The HKU study published Tuesday examined how the two COVID vaccines deployed in Hong Kong held up during one of the world’s deadliest COVID-19 waves.

The study found that three doses of the vaccine from private Chinese vaccine maker Sinovac were 98.3% effective in preventing COVID deaths in people over the age of 60. By comparison, BioNTech was 98.1% effective against death after three doses among the same age group. While the two vaccines were evenly matched after three doses, the third dose is key: after two doses, Sinovac was just 77.4% effective against deaths in people over 60; two doses of BioNTech protected against 92.3% of deaths.

HKU’s study was based on data from the city’s ongoing fifth wave of COVID, from the beginning of this year through March 8. The outbreak, Hong Kong’s worst, has infected 1 million and killed nearly 6,000. The surge in COVID cases has been especially deadly among Hong Kong’s elderly population, which has an alarming low vaccination rate for a wealthy territory with a ready supply of jabs. Just 39% of people above age 80 have been fully vaccinated. As of Monday, of the 5,906 people killed in the wave, 4,182 are Hong Kongers age 80 and up.

The HKU study was the first to break down Hong Kong’s hospitalizations and deaths by vaccination type and number of doses, providing a clearer picture of how the vaccines performed against one another and how inoculated people fared compared with the unvaccinated.

“The top line message is get triple jabbed,” said Gabriel Leung, Hong Kong University’s dean of medicine said in a press conference on Tuesday. “It really does not matter which [vaccine] it is for severe or fatal outcomes.”

The effectiveness of three Sinovac jabs against deaths will be welcome news in mainland China, which has relied on the Sinovac jab, as well as a similar vaccine from state-owned Sinopharm, for its own vaccine campaign.

Mainland China continues to impose a strict “dynamic COVID zero” strategy of stamping out all infections, and the costs of its approach are rising as the country battles its largest COVID wave since early 2020. On Tuesday, China recorded 4,770 new COVID-19 infections, more than double the cases from Monday. Tens of millions of people across the country are under lockdown to contain the spread of the virus. Chinese authorities also shut down factories and key transportation routes in Shenzhen, a manufacturing hub in southern China, for a week.

Experts have long assumed that China would need to approve or develop an mRNA vaccine to relax its strict virus control measures and reopen. In January, the European Chamber of Commerce in Hong Kong predicted that the most likely scenario for China to reemerge from COVID isolation would be if China developed its own mRNA vaccine and distributed the jab to its 1.4 billion citizens, according to Bloomberg.

Beijing has blocked BioNTech’s mRNA vaccine from being sold in China for over a year even though the jab is highly effective and BioNTech’s Chinese partner Fosun Pharma has applied for distribution approval. China’s failure to approve BioNTech’s jab has puzzled experts since early trials showed that BioNTech’s vaccine was 95% effective in stopping COVID-19 infections compared with Sinovac’s 50% efficacy. Domestic vaccine makers have tried to fill China’s mRNA gap, but a homegrown mRNA jab is still a distant goal for Beijing.

China will likely battle a surge of infections whenever it eases COVID measures, and experts believed mRNA vaccines would better arm China against the worst outcomes of the inevitable COVID outbreak.

The HKU study undercuts those assumptions to some extent.

It “may be the case” that Sinovac’s high efficacy against death in Hong Kong will convince the mainland that it does not need mRNA vaccines to reopen, says Zhengming Chen, an epidemiologist at Oxford University. Instead, it must ensure that its population is triple-jabbed, which is no small task. Zeng Yixin, a vice minister of the National Health Commission, said last week that 20% of people in China above age 80 have been boosted, compared with 48% of people over 70 and 56% of people over 60.

Chen cautioned that more detailed data on how Sinovac and BioNTech performed in Hong Kong are needed before fully reconsidering China’s reopening prospects; the HKU study only included two age brackets: age 20 to 60 and over 60.

What’s more, past studies suggest Sinovac’s COVID jab loses efficacy faster than BioNTech’s vaccine, as Ben Cowling, an epidemiologist at Hong Kong University, pointed out on Twitter. Waning immunity for those double-vaxxed and boosted with Sinovac could pose a risk to China’s reopening efforts, as only those recently boosted would be better protected in the event of a large-scale outbreak.

Ultimately, China’s lack of an mRNA vaccine is still likely to slow its reopening efforts because COVID-related deaths aren’t the only concern.

“China’s reopening calculation is about disruption due to sickness and the perception of a circulating health threat,” says Nicholas Thomas, a professor in global health governance at the City University of Hong Kong. “And it is in preventing illness that Pfizer and other mRNA-based vaccines still have a critical edge.”

An even bigger risk at the moment is that—like Hong Kong—so many of China’s elderly have had no jab at all.

Last week, Zeng said in a press conference that only half of China’s population above the age of 80 had been fully vaccinated.

“In China, a large proportion of the elderly and those with underlying diseases are not fully vaccinated due to fear of the side effects,” Zhang Wenhong, a top epidemiologist in China, said in a post on Chinese social media last week. “If widespread infection were to occur, there will be hell to pay.”