Beijing is starting to offer residents booster jabs for Covid-19, local media said Wednesday, as the Chinese capital gears up to host a tightly controlled Winter Olympics in February.

China has fully vaccinated more than one billion people -- more than 78 percent of the population -- as of mid-September, according to the National Health Commission. 

But there are concerns over the efficacy of Chinese vaccines against the Delta variant of the coronavirus. 

Covid was first detected in the central city of Wuhan in late 2019 and has since swept the globe, but strict border controls have mostly reduced the number of daily cases in China to a trickle.

Boosters will be offered to residents aged over 18 who have been fully vaccinated for at least six months, according to a report in the state-run Beijing News.

The move comes as Beijing is set to host the Winter Olympics under strict rules that will see athletes live and compete in a "closed loop" and only Chinese spectators allowed to watch events.

Athletes must be vaccinated or face 21-day quarantine upon entry to China.

Chinese health experts have said the country needs to reach around 85 percent vaccination coverage to achieve herd immunity -- a goal that authorities are trying to achieve by the end of this year. 

Booster shot programmes have been announced in at least 10 other Chinese provinces in recent weeks, including cities in Fujian province and the Xinjiang region, where local outbreaks have been reported in the past month.

Authorities show no sign of loosening their zero-tolerance approach in the run-up to the international sporting event.

A fresh outbreak involving a tour group has spread to at least five provinces -- as well as one case in the capital.

That has prompted the closure of public transport, entertainment venues, schools and tourist sites in a handful of cities in Inner Mongolia and Gansu province.