LONDON, Dec 3 (Reuters) - Health officials in Britain said on Friday they were investigating a rise this week in the number of COVID-19 tests results that have a trait that is associated with the newly identified Omicron coronavirus variant of concern. One of Omicron"s mutations produces S-gene target failure (SGTF) in about half the PCR tests in use in Britain, allowing it to be distinguished from Delta and providing a clue over the spread of Omicron before full genomic sequencing. read moreAlthough a useful early indicator of the spread of Omicron, SGTF is associated with other variants too, such as Alpha. "The proportion of test results displaying SGTF has been very low in recent months but an increase has been observed in the past week," the UK Health Security Agency (UKHSA) said. "This is still a very small number of cases but is being investigated carefully to understand whether it is related to travel, any other variant or whether there is evidence of spread of Omicron beginning in the community." Omicron has mutations that are associated with reduced vaccine efficacy. UKHSA said the fully vaccinated accounted for 60% of the 20 Omicron infections traced where vaccination status was known, with the unvaccinated accounting for 30%, and 10% having had one dose. "None of the cases of (of Omicron) are known to have been hospitalised or died," UKHSA said.