Madrid's health minister, Enrique Ruiz Escudero. | Efe

Madrid, Catalonia and the Balearics have been the first regions to detect cases of the so-called "stealth Omicron" (the BA.2 subvariant), the advance of which, for the moment, is being treated with caution but without special concern among health authorities and experts.

This subvariant has spread to some forty countries, including Denmark, the UK and India, which is why the World Health Organization has requested that its development is monitored. Up to five cases have been detected in Spain, according to data from the three regions on Thursday. This figure is expected to increase over the coming days. It is assumed to be highly transmissible but is more difficult to detect in certain diagnostic tests; hence, the "stealth".

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In Madrid, the microbiology service at the Gregorio Marañón University Hospital detected the first two cases. The regional health minister, Enrique Ruiz Escudero, says that at present, and with due caution, "we are not particularly worried". "We are monitoring the people who have been infected and we will make decisions, if we have to."

Otherwise, Omicron continues to dominate transmission, with indicators having softened over the past twelve days. However, there are questions about incidence data, health authorities aware of the underreporting in the general statistics of positive cases that are detected by self-diagnostic tests.

* Earlier this week, Son Espases in Palma suggested that up to ten cases of BA.2 had been detected.