial geographical variability, with higher prevalence around Madrid (>10%) and lower in coastal areas (<3%). Seroprevalence among 195 participants with positive PCR more than 14 days before the study visit ranged from 87•6% (81•1-92•1; both tests positive) to 91•8% (86•3-95•3; either test positive). In 7273 individuals with anosmia or at least three symptoms, seroprevalence ranged from 15•3% (13•8-16•8) to 19•3% (17•7-21•0). Around a third of seropositive participants were asymptomatic, ranging from 21•9% (19•1-24•9) to 35•8% (33•1-38•5). Only 19•5% (16•3-23•2) of symptomatic participants who were seropositive by both the point-of-care test and immunoassay reported a previous PCR test.Interpretation The majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hotspot areas. Most PCR-confirmed cases have detectable antibodies, but a substantial proportion of people with symptoms compatible with COVID-19 did not have a PCR test and at least a third of infections determined by serology were asymptomatic. These results emphasise the need for maintaining public health measures to avoid a new epidemic wave.

Our findings strongly suggest that monotherapy with levofloxacin is a safe and effective treatment for legionnaires disease, including in patients with severe disease. In these patients, levofloxacin appears to be more effective than clarithromycin.

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Fluoroquinolone efflux was studied in 47 Staphylococcus aureus clinical strains with MICs of ciprofloxacin (CFX) of <2 g/ml. Forty-three strains were wild type for gyrA, gyrB, and grlA quinolone resistance-determining regions and for norA and its promoter region. Forty of these strains (MICs of CFX, 0.1 to 0.2 g/ml) did not show efflux of fluoroquinolones. Three strains (MICs of CFX, 1 to 2 g/ml) showed efflux. These results suggest that efflux can appear in S. aureus clinical strains in the absence of mutations in norA and its promoter.