BEIJING (BLOOMBERG) - China's capital is experiencing its worst Covid-19 outbreak in more than eight months, fuelled by tourists returning from northern provinces where the fall colours were in full bloom.

Beijing has reported about 20 infections so far in the latest flare-up, which was initially tied to a couple of retired university lecturers from Shanghai who went on a road trip through the nation's scenic northwest in early October.

A second couple from Beijing was so set on having fun once they returned that they ignored persistent fevers and ultimately exposed hundreds of people to the virus.

The latest eruption was in part set off by the second couple who failed to report to Beijing's health authorities in a timely fashion and played mahjong with friends despite having high fevers. The initial cluster of Covid-19 infections spiralled into a nationwide outbreak in less than a week's time.

The situation in the closely-guarded capital city is the worst it's been since an outbreak that started in a residential compound in a southern suburb spread to more than 30 people.

Before that, a flare-up tied to a fresh produce market in June 2020 ultimately led to more than 300 infections.

Cross-country travellers, and patients who don't stay home despite feeling ill, are fanning the outbreak that's driven by the highly infectious Delta variant.

Their activity is complicating the nation's efforts to eradicate coronavirus within its boundaries and maintain its Covid Zero approach.

The country is currently battling its fourth delta outbreak in the past five months.

Stuck in lockdown

Infections proliferated among tourists that flocked to areas like Inner Mongolia and Gansu to enjoy goldening poplars and far-stretching deserts at the turn of fall.

Many unknowingly transmitted the virus to others after they returned to their hometowns.

Nearly 10,000 tourists are currently stuck in a locked down county bordering Mongolia, where the number of infections is nearing 100 after just 10 days.

China's top health official, Ma Xiaowei, visited the county of Ejin and the places where the stranded travellers are staying. He called on local government draft plans to send them home safety, according to a statement published on the National Health Commission's website on Wednesday (Oct 27).

Nationwide, China reported 54 infections on Wednesday, the most in a single day since the outbreak began last week.

In Beijing, the returning couple created a tremendous hurdle for health officials.

They purchased medicine to combat high temperatures and other symptoms, then invited friends over to play mahjong. They went to several banks and supermarkets, dined inside at restaurants and drove their Mercedes-Benz to a local dealer for maintenance.

Others who travelled or played mahjong with them also got infected.

The police in Beijing have opened an criminal investigation to see if their behaviour violated public health emergency response regulations.

The municipal health commission, in exasperation, asked people not to self-medicate for fevers. Instead, they should go to designated clinics for screening.

Health workers in Beijing identified as many as 1,534 of the couple's close contacts.

The government closed down apartment buildings, shops, banks, markets, hospitals and car dealers that were exposed to the virus.

The city is also tightening checks at its borders and all but banned entry for people from other areas where infections have been reported.

Those entering Beijing must produce a negative Covid-19 test from the past two days and undergo health monitoring in the city.

Access to parks, museums and theaters has been limited, and organisers of local events have been asked to cancel or move them online. Poorly ventilated and crowded community mahjong dens have been closed.