Across Asia, Chinese vaccines have played a crucial role in immunising people against Covid-19, with millions receiving either a Sinovac or Sinopharm jab.
By Tessa Wong
BBC News
But in recent weeks, concerns have grown about their efficacy. Now, some Asian countries which made Chinese vaccines a key plank in their immunisation programmes have announced they will use other jabs.
The move has raised questions, not only about whether China's vaccines can be trusted, but also about its attempts at vaccine diplomacy in Asia.
What's happening in Thailand and Indonesia?
Last week, Thailand announced it was changing its vaccine policy - instead of receiving two Sinovac shots, residents will now get a mix of Sinovac and AstraZeneca.
Healthcare workers who are already fully vaccinated with Sinovac will also get a different jab as a booster shot.
Indonesia announced a similar move the previous week, saying it was giving Moderna booster shots to healthcare workers immunised with Sinovac.
The decisions followed reports that hundreds of fully vaccinated healthcare workers had caught Covid, with some of them - two in Thailand and 30 in Indonesia - dying.
Both countries, which have seen slow rollouts of their vaccination programmes, have been battling new outbreaks. Thailand is now reporting record high numbers of infections and deaths, while Indonesia - the new epicentre of Covid in Asia - has seen overcrowded hospitals and oxygen shortages.
The two countries said they were making the switch to increase protection, and Thai officials cited local studies which showed mixing vaccines could boost immunity.
Indonesia's tourism minister Sandiaga Uno also recently told the BBC the Sinovac vaccine was "quite effective".
But by choosing to switch vaccines, the Thai and Indonesian governments were essentially "saying they are concerned about vaccine failure", said Dale Fisher, who heads the World Health Organization's Global Outbreak Alert and Response Network.
However, he also cautioned that there was not enough information about the healthcare workers' infections and deaths, and urged authorities to conduct a "thorough investigation".
Sinovac has yet to comment.
Since then, Malaysia announced it was switching to Pfizer's vaccine after it finished its Sinovac supplies.
But other countries like the Philippines and Cambodia are continuing to use Chinese vaccines.
Are the Chinese vaccines effective?
In clinical trials across the world, Sinovac and Sinopharm's inactivated virus vaccines have been shown to be 50% to 79% effective in preventing symptomatic Covid infection.
But they are still highly effective in preventing Covid hospitalisations or deaths - studies found Sinovac's jab was 100% effective in Brazil and 96 to 98% effective among Indonesian medical workers.
The fact that there are still numerous breakthrough infections in fully vaccinated people could be due to several factors, says epidemiologist Professor Benjamin Cowling, of Hong Kong University.
One is that the Chinese vaccines, like many other vaccines, may wane in efficacy over time. A Thai study released this week found that antibodies in those fully vaccinated with Sinovac decline by half every 40 days.
Another is that the clinical trials had smaller datasets compared to real-world infections, particularly in Indonesia which is seeing soaring daily infection numbers in the tens of thousands.
It could also be due to the more infectious Delta variant, which has been detected in 60% of recent cases in Indonesia and 26% of cases in Thailand's capital Bangkok.
There is no public data yet on the Chinese vaccines' efficacy against any of the Covid variants. But preliminary studies have suggested that inactivated virus vaccines, like Sinopharm and Sinovac's, could offer 20% less protection against the Delta variant than against the original virus, according to Prof Cowling.
No vaccine is fully effective in preventing Covid infection, he said, and while the Chinese vaccines "are not 100% effective, they are still saving many lives". Experts stress that breakthrough infections do not mean vaccines are pointless, as immunisation helps to stop people from getting very sick with Covid-19.
There are also no reports yet of breakthrough infections in China, where more than 630 million have taken at least one shot of a Chinese vaccine. It is not known how many of them are fully vaccinated.
But the virus is thought to be controlled in China, which is reporting low daily infection rates and has moved swiftly to stamp out local outbreaks.
How does this affect China's vaccine diplomacy?
As the region that's received the most number of Chinese shots, Asia has been a key linchpin in China's vaccine diplomacy strategy.
More than 30 Asian countries have bought jabs or received donated shots. Indonesia is one of the biggest buyers of Sinovac vaccines in the world having ordered 125 million doses.
China's eagerness to sell or donate vaccines has been "an effort to change the narrative away from the fact that infections were first detected in Wuhan, and to show that it's a scientific powerhouse", said China expert Ian Chong, of the National University of Singapore.
With richer countries monopolising many of the early orders of other vaccines, many countries in Asia - particularly poorer ones - welcomed the Chinese jabs.
"The standard thinking was that 'some protection is better than no protection', even though at the time efficacy data wasn't great," said Dr Chong.
Thailand, for instance, had initially counted on a local firm owned by the king to produce the bulk of its vaccines, but the slow delivery timeframe forced the government to seek other sources after fresh Covid outbreaks this year.
Besides locally manufactured AstraZeneca vaccines, it has ended up relying mostly on Sinovac's jab for now, because the Chinese firm was one of the first to deliver.
Thailand and Indonesia's decisions to switch to other vaccines "could potentially puncture the image of success, bursting the bubble of effectiveness of Chinese vaccines, and in effect calls into question the technical prowess of China," said Dr Chong.
The Chinese government has yet to comment, but in the past has insisted that its vaccines are effective.
How is the public reacting?
Both Thailand and Indonesia's governments are facing mounting criticism over their slow vaccination rollouts and worsening Covid situations.
In Thailand, the outrage has been further fuelled by a leaked health ministry document quoting an official who opposed giving medical workers a Pfizer booster shot as it would be "an admission that Sinovac can't give protection".
"There is a lot of anger among the Thai public, they're saying 'why don't you care about healthcare workers', 'this should not be a factor'. Many people have deep concerns about the government's communications and reliance on Sinovac," said Dr Arm Tungnirun, director of the Chinese Studies Center at Chulalongkorn University.
"Right now there is an increasing number of people who reject Sinovac, who believe it's not effective. There's a big distrust in the Thai government, and the vaccine issue has become heavily politicised."
On Sunday, hundreds of protesters marched in Bangkok calling for the prime minister's resignation over his handling of the crisis, and also demanded that mRNA vaccines such as Pfizer or Moderna's be brought in .
There are fears that the latest reports of breakthrough infections will fuel overall scepticism in vaccines. In Indonesia, religious social media influencers and conspiracy theorists have already been spreading anti-vaccine messages laced with anti-Chinese sentiment.
Experts are urging tighter infection controls and greater efforts in combating misinformation online.
Says Prof Cowling: "It's great that we are using [the Chinese vaccines] but we can't expect too much of them.
"We have to recognise that there will be breakthrough infections and be ready to deal with them, because they can damage confidence in the vaccines."
