China became the country to have administered Covid-19 vaccinations to more people than any other this week, but health authorities will need to accelerate the rollout to meet a target to inoculate 40% of its population by the end of June.
China had administered a total of 243.91 million doses as of April 28, surpassing 234.6 million shots the United States has given.
But with population of 1.4 billion people, China has administered just 17.4 doses per 100 people, far behind the 71.1 administered in the United States, which has a population less than a quarter the size.
China has given around 4.4 million doses per day on average so far this month and the pace would need to pick up to at least 5.0 million doses to reach its end of June target.
Achieving that will test China's vaccine production capability, as some parts of the country are already grappling with tight supplies, a health official said. He said the supply crunch will ease "from May, especially after June" as production is being stepped up.
China's complete reliance on locally developed vaccines could complicate the country's immunisation drive, as insufficient data on their efficacy has been released, and they have shown modest efficacy so far.
China approved five domestically developed vaccines, and four of them reported efficacy rate of between 50.7% and 83.5% against symptomatic Covid-19 disease, lower than readings from rival shots developed by Moderna (MRNA.O), Pfizer (PFE.N) and its partner BioNTech .
The fifth Chinese vaccine, developed by a state microbiology research agency, has yet to release efficacy data, and Sinovac (SVA.O) is the only vaccine maker that has released detailed data so far.
"This is going to be a problem going forward in getting people to fully trust the vaccines," said Nicholas Thomas, professor in health security at the City University of Hong Kong.
While real world data showed some of the Chinese vaccines are effective in preventing symptomatic infection and hospitalisations, there is no sufficient data yet to decide how much these vaccines help reduce transmission.
Despite record inoculation numbers, it's hard to measure them with real world data in China, as the country has largely stamped out local transmission through strict quarantine measures and routine screening tests.
That leaves China to rely on data from coronavirus-hit countries that use its vaccines such as Chile, Brazil and Indonesia.
Wu Zunyou, chief epidemiologist at the Chinese Center For Disease Control and Prevention, said earlier this month that "whichever country gets the vaccination rate up to herd immunity first is likely to open up to the world".
Experts caution that many estimates on inoculation targets required to achieve herd immunity could be oversimplified in the absence of enough scientific data on the vaccines' ability to cut transmission. In general, they said using vaccines with modest efficacy would require more people to be inoculated.
Benjamin Cowling, an infectious disease expert from the University of Hong Kong, estimates China would likely need to around 100% vaccine coverage of the population to achieve herd immunity, using vaccines currently available to it.
But the rate could go down to 70-80% when it uses vaccines with efficacy of higher than 90%, Cowling said.
Chinese researchers are already testing various ways to boost their products' efficacy including a booster shot and a mix and match of different vaccines, while BioNTech expects its vaccine to be approved in China by July at the latest.
Cowling cautioned that herd immunity does not need to be the ultimate goal of a vaccination campaign.
"We could aim to have 100% vaccination coverage... Then everybody would have their individual protection against severe disease, and Covid-19 would not pose a public health threat even if it was able to circulate and cause mild infections," he said.