SINGAPORE (NYTIMES) - The arrival of the Chinese vaccines was supposed to help stop the spread of the coronavirus in South-east Asia.

Instead, countries across the region are quickly turning elsewhere to look for shots.

Residents in Thailand vaccinated with one dose of China's Sinovac are now given the AstraZeneca shot three to four weeks later. In Indonesia, officials are administering the Moderna vaccine as a booster to healthcare workers who had received two doses of Sinovac.

Malaysia's health minister said the country would stop using Sinovac once its supply ran out. Even Cambodia, one of China's strongest allies, has started using AstraZeneca as a booster for front-line workers who had taken Chinese vaccines.

Few places benefited from China's vaccine diplomacy as much as South-east Asia, a region of more than 650 million that has struggled to secure doses from Western drugmakers. Several of these countries have recorded some of the fastest-growing number of cases in the world, underscoring the desperate need for inoculations.

China, eager to build goodwill, stepped in, promising to provide more than 255 million doses, according to Bridge Consulting, a Beijing-based research company.

Half a year in, however, that campaign has lost some of its lustre. Officials in several countries have raised doubts about the efficacy of Chinese vaccines, especially against the more transmissible Delta variant. Indonesia, which was early to accept Chinese shots, was recently the epicentre of the virus. Others have complained about the conditions that accompanied Chinese donations or sales.

The setback to China's vaccine campaign has created a diplomatic opening for the United States when relations between the two countries are increasingly fraught, in part because of the coronavirus.

China has criticised the US' handling of the crisis at home and even claimed, with no evidence, that the pandemic originated in a military lab at Fort Detrick, Maryland, not in Wuhan, China, where the first cases emerged in late 2019.

As more countries turn away from Chinese shots, vaccine aid from the United States offers an opportunity to restore relations in a region that US officials have mostly ignored for years while China extended its influence. The Joe Biden administration has dispatched a crowd of senior officials, including Vice-President Kamala Harris, who is scheduled to arrive on Sunday to visit Singapore and Vietnam.

It has also, at last, made its own vaccine pledges to South-east Asia, emphasising that the US contribution of roughly 23 million shots comes as of this week with "no strings attached", an implicit reference to China.

Several countries in the region have been eager to receive the more effective Western doses. Although these remain far outnumbered by Chinese shots, they present an attractive alternative.

China's "early head-start advantage has lost its magic already", said Ms Hoang Thi Ha, a researcher with the Asean studies centre of the ISEAS - Yusof Ishak Institute in Singapore.

For most of the year, many developing countries in South-east Asia did not have much of a choice when it came to vaccines. They struggled to acquire doses, many of which were being made by richer nations that have been accused of hoarding them.

China sought to fill those needs. Chinese Foreign Minister Wang Yi travelled through the region in January, promising to help fight the pandemic. In April, he declared that South-east Asia was a priority for Beijing. About one-third of the 33 million doses that China has distributed free worldwide were sent to the region, according to the figures provided by Bridge Consulting.

Much of Beijing's focus has been on the more populous countries, such as Indonesia and the Philippines, and its longstanding allies, like Cambodia and Laos.

Indonesia was China's biggest customer in the region, buying 125 million doses from Sinovac. The Philippines obtained 25 million Sinovac shots after its president, Mr Rodrigo Duterte, said he had turned to Mr Xi Jinping, China's top leader, for help. Cambodia received more than 2.2 million of China's Sinopharm doses. It has inoculated roughly 41 per cent of its population, achieving the second-highest vaccination rate in the region, after Singapore.

Then, signs started emerging that the Chinese vaccines were not as effective as hoped. Indonesia found that 10 per cent of its healthcare workers had become infected with Covid-19 as at July, despite being fully vaccinated with the Sinovac shot, according to the Indonesian Hospital Association.

In July, a virus expert at Chulalongkorn University in Bangkok said a study of people who had received two doses of the Sinovac vaccine showed that their level of antibodies, 70 per cent, was "barely efficacious" against the Alpha variant of the coronavirus, first detected in Britain, or against the Delta variant, first detected in India.

The governments in both Indonesia and Thailand decided that they had to make a switch to other vaccines, like those provided by the United States, Britain and Russia.

"Now that they have more choices, they can make other decisions," said Ms Nadege Rolland, senior fellow at the National Bureau of Asian Research in Washington. "I don't think it's politically motivated. I think it's pragmatic."

Ms Yaowares Wasuwat, a noodle seller in Thailand's Bangsaen Chonburi province, said that she hoped to get the AstraZeneca vaccine for her second shot after being inoculated with Sinovac, but that she would take whatever was available.

"I have nothing to lose," she said. "The economy is so bad; we are gasping for air. It's like dying while living, so just take whatever protection we can."

China's early moves in the region stand in marked contrast with the United States, which was slow to provide assistance.

The calculus has now changed under President Biden. Both Secretary of Defence Lloyd Austin and Secretary of State Antony Blinken had meetings with top officials in South-east Asia in recent weeks. They noted the donations of roughly 20 million shots.

After Mr Austin visited the Philippines, Manila restored a defence agreement that had been stuck in limbo for more than a year after Mr Duterte threatened to terminate it. The agreement, which would continue to allow US troops and equipment to be moved in and out of the Philippines, could thwart China's goal to push the US military out of the region.

Part of the reason for Mr Duterte's turnaround: the delivery of millions of doses of Johnson & Johnson and Moderna vaccines.

Still, some South-east Asian analysts have misgivings about Washington's belated vaccine diplomacy.

"The fact remains that the US was really slow off the bat," said Ms Elina Noor, director of political-security affairs at the Asia Society Policy Institute. "And given that rich countries were hoarding vaccines when they became available, I think that sour taste still lingers."

China continues to be seen to be a reliable supplier for the vaccines it has produced. It has delivered 86 per cent of the doses that it has promised to sell. And there remain concerns that the American companies have been slow to make deliveries. For those reasons, most South-east Asian countries have not openly criticised China - and have not abandoned Chinese vaccines.

Anti-China sentiment runs high in Vietnam, but the country accepted a donation of 500,000 doses of Sinopharm in June, causing a backlash among citizens who said they did not trust the quality of Chinese shots.

"Even right in the middle of this emergency, I have no reason to trade my life or my family's for a Chinese vaccine," said Mr Nguyen Hoang Vy, a manager for healthcare operations at a hospital in the city of Ho Chi Minh.

It later emerged that the donated Sinopharm shots were meant for priority groups outlined by Beijing, deepening the cynicism towards China.

"There are always some conditions attached," said Dr Huong Le Thu, a senior analyst at the Australian Strategic Policy Institute who specialises in South-east Asia, referring to China's vaccine deals.

Vietnam continues to battle an outbreak, and vaccines remain in short supply. Despite the earlier public anger, a private Vietnamese company acquired five million doses of Sinopharm for distribution, which the local authorities began to administer this month.