The coronavirus disease 2019 (COVID-19) pandemic has affected the world radically since 2020. Spain was one of the European countries with the highest incidence during the first wave. As a part of a consortium to monitor and study the evolution of the epidemic, we sequenced 2,170 samples, diagnosed mostly before lockdown measures. Here, we identified at least 500 introductions from multiple international sources and documented the early rise of two dominant Spanish epidemic clades (SECs), probably amplified by superspreading events. Both SECs were related closely to the initial Asian variants of SARS-CoV-2 and spread widely across Spain. We inferred a substantial reduction in the effective reproductive number of both SECs due to public-health interventions ( R e < 1), also reflected in the replacement of SECs by a new variant over the summer of 2020. In summary, we reveal a notable difference in the initial genetic makeup of SARS-CoV-2 in Spain compared with other European countries and show evidence to support the effectiveness of lockdown measures in controlling virus spread, even for the most successful genetic variants.

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Ulcerative Colitis from Genetics to Complications 42 of the total. Only about 50-100 bacteria were found in most individuals regardless of their number. On the other hand, it has been observed that the changes that occur over the lifetime of a healthy person are minimal, the most dramatic being at the beginning of life, since initial colonization after birth until weaning, when the microbiota begins to transform and becomes gradually very similar to that of an adult. These changes are determined by the geographic location with their characteristic eating habits, by the type of birth (caesarean versus vaginal delivery), and the type of feeding after birth (breast vs. bottle). Eventually, the microbiota is composed mostly (> 90%) by bacteria of the genera Bacteroides and Bifidobacterium, and the rest by Eubacterium, Lactobacillus, coliforms, Streptococcus, Clostridium, and a variable number of yeasts. As an indirect consequence of the large number and variety of microorganisms, the balance achieved between the components of the microbiota and the competition for nutritional resources exert a protective effect on the host by preventing colonization and proliferation of potential pathogens. This huge set of microorganisms contains approximately three million of unique genes, i.e. 150 times more genes than our own genome. This metagenome or microbiome is essential for proper homeostasis and good health, as it provides metabolic pathways that allow implementations that we would be unable to perform without the assistance of the intestinal microbiota, like digestion of some sugars, production of vitamins or removal of hydrogen in the distal intestine. Recent studies have demonstrated a common set of around 500,000 unique genes shared by the microbiota of different subjects, in other words, a common metagenome. For all this, it has been suggested that the microbiota serves as an organ for human beings (Zhu et al., 2010). In addition to these functions, the intestinal epithelium requires intestinal flora and the substances excreted by it in order to develop properly, given that these metabolic byproducts promote growth and epithelial differentiation.