China is racing to contain its worst coronavirus outbreak in months, as health officials blamed the highly infectious Delta variant for a surge in infections spanning 14 provinces.
China reported 328 symptomatic infections in July, almost equal to the total number of local cases from February to June.
"The main strain circulating at present is the Delta variant ... which poses an even greater challenge to virus prevention and control work," said Mi Feng, spokesman for the National Health Commission (NHC).
The outbreak is geographically the largest to hit China in several months, challenging the country's early success in snuffing out the pandemic within its borders after Covid-19 seeped out of Wuhan.
But that record has been thrown into jeopardy after the fast-spreading Delta variant broke out at Nanjing airport in eastern Jiangsu province earlier this month.
More than 260 infections nationwide have been linked to the cluster in Nanjing, where nine cabin cleaners at an international airport tested positive on 20 July.
Hundreds of thousands have already been locked down in Jiangsu province, while Nanjing has tested all 9.2 million residents twice.
The contagiousness of the Delta variant combined with the peak tourist season and high passenger circulation at the airport has led to the rapid spread of this outbreak, NHC official He Qinghua told reporters.
Fresh cases reported today in two more regions - Fujian province and the sprawling megacity of Chongqing - included one patient who visited the tourist city of Xi'an, Shaanxi province, and an international cargo crew member who recently travelled from abroad, authorities said.
Officials in one Chongqing district ordered emergency mass testing late yesterday for people who had visited venues linked to confirmed cases.
After one asymptomatic case was discovered in Zhengzhou, the epicentre of recent deadly floods in central Henan province, city officials ordered mass testing of all 10 million residents.
The head of the city health commission was also sacked.
The tourist city of Zhangjiajie in Hunan province locked down all 1.5 million residents and shut all tourist attractions yesterday, according to an official notice.
Health officials said the virus was likely brought there via the Nanjing cluster, according to preliminary investigations.
Officials are now scrambling to track people nationwide who recently travelled from Nanjing or Zhangjiajie, and have urged tourists not to travel to areas where cases have been found.
After reports that some people sickened in the latest cluster were vaccinated, health officials said this was "normal" and stressed the importance of vaccination alongside strict measures.
"The Covid vaccine's protection against the Delta variant may have somewhat declined, but the current vaccine still has a good preventative and protective effect against the Delta variant," said Feng Zijian, virologist at the Chinese Centre for Disease Control and Prevention.
More than 1.6 billion vaccine doses have so far been administered nationwide as of yesterday, the NHC said. It does not provide figures on how many people have been fully vaccinated.
Health officials have said they are aiming for 80% of the population to be fully vaccinated by year-end.
Virus is evolving - WHO
Worldwide, coronavirus infections are once again on the upswing, with the World Health Organization announcing an 80% average increase over the past four weeks in five of the health agency's six regions, a jump largely fuelled by the Delta variant.
First detected in India, it has now reached 132 countries and territories.
"Delta is a warning: it's a warning that the virus is evolving but it is also a call to action that we need to move now before more dangerous variants emerge," the WHO's emergencies director Michael Ryan told a press conference.
He stressed that the "game plan" still works, namely physical distancing, wearing masks, hand hygiene and vaccination.
But both high and low-income countries are struggling to gain the upper hand against Delta, with the vastly unequal sprint for shots leaving plenty of room for variants to wreak havoc and further evolve.
Meanwhile in Australia, where only about 14% of the population is jabbed, the third-largest city of Brisbane and other parts of Queensland state are to enter a snap Covid-19 lockdown today as a cluster of the Delta variant bubbled into six new cases.
"The only way to beat the Delta strain is to move quickly, to be fast and to be strong," Queensland's Deputy Premier Steven Miles said while informing millions they will be under three days of strict stay-at-home orders.
The race for vaccines to triumph over variants appeared to suffer a blow as the US Centers for Disease Control released an analysis that found fully immunised people with so-called breakthrough infections of the Delta variant can spread the disease as easily as unvaccinated people.
While the jabs remain effective against severe disease and death, the US government agency said in a leaked internal CDC document "the war has changed" as a result of Delta.
An analysis of a superspreading event in the northeastern state of Massachusetts found three-quarters of the people sickened were vaccinated, according to a report the CDC published yesterday.
The outbreak related to 4 July festivities, with the latest number of people infected swelling to 900, according to local reports. The findings were used to justify a return to masks for vaccinated people in high-risk areas.
"As a vaccinated person, if you have one of these breakthrough infections, you may have mild symptoms, you may have no symptoms, but based on what we're seeing here you could be contagious to other people," Celine Gounder, an infectious diseases physician and professor at New York University, told AFP.
According to the leaked CDC document, a review of findings from other countries showed that while the original SARS-CoV-2 was as contagious as the common cold, each person with Delta infects on average eight others, making it as transmissible as chickenpox but still less than measles.
Reports from Canada, Scotland and Singapore suggest Delta infections may also be more severe, resulting in more hospitalisations.