The World Health Organization’s struggles have opened the door for China to start ramping up its vaccine diplomacy, with Foreign Minister Wang Yi pledging last week to hand out more than a million doses during a swing through Southeast Asia.

For months, the World Health Organization (WHO) has called on countries to come together to ensure a fair distribution of coronavirus disease 2019 (COVID-19) vaccines among rich and poor nations. Now it’s starting to lose patience.

On Monday, WHO Director-General Tedros Adhanom Ghebreyesus said drug manufacturers had prioritized regulatory approval in rich countries, where profits are highest, rather than submitting full dossiers to get the greenlight from the global health body. He said that could delay distribution through COVAX, a WHO-backed initiative that aims to supply vaccines to poorer countries.

“The world is on the brink of a catastrophic moral failure,” Mr. Tedros said. “Even as they speak the language of equitable access, some countries and companies continue to prioritize bilateral deals—going around COVAX, driving up prices, and attempting to jump to the front of the queue. This is wrong.”

The WHO’s struggles have opened the door for China to start ramping up its vaccine diplomacy, with Foreign Minister Wang Yi pledging last week to hand out more than a million doses during a swing through Southeast Asia. That amounted to a geopolitical win just before the inauguration of President-elect Joseph R. Biden, Jr., who has vowed to put the US back in the WHO following Donald J. Trump’s withdrawal from the organization last year.

“China’s ‘mask diplomacy’ in 2020 is being followed in 2021 by ‘vaccine diplomacy,’” said Ian Storey, a senior fellow at the ISEAS-Yusof Ishak Institute. “The aims remain the same: to win friends and influence countries in Southeast Asia and bury the memory that the pandemic started in China a year ago.”

Antony Blinken, Mr. Biden’s pick for secretary of state, told lawmakers on Tuesday the US is preparing to join COVAX and look at “how we can help make sure the vaccine is equitably distributed.” Mr. Biden officially takes over on Wednesday in the US.

China’s vaccines have received some high-profile endorsements, with Indonesian President Joko Widodo receiving the Sinovac Biotech Ltd. shot on live television last week in the world’s fourth-most populous nation despite inconsistent efficacy data. Brazil also began distributing six million Sinovac doses on Monday—an about-face for President Jair Bolsonaro, who had been an outspoken critic of Chinese vaccines last year.

Cambodian Prime Minister Hun Sen, who last month said his country wouldn’t use any vaccines that weren’t approved by the WHO, last week reversed course and accepted one million vaccine doses from China. He cited widespread use in places like Indonesia, Egypt, and China, noting that Mr. Wang had received the vaccine and is still “in good health and can travel places.”

“For the need to defend our nation and protect our people from this deadly epidemic, we can no longer wait,” Hun Sen said in a message published in a cabinet newsletter on Friday. “We are reversing what I said last time about accepting only vaccines recognized by the World Health Organization.”

Because they lack regulatory bodies with the capacity to scrutinize scientific data, many developing countries have traditionally relied on the WHO’s list of approved vaccines to know which shots they can permit for local vaccination drives.

At the end of 2020, the Pfizer Inc.-BioNTech SE vaccine was the first, and so far only, shot to receive emergency validation from the WHO since the outbreak began a year ago. With no low-income countries producing their own vaccines, richer nations have secured 85% of Pfizer’s vaccine and all of Moderna Inc.’s, according to London-based research firm Airfinity Ltd.

While China has pledged to support the WHO’s efforts, its vaccines are not among those procured by COVAX. A spokesperson for Sinovac said the company has begun submitting data to the WHO for a pre-qualification of its coronavirus vaccine, known as CoronaVac. A group of WHO inspectors has also traveled to China and will inspect its production facilities after completing quarantine, the spokesperson said.

COVAX still plans to distribute two billion doses around the world by the end of this year, with enough to protect 3% of the population in all participating countries by July, according to an emailed response to questions. The facility has said it will consider procuring any candidate vaccine that meets global standards set by the WHO.

Among the 11 candidates that it can tap for distribution, two—Moderna Inc.’s shot and the one developed by AstraZeneca Plc and the University of Oxford—are ready for rollout and are being administered in countries like the US and UK. It’s unclear why COVAX has not yet started distributing those vaccines as well.

Mr. Tedros’s statements castigating companies for prioritizing rich countries where they can make the most profit indicates that the global health body sees the delay as stemming from the companies.

AstraZeneca said on Dec. 30 that it was seeking the WHO’s greenlight, known as the body’s Emergency Use Listing, “for an accelerated pathway to vaccine availability in low and middle-income countries.” A company spokesperson did not immediately respond to a request for comment on what stage the process is at.

COVAX’s rollout could begin “as early as February pending favorable regulatory outcomes and the readiness of health systems and national regulatory systems in individual participating economies,” said Iryna Mazur, a spokesperson at Gavi, the Vaccine Alliance, which is co-leading COVAX.

Thailand bought two million doses from Sinovac, and China promised to donate a total of 800,000 doses to the Philippines and Myanmar during Mr. Wang’s diplomatic push last week.

During a visit to Manila, Mr. Wang drew praise from Philippine officials after committing to completing China-funded infrastructure projects including a $400 million bridge and a $940 million cargo railway project.

Philippine President Rodrigo R. Duterte this week chided a group of senators who scrutinized the government’s plans to buy Sinovac, after previously threatening to terminate a Visiting Forces Agreement with the US if it failed to deliver at least 20 million vaccines immediately. “No vaccine, no stay here,” Mr. Duterte said last month of the military deal—a threat he has made before without following through on.

“Coronavirus vaccines have clearly become a political football in the increasing US-China cold war,” said Paul Chambers of Naresuan University’s Center of Asean Community Studies, who has researched geopolitics in Southeast Asia for about two decades. “The daunting delay in the launching of COVAX is exactly the opportunity that China is using to initiate and expand its supply of Sinovac to developing countries.” — Bloomberg