BEIJING (BLOOMBERG) - China is confronting its broadest coronavirus outbreak since the pathogen emerged in late 2019 after the Delta variant broke through the country's defences, with cases now in 14 of 32 provinces.

While the overall number of infections - more than 300 so far - is much lower than outbreaks elsewhere, the wide spread indicates that the variant has been on the loose for some time and is alarming officials who wield the strictest containment measures in the world.

It's the biggest challenge for the world's second-largest economy since the virus was first reported in the central Chinese city of Wuhan in December 2019.

China's strict regulations, including mass testing, aggressive contact tracing, quarantines and occasional lockdowns, crushed more than 30 previous flareups.

The arrival of the more infectious Delta variant, however, is testing that approach. The new strain may be exploiting the population's recent willingness to lower their guards when it comes to masking and distancing, since much of the country has been Covid-free for months.

That, along with increased travel during the summer months, created a perfect storm for Delta to gain a foothold.

The initial infection arrived via an overseas flight from Moscow into the eastern Chinese city of Nanjing in mid-July, and took hold there among the airport cleaning staff.

China reported 99 infections on Monday (Aug 2) alone, including 44 people who tested positive but have no symptoms. By number of cases, it is the biggest outbreak since the flareup in Hebei in northern China in January, when 2,000 people were infected.

The broad spread is more concerning, with infections having reached the highly protected capital, Beijing, and as far as Hainan province in the south, 1,900km away from Nanjing.

Local governments in major cities including Beijing have now tested millions of residents, while cordoning off residential compounds and placing close contacts under quarantine.

The central city of Zhuzhou in Hunan province ordered over 1.2 million residents on Monday to stay home under strict lockdown for the next three days as it rolls out a citywide testing and vaccination campaign, according to an official statement.

In the tourist destination of Zhangjiajie, near Zhuzhou, an outbreak spread last month among theatre patrons who then brought the virus back to their homes around the country. Zhangjiajie locked down all 1.5 million residents on Friday.

It remains to be seen if the country's vaccination rate, close to 60 per cent and among the highest in the world, can slow Delta's spread and keep serious illness and death at bay.

Most of those infected in Nanjing have been immunised. The shots do appear to protect against serious disease, with 4 per cent of those infected battling severe disease so far. Many of them have pre-existing conditions such as asthma, diabetes or high blood pressure, said Mr Guo Yanhong, an official with the National Health Commission, at a briefing in Beijing on Saturday.

While all Covid vaccines have seen their effectiveness dented by Delta, concerns are high that non-MRNA vaccines like the Chinese ones and AstraZeneca Plc's shot will be less able to slow transmission.

State-owned Sinopharm said its inactivated Covid-19 shot, a mainstay for the Chinese population, is 68 per cent effective against Delta, citing a study in Sri Lanka.

Sinovac Biotech Ltd, the other major Chinese supplier of shots, said that sera samples from people inoculated with its inactivated vaccine can still neutralise the Delta strain in laboratory studies, state media Global Times reported, without more detail.

All Covid vaccines have seen their effectiveness dented by Delta, showing that immunisations alone won't bring the outbreak to heel.

Last week, the US Centres for Disease Control and Prevention reversed its earlier position and said fully vaccinated people should go back to wearing masks indoors in places where infections are rising.

"Delta accounts for 80 per cent of cases in the US, and they re-instituted a requirement for masks," said Dr Wang Huaqing, chief immunisation expert at the Chinese Center for Disease Control and Prevention, at the Saturday briefing. "That means Delta's spread is severe and personal protection can not be slackened even with vaccination."

Adding to the concern is a separate cluster in the central Chinese city of Zhengzhou - also of the Delta strain - where hospital and cleaning staff have been infected.

Cases were reported in the broader Henan province as well, where the ability to curb the virus's spread may be weakened due to the fallout from torrential rain that has killed nearly a hundred people and destroyed infrastructure.

Residents in Nanjing, where the recent outbreak began, have been placed under lockdown. Also affected are those living in Zhangjiajie, a scenic area famous for its verdant mountain ridges, where a live outdoor performance a week ago with more than 3,000 spectators fuelled the virus's spread.

Officials in the Chinese capital of Beijing, which has detected five Delta cases, vowed to cut off the virus' transmission with "fastest pace, strictest measures and the most decisive actions."

It will tighten entry restrictions for those travelling from places currently battling outbreaks, and government and state company employees have been barred from leaving the city.