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In recent weeks, a string of vaccine makers including Pfizer, Moderna, and AstraZeneca have released promising phase III trial data, injecting the world with hope that the COVID-19 pandemic will come to an end.

On Wednesday, the U.K. became the first western country to approve a vaccine for COVID-19, authorizing Pfizer to distribute its vaccine on an emergency basis to frontline medical workers and other high-risk groups. The U.S. and the European Union are expected to follow suit in coming days or weeks and give vaccine makers the green light to start distributing their candidates.

But amid the optimism, experts have raised some concerns about the accessibility of Pfizer and Moderna's candidates, which require costly cold-chain distribution mechanisms, and about the quality of the data provided by British vaccine maker AstraZeneca.

At the same time, China's two leading vaccine makers—the private Sinovac and state-owned Sinopharm—appear close to rolling out their vaccines in coming weeks with candidates that don't require expensive cold chain networks.

Western vaccine makers appear to have taken the lead in the global race to produce a vaccine, but Chinese vaccine makers may help fill in the gaps they leave behind.

Data limits

In mid-November, American firms Pfizer and Moderna each announced preliminary trial results for their experimental vaccines indicating that they were over 90% effective in preventing infections of COVID-19. The results far exceeded expectations, given that the U.S. Food and Drug Administration said it would require a vaccine to be just 50% effective or better to gain approval.

Still, Pfizer and Moderna’s vaccines are based on newly-developed mRNA technology that requires the candidates to be kept at sub-zero temperatures during shipments. Such temperature restrictions will make it difficult for lower- and middle-income countries to access the vaccines because they will need cold-storage infrastructure to distribute the doses.

In Indonesia, for example, the country's state-owned vaccine manufacturer Bio Farma said that Indonesia, the world's fourth-most populous nation, effectively cannot purchase Pfizer's vaccine given the logistical challenges in distributing it.

"Indonesia does not have such [cold-chain] capabilities, and it is hazardous if this vaccine is not stored at the proper temperature," Honesti Basyir, head Bio Farma, told Indonesian news outlet the Jakarta Globe on Nov. 22. In India, the country's 28,000-unit cold chain network used for distributing vaccines only handles vaccines between 2 and 8 degrees Celsius.

Pfizer’s vaccine, which was originally developed by the German firm BioNTech, must be shipped at -70 Celsius (-94 Fahrenheit), colder than average winter temperatures at the South Pole. Moderna’s needs to be kept at -20 Celsius (-4 Fahrenheit), roughly equivalent to the temperature of a home freezer.

Pfizer has developed its own shipping containers that keep its vaccines at -70 Celsius for up to ten days. Otherwise, keeping the vaccines cold for a longer period of time requires an extensive network of airport freezer warehouses, refrigerated trucks, and on-site freezers that can cost up to $10,000.

These freezer-chain fears partly dissipated when, weeks later, British pharmaceutical firm AstraZeneca announced that its vaccine candidate was also up to 90% effective in phase III trial results. AstraZeneca's vaccine candidate, based on adenoviral technology, which induces an immune response by infecting cells with a weakened form of a live virus, only needs to be kept between 2 and 8 degrees Celsius (36 to 47 degrees Fahrenheit), roughly the temperature of a home refrigerator, making it easier to distribute.

But in the days after AstraZeneca’s announcement, experts raised questions over the "transparency and rigor" of AstraZeneca’s data. Scientists questioned why the most effective results emerged from an apparent dosing error, and how datasets from multiple countries were combined and tabulated into the final results. In response, a spokesperson for AstraZeneca said its trials were conducted to the "highest standards." Still, amid the questions over its trial data, AstraZeneca CEO Pascal Soriot pledged that the company would conduct an additional study to validate its results

AstraZeneca’s woes and the potential distribution limitations of the Pfizer and Moderna vaccines have left a hole in the global supply that may be filled by Chinese vaccine makers, says Yanzhong Huang, senior fellow for global health at the Council on Foreign Relations.

Sinovac and Sinopharm, China’s two leading vaccine makers, are currently testing three vaccine candidates in phase III trials with candidates that could be distributed without cold storage chains. Sinovac's candidate, called CoronaVac, uses inactivated forms of COVID-19 to induce immune responses. Sinopharm's two unnamed candidates are both based on similar technology.

Beijing "absolutely" sees these Chinese-made vaccines as a potential substitution for AstraZeneca's candidate, says Huang. In state media outlets, Beijing is presenting Sinovac and Sinopharm's candidates as "particularly attractive to lower and middle income countries, especially the ones who cannot afford or don't have the capacity to sustain cold chain to distribute vaccines," Huang says.

Because they rely on inactivated forms of the virus, Sinovac's and Sinopharm's candidates are more stable in comparison to vaccines based on mRNA technology, meaning they don't need to be kept quite as cold. Similar to AstraZeneca, Sinovac says its vaccine can be stored and shipped at temperatures ranging from 2 to 8 degrees Celsius. Chinese state media reports that Sinopharm's candidates can likely be stored at a similar 2 to 8 degree Celsius range. Sinopharm declined Fortune's request for confirmation.

Sinovac and Sinopharm

Sinovac and Sinopharm have both recently indicated that they are nearing the finish line in the vaccine development process.

Sinovac, which is conducting large-scale clinical trials in Brazil and Indonesia, said last week that it expected to release clinical data from its CoronaVac candidate in Brazil in a matter of days. The Butantan Institute, a vaccine research center and Sinovac's partner in Brazil, told reporters on Thursday that it expects Sinovac to publish clinical data before Dec. 15. Joao Doria, governor of Brazil's Sao Paolo state, also said Thursday he aims to begin distributing CoronaVac by January. Sinovac did not respond to Fortune's request for comment on its trial data.

Sinovac published results of its phase II trial in mid-November that suggest CoronaVac induced immune responses among volunteers and may offer COVID-19 protection. Researchers noted, however, that Pfizer and Moderna's vaccines induced stronger immune responses.

Meanwhile, on Nov. 25, Chinese state-owned vaccine maker Sinopharm asked regulatory authorities in China to officially make its vaccine available to the Chinese public. In an interview with state-run news outlet Xinhua, a Sinopharm executive did not specify when the company expects to receive a response from Chinese regulators.

Sinopharm has not released any data to the public on its phase III trials, but says it has already distributed the vaccine to nearly a million people in China under the country’s controversial emergency-use program.

Of the two drugmakers' candidates, Sinovac's CoronaVac is likely China’s most "promising" vaccine, with the best chances of getting approved and distributed internationally because Sinovac has released more data, says Huang.

Authorities in Brazil, Turkey, and Indonesia have already struck deals with Sinovac to purchase tens of millions of doses of CoronaVac if phase III trials prove successful.

Admiral Craig Fuller, the U.S.'s top commander in Central and South America, told reporters this week that while the U.S. is focused "on taking care of the U.S. first," China is poised to deploy its vaccines around the world.

Still, Sinopharm may have a built-in edge given its established distribution networks within China and the fact that it is testing its vaccine in a broader array of countries than other Chinese vaccine makers.

"[Sinopharm's] large scale rollout [via China's emergency use program] also means that there is already a large distribution system, which has already been stress-tested," says Nicholas Thomas, a vaccine expert and health governance professor at the City University of Hong Kong. "This is a major advantage in the future distribution." Sinopharm is also already distributing vaccines to the United Arab Emirates via that country's emergency use program, and is testing, or plans to conduct trials, in at least ten countries including Egypt, Jordan, and Argentina.

Home stretch

The U.S.'s apparent success in producing effective COVID-19 vaccines has turned the global race to develop an immunization for the disease into an all-out sprint. Globally, China does not want to be seen as losing in a scientific competition with its chief rival, the U.S. Domestically, China is under pressure to quickly roll out vaccines so it can loosen travel restrictions and cease the cycle of sporadic lockdowns when new outbreaks arise.

"If the U.S. manages to mass produce their vaccines for their populations by say, May, it would make China look bad if they don’t have their population vaccinated," says Huang. "They have strong incentives to get vaccines to their populations as soon as they can."

If and when Sinovac or Sinopharm's vaccines come to the market, Thomas says, the next big phase in the vaccine race will be a bifurcation of nations based on which vaccines they seek, China's or ones made by Western countries.

"There is a significant commercial advantage to be gained if the public prefers one manufacturer over another," says Thomas. "The competition at the global level is only just beginning."