MADRID, Dec 2 (Reuters) - Regional authorities in Madrid said on Thursday they had detected Spain"s first case of the Omicron coronavirus variant in a vaccinated person without links to risk countries, and were investigating two similar suspected cases. It was the fifth confirmed case of the variant in Spain, Health Secretary Silvia Calzon told a news conference later in the day - but the first showing Omicron is already circulating independently in the country. We"re already seeing a handful of cases where transmission happened within a household," Calzon said of the 78 confirmed Omicron cases sequenced in the European Union. "But we still don"t know the effectiveness of vaccines against this variant." On Wednesday, the northeastern region of Catalonia said it had detected potential traces of the Omicron coronavirus variant in sewage samples from late November. read more The 62-year-old man in Madrid, who has shown mild symptoms since Nov. 29, is isolating at home while his closest contacts are under quarantine, authorities said. The several other cases being investigated also presented only mild symptoms, and had no history of travel to at-risk countries. Spain plans to begin vaccinating children aged 5-11 as soon as possible, with 1.3 million doses of Pfizer"s paediatric vaccine due to arrive on Dec. 13, Health Minister Carolina Darias told reporters. A second shipment containing approximately 2 million doses is expected in January, she added. Spain"s 14-day coronavirus infection rate rose again on Thursday to 234 per 100,000 people, having doubled since Nov. 19 - but it remains substantially lower than in some EU countries such as the Netherlands, Belgium or Germany. The Health Ministry on Thursday added 14,500 cases to its tally of COVID-19 infections since the pandemic began. The death toll rose by 42 to 88,122.