Last month, a shipment of Sinovac coronavirus vaccine doses arrived without fanfare in Singapore from China.

Today, the vaccine sits unused in a storage facility. The wealthy city-state is moving ahead with Pfizer-BioNTech and Moderna shots for its coronavirus immunization program, with officials saying Sinovac needs to provide more data before they will consider rolling out its doses.

The case highlights the limitations of Beijing’s vaccine diplomacy. China’s lack of transparency in its clinical trials has hurt public confidence, even as national leaders from Indonesia to Sierra Leone have gotten the shots to rally their populations to do the same.

China’s coronavirus vaccine makers Sinovac and Sinopharm were among the earliest in the world to begin clinical trials last year. It remains unclear why they still have not published the data from the studies, even after dozens of governments have greenlighted their vaccines for emergency use.

“It’s extremely unusual,” said Peter English, a British expert in communicable-disease control, of the widespread use of these vaccines before the publication of peer-reviewed data. “It leaves a lot of questions.”

There have been signs the Sinovac and Sinopharm vaccines are less effective than hoped, even as Beijing is pushing for foreigners to use them and enjoy streamlined access to resume travel to China. This week, Sinopharm’s distributor in the United Arab Emirates said a “very small number” of people are being invited to take a third dose of the vaccine after insufficient antibody response from the first two doses.

State-owned pharma giant Sinopharm has self-reported a 79 percent efficacy rate. Smaller rival Sinovac’s efficacy rate has varied in trials from 50.4 percent in Brazil — barely above the 50 percent threshold that governments find usable — to over 80 percent in Turkey.

The Chinese Center for Disease Control and Prevention, Sinopharm and Sinovac did not respond to requests for comment.

China’s coronavirus vaccines remain oversubscribed by developing countries, as wealthier nations hoard the most effective vaccines made by U.S. biotech company Moderna and U.S. pharmaceutical firm Pfizer with its German partner BioNTech.

In Singapore’s case, the government has the luxury of a small population of 5.7 million people to immunize and more effective vaccines at hand.

Chong Ja Ian, an associate professor of political science at the National University of Singapore, said Singapore’s government did not turn down the Sinovac vaccine, which would be an affront to Beijing, but regulators also couldn’t approve its use on such limited data.

“Singapore has options, unlike some of the countries who have received Sinovac,” Chong said.

Other countries with larger populations and fewer options have accepted China’s vaccines, sometimes after considerable griping from top leaders.

Brazil, South America’s most populous country, has adopted Sinovac’s vaccine after initial resistance from President Jair Bolsonaro. The Philippines accepted a donation of Sinovac vaccines from China last month, despite President Rodrigo Duterte saying he personally wants to take a different Chinese vaccine, according to Reuters.

Vaccine makers usually release details of their Phase 3 clinical trials in peer-reviewed journals before the vaccines gain regulatory approval. Pfizer-BioNTech and Moderna published theirs in the New England Journal of Medicine in December.

Sinopharm and Sinovac have self-reported some key results, but they have not published the underlying data in a journal, which would require vetting by third-party experts.

Chinese government officials and industry executives have largely deflected questions about when they will release the data. In an interview with state-run tabloid Global Times this month, Shao Yiming, a vaccine expert with China’s CDC, claimed that the countries where Sinovac and Sinopharm ran trials — including Brazil and the UAE — must be the ones to release it.

“Whether to release the clinical trial data, when and how, must be decided by the foreign institutions,” Shao said. “China has no power to decide.”

At a Beijing news conference on March 15, health regulators ignored a question on when data will be released, according to the transcript. Sinopharm and Sinovac executives also have not addressed the question.

More than 60 countries have approved one of China’s coronavirus vaccines for market use or emergency use, according to the China International Development Cooperation Agency.

Malaysia began using Sinovac’s vaccine this month, after kicking off its vaccination drive with limited supplies of vaccines from Pfizer-BioNTech and British-Swedish firm AstraZeneca, which developed a shot with Oxford University. Malaysia’s science minister, Khairy Jamaluddin, took the first shot of the Sinovac vaccine to shore up confidence.

“I want to say, it is safe,” he wrote in Chinese on Twitter after receiving the first dose. “I feel great. And my Chinese has suddenly gotten better.”

Chinese leader Xi Jinping has yet to publicly take a vaccine. Sinopharm said this month more than 5,000 senior officials attending the country’s annual legislative session had been vaccinated.

In Hong Kong, vaccine mistrust has jumped after seven people died following Sinovac doses and one after receiving the Pfizer-BioNTech vaccine, although no causation has been established.

The Chinese territory has expanded its vaccine program to include anyone over 30, who can choose from either the Pfizer-Bio­NTech vaccine or Sinovac’s, after priority groups were reluctant to take up the shots. That is a situation Singapore is trying to avoid, experts say, as it could delay the reopening of borders and the economy.

English, the communicable-disease expert, said vaccine makers typically publish clinical trial results transparently to build public trust. This is especially critical during a pandemic, where public indecision about whether to get a vaccine costs lives.

Singapore’s ministry of health said that it received its first shipment of Sinovac doses on Feb. 23. The country’s Health Sciences Authority said this week it has begun reviewing the data that Sinovac had submitted, but it needs more information from the company.

Regulators are “still waiting for the company to submit the data,” the Health Sciences Authority said.

Alicia Chen in Taipei, Taiwan, and Lyric Li in Seoul contributed to this report.