China battles biggest Covid-19 outbreak in months as US ramps up vaccine push

China is battling its biggest Covid-19 surge in months fuelled by the delta variant, while the US is intensifying efforts to combat the strain which officials believe is more contagious than chicken pox after a fast-spreading outbreak in Massachusetts.

Hundreds of thousands of people in Jiangsu province were in a coronavirus lockdown on Friday after a cluster of infections were detected in the provincial capital Nanjing.

The outbreak, driven by the fast-spreading delta variant, was linked to airport workers who cleaned a plane from Russia earlier this month. By Friday, infections were detected in Beijing and five provinces.

At least 206 infections across China have been linked to the cluster, and the outbreak is geographically the largest in several months.

In Beijing's Changping district, where two locally transmitted cases have been found, 41,000 people in nine housing communities were placed under lockdown Thursday, according to city officials.

It comes as a leaked document from the US Centers for Disease Control (CDC) suggested the delta variant causes more severe illness than earlier variants and spreads as easily as chickenpox.

The document, seen by the New York Times and the Washington Post, argues US health officials must “acknowledge the war has changed” and intensify efforts to halt the spread.

The agency is concerned that cases of the variant appear to be spread among vaccinated people. On Friday, the CDC published data following an outbreak in Cape Cod, Massachusetts, in which 27 vaccinated people infected with the delta variant appeared to carry as much virus as 84 unvaccinated people who later become infected.

It reflects the difficulties the US faces as it struggles to persuade more Americans to get the Covid-19 vaccine and take up prevention measures such as wearing face masks as cases surge and new research suggests vaccinated people can spread the virus.

Earlier this week, the CDC warned Americans that vaccinated people with the delta variant carry just as much virus in the nose and throat as unvaccinated people.

The health agency changed its guidelines on Tuesday to recommend that vaccinated Americans return to wearing face masks in areas of the country where transmission is high.

But the internal CDC document suggests the measures may not go far enough. “Given higher transmissibility and current vaccine coverage, universal masking is essential,” the document said.

Meanwhile, local media reported that most of the early Nanjing patients, where China's most recent outbreak occurred, were vaccinated, leading commentators online to question the efficacy of domestic vaccines.

"If the goal is to slow down the spread and reduce the fatality rate, [Chinese vaccines] can afford a certain degree of protection," top Shanghai infectious disease expert Zhang Wenhong said in a social media post Thursday.

China's top disciplinary watchdog has blamed Nanjing airport officials for "poor supervision and unprofessional management" including not separating cleaning staff who worked on international flights from those on domestic flights.