A health official in China said on Sunday that the newest COVID-19 outbreak is increasingly likely to spread, as officials encouraged all regions to increase monitoring and called for a reduction in cross-province travel.

China has mostly suppressed the virus, but it is keen to eradicate any irregular small outbreaks, especially in the run-up to the 2022 Winter Olympics, which will take place in February.

Over the previous week, more than 100 locally transmitted cases have been confirmed throughout 11 provincial areas, the majority of which have been linked to 13 separate tour groups.

According to Mi Feng, a spokesman for the National Health Commission, there is a growing possibility that the outbreak may spread further, aided by “seasonal circumstances.”

According to commission deputy director Wu Liangyou, the Delta variation causing the outbreak is likewise highly transmissible, and sequencing revealed that it was distinct from the source of an earlier outbreak, implying that the current cases came from a foreign source.

Authorities have prohibited travel agencies from planning cross-provincial tours that include areas believed to be at higher risk of virus transmission, and some travel services linking several tourist attractions have been suspended nationally.

The capital Beijing has said it will impose strict restriction on travels to the city by people who have been to counties with at least one infection.

Health authorities also said on Sunday that about 75.6 percent of China’s population had received complete vaccine doses as of Oct. 23, or some 1.068 billion people.

China is giving booster shots to adults whose last dose was at least six months earlier, with priority groups including essential workers, older people and those with weaker immune systems.

Data showed antibodies elicited by vaccines, including the most-used shots from Sinovac and Sinopharm, declined within months.

Wang Huaqing, chief expert for the immunization program at the Chinese Center for Disease Control and Prevention, said China would not keep giving people booster shots indefinitely.

“Even if it needs to be strengthened later, the number of boosters is limited,” Wang told the briefing.

“We hope in the future there will be better vaccines and better vaccination procedures to achieve solid protection among the public.”