Gao’s comments conflicted with China’s state narrative about its homegrown vaccines, which top Chinese officials have long touted as highly effective and powerful tools in bringing an end to the COVID-19 pandemic.

Gao later backtracked, telling the Chinese media outlet the Global Times that reports of him criticizing the effectiveness of Chinese vaccines were due to “a complete misunderstanding.” Rather, he said, he was issuing a global call for vaccine makers around the world to explore new ways to boost vaccine efficacy.

“I suggest that we can consider adjusting the vaccination process, such as the number of doses and intervals and adopting sequential vaccination with different types of vaccines,” Gao told the Global Times.

The “sequential vaccination” technique, or mixing doses from two different vaccines, is one already being explored in clinical trials by global COVID-19 vaccine makers, including those in China. Gao did not comment on the trials and did not clarify if China’s government would recommend mixing different vaccine doses without conducting trials first.

Mixing vaccine doses may offer China a chance to increase the efficacy of the vaccines it is distributing at home, which could boost its drive toward herd immunity. At the same time, any effort to mix doses may be thwarted by China’s own reluctance to approve the proven COVID-19 shot from German maker BioNTech.

The herd immunity challenge

China’s government reported Sunday that it has distributed 165 million vaccine doses to its citizens and is now second only to the U.S.’s 187 million doses in terms of total shots delivered.

But China still faces a long road to achieving herd immunity through vaccines, which may require vaccinating upwards of 1 billion people.

The efforts are complicated by Chinese-made vaccines offering less protection against COVID-19 than those from drugmakers like Pfizer and Moderna that are in use in places like the U.S. Pfizer, which developed its vaccine in partnership with Germany’s BioNTech and China’s Fosun Pharma, says the jab is 95% effective in preventing COVID-19 infections, while Moderna’s vaccine is 94% effective in protecting against the virus.

China’s five approved vaccines all fail to match those high standards.

Private firm Sinovac is the only Chinese COVID vaccine maker to publish Phase III clinical data so far. The data, released over the weekend, has not been peer-reviewed but says that Sinovac’s vaccine is 50.34% effective in preventing symptomatic COVID-19 infections.

State-owned firm Sinopharm has two vaccines approved in the Chinese market. Sinopharm reports that the two vaccines are 79% and 73% effective against COVID-19 but hasn’t released data to back up the claims.

There are also some indications that Sinopharm’s vaccine may offer only limited protection against the virus. The United Arab Emirates, which has relied on Sinopharm’s 79% effective jab in its campaign, is reportedly offering a third dose of the Sinopharm vaccine to some people in order to boost the vaccine’s efficacy. UAE officials had found that two jabs of the vaccine did not induce a strong enough immune response among some people who received the vaccine.

China has also approved a one-shot vaccine from private maker CanSino Biologics that is 65.7% effective in preventing COVID-19 infections, and another three-dose vaccine from Anhui Zhifei Longcom with unknown efficacy against COVID-19.

Mixed trials

One mixed trial involving a Chinese vaccine is already underway on Chinese soil in Hong Kong.

In March, Hong Kong University’s Department of Medicine announced that it was recruiting participants for a trial that would mix doses of China’s Sinovac COVID-19 vaccine and Germany’s BioNTech vaccine.

The trial, which will involve roughly 150 volunteers over the age of 18, will give participants four different vaccine regimens. One group will get a BioNTech dose followed by the Sinovac dose, while other groups will get full regimens of either BioNTech’s vaccine, Sinovac’s shot, or placebo jabs.

Dr. Ivan Hung, a professor at Hong Kong University and adviser to the city’s COVID-19 response, said in late March that the research will be important to determine if mixing the vaccine jabs boosts immunity against COVID-19. But he said that because of a lack of clinical data he does not currently recommend that Hong Kong mix vaccine doses.

The outcome of the trial may have implications for China.

China has secured 100 million doses of BioNTech’s mRNA vaccine, but regulators have yet to approve it for distribution within China even though the shot has been approved in Hong Kong and dozens of other countries around the world.

Experts told Fortune in March that China may be delaying its approval due to a preference for China-made jabs.

But Gao’s comments hint that China’s attitude toward mRNA vaccines like BioNTech’s may be changing.

On Saturday, Gao told the conference that China should “consider the benefits” of mRNA vaccines and “not ignore” them.

Other mixed-vaccine trials are also underway in other parts of the world using different types of vaccines.

China’s CanSino Biologics is reportedly involved in discussions with Russia’s Sputnik 5 vaccine makers to conduct a two-dose mixed trial using CanSino’s one-shot adenovirus shot and Sputnik’s viral vector jab. In the U.K., AstraZeneca and BioNTech also launched the world’s first mixed-vaccine trial involving over 800 volunteers.

China, at least, appears to be excited about the possibilities such trials represent.

“It’s now under formal consideration whether we should use different vaccines from different technical lines for the immunization process,” Gao said Saturday.