On Thursday, Hong Kong recorded 164 cases of COVID-19, the most infections in one day since the beginning of the pandemic.

“Most of the infected cases are Omicron,” Carrie Lam, Hong Kong’s chief executive, said in a press conference on Thursday. “We are still in a severe stage of the pandemic, and there’s been major outbreaks in the community.”

The record infections come as the city doubles down on its COVID-zero, or “dynamic zero,” strategy that does not tolerate even one infection within its borders. Last year, Hong Kong went seven months without recording community spread of COVID-19. That streak came to an end on Dec. 31, pushing the city to impose some of its harshest social distancing restrictions since the beginning of the pandemic. Since Jan. 5, Hong Kong has closed all sporting and entertainment venues; shut down all bars, restaurants, and shops after 6 p.m.; placed thousands of people into government-run quarantine camps; locked down crowded apartment complexes; banned all flights from eight nations; and culled thousands of pet hamsters that were suspected of transmitting the virus.

But the measures have not appeared to hold up against the Omicron BA.2 substrain, which is often dubbed “stealth Omicron” due to a now-debunked theory that it was harder to spot on PCR tests. The variant has rapidly spread in individual apartment buildings, infecting hundreds of people living in the same block and prompting authorities to seal residents in the complexes for up to a week. Scientists believe the substrain could be as much as 30% more transmissible than Omicron’s original strain, and some say it will likely outpace Omicron’s original strain and rise to become the next variant of concern.

But little else is known about stealth Omicron, and the World Health Organization this week called for scientists to research how it behaves.

If the substrain spreads further in Hong Kong, the city may provide some clues.

For one, scientists are not sure what exactly makes stealth Omicron more transmissible. It might be more infectious or simply better at evading vaccines. Among vaccinated populations, it is difficult to tell the difference. Hong Kong has fully vaccinated 64% of its population, similar to the U.S. But its elderly population has lagged behind, with a fully vaccinated rate among people over 80 of 20%.

The low vaccination rate means that if the substrain spreads among unvaccinated populations in Hong Kong, scientists can gauge its transmission versus how quickly it’s spreading in more vaccinated populations elsewhere. The evidence could provide clues about stealth Omicron’s ability to evade vaccines versus its base level of contagiousness.

Gabriel Leung, Hong Kong’s top epidemiologist and adviser to the government on the pandemic, tells Stat News that only 16% of people in Hong Kong’s nursing homes are vaccinated. He explained that if different strains of Omicron spread in different nursing homes, scientists could better understand the differences in how quickly, say, Omicron variant BA.1 and BA.2 spread. “Ironically, and, paradoxically, Hong Kong’s nursing homes…would be the perfect laboratory to sort this out,” Leung told Stat News.

Michael Diamond, a viral immunologist at Washington University in St. Louis, also told Stat News that because so many Hong Kongers have been neither vaccinated nor previously infected, the city could provide important clues about how deadly stealth Omicron is.

“The key measure of whether this is less pathogenic [or less severe]—apart from the animal studies—is in naive people, meaning those who have never been vaccinated and never been infected. Are they getting severe disease or not?” he said.

Hong Kong, meanwhile, is hoping to halt the spread of the variant as soon as possible to avoid becoming the subject of scientific inquiry.

“We are racing against Omicron,” Lam said Thursday. “We do have the ability to stop the spread of the disease in Hong Kong…[but with this variant] exponential growth in the number of cases in the community can happen at any time.”