BANGKOK, Jan 6 (Reuters) - Thailand on Thursday raised its COVID-19 alert level following rising infections driven by the spread of the Omicron variant of the coronavirus, a senior health official said. The change, from level three to four, sets a pretext for possible measures that could follow, such as closing high-risk areas and placing restrictions on domestic travel or public gatherings, said Kiattiphum Wongrajit, permanent secretary of the health ministry.