More than 1.01 billion people have been fully inoculated with Covid-19 vaccines in China, but the highly transmissible Delta variant is still challenging the country's target to build herd immunity.

More than 2.16 billion doses had been administered and 1 billion people fully inoculated by Wednesday, Lei Zhenglong, National Health Commission disease prevention chief, said on Thursday.

That total included 170 million doses given to 95 million teenagers and 390 million doses taken by 200 million people aged over 60.

"We rank the top in the world in terms of total number of doses and number of people covered, and have one of the highest vaccination rates," Lei said.

According to Our World in Data, 54 per cent of the US population, 30.7 per cent of people in the European Union, 13.5 per cent of India's population have been fully vaccinated. Malta tops the world with 82.9 per cent of its 500,000 or so people fully vaccinated.

Meanwhile, in Beijing 20.3 million people have had at least one dose, with 19.5 million fully inoculated. More than 97.4 per cent of the adult population, or 89 per cent of the total population, of the Chinese capital has completed a full regimen, according to Beijing's health commission.

The figures mean that 72 per cent of China's 1.4 billion people are fully vaccinated, reaching a target originally set when China started the national vaccination campaign in December.

At first, China's public health experts thought a 70-80 per cent vaccination rate would result in enough of an immunity barrier to protect unvaccinated people and limit virus transmission in the community.

But the threshold has been revised up to 85 per cent as the highly transmissible Delta variant has spread, according to top respiratory expert Zhong Nanshan and epidemiologist Shao Yiming.

• China races to squash new COVID-19 cluster among schoolchildren
• Chinese researchers claim Sinovac booster improves immune response to COVID Delta variant

Wang Huaqing, chief immunization expert at the Chinese Centre for Disease Control and Prevention, said the vaccination of more than 1 billion people laid the foundation for controlling the epidemic, but Delta was challenging the herd immunity goal.

"In terms of populations, based on past experience, the vaccine has a greater effect if the vaccination rate is high ... but with the highly infectious Delta variant, we have seen a weakening of protection in individual vaccines, which poses a challenge in establishing herd immunity," Wang said.

The Chinese inoculation campaign has relied heavily on inactivated vaccines, which have been shown in various studies to be less potent against Delta and to wane as time passes.

Wang said China needed to take more action to meet the challenge, including strengthening surveillance of the virus and the disease, and reinforcing evaluations of vaccine efficacy. The immunization strategy against Covid-19 needed to be improved and "better" vaccines need to be developed to prevent morbidity.

"At this point in time, we cannot relax our vigilance," Wang said.

The Chinese health authorities aimed to have 1.1 billion people vaccinated by the end of October. By then key groups - the over-60s, those at high risk of infection and people who need to travel to countries with high infection rates - will also have received a booster shot.

The high vaccination rate is not expected to lead to immediate change in China's zero-tolerance strategy, characterized by strict quarantine, rigorous contact tracing and large-scale testing.

Zheng Zhongwei, a NHC official in charge of vaccine development, said earlier this month that those control measures would remain in place until the authorities were confident that its vaccination campaign had been effective enough to ease restrictions.

Wang from the China CDC urged eligible teenagers and the elderly to get vaccinated soon so that the vaccines could be "truly effective". Those under 12 years old should also be inoculated, he said.

China has approved the shots for children as young as three but vaccination started only last month for 12-17-year-olds.

Copyright (c) 2021. South China Morning Post Publishers Ltd. All rights reserved.