But a study published to British medical journal The Lancet last week found that Omicron BA.2, or “stealth Omicron”—currently the world’s dominant variant—is not as mild as one might think, at least not in unvaccinated children.

The study, which has yet to be peer-reviewed, examined the medical records of all Hong Kong children ages 0-11 who were hospitalized with COVID during the Omicron BA.2 dominant wave, Feb. 5 to Feb. 28, and compared them to the records of children who had been hospitalized during times of other COVID variant dominance, as well as influenza and parainfluenza dominance before the pandemic.

It found that, among children hospitalized with COVID during the Omicron BA.2 wave:

• Deaths were higher than they were during other COVID variants, influenza, and parainfluenza.
• Pediatric ICU admissions were higher than they were during other COVID variants and influenza.
• Cases of neurological complications, including seizures, were elevated when compared to influenza and parainfluenza.
• Rates of croup were higher than they were during other COVID variants and influenza.
• The need for mechanical ventilation and oxygen was higher than it was during other COVID variants and the flu.

The authors’ conclusion: Omicron BA.2 “is not mild, as evident by fatality and severe complications of the uninfected and unvaccinated children.”

Rates of COVID in Hong Kong were incredibly low for first two years of the pandemic, thanks to the government’s “zero COVID” policy with strict interventions. As ABC News reported last month, the region was a “poster child for COVID control, after going long stretches of last year without a single reported infection.” Now stealth Omicron is sweeping the country, and its death rate is the highest in the developed world, according to ABC.

Vaccines weren’t approved for children in Hong Kong until mid-January. The vast majority of children in the study hadn’t been previously infected or vaccinated, according to the study, giving scientists a rare glimpse at the effects of “stealth Omicron” on a population with little to no previous exposure to the virus.

Hardest hit among Hong Kong’s population of hospitalized children were those 5 years and older, who comprised 80% of the 1,147 hospitalized in February. Of the four deaths that occurred, three of the children were under the age of 5. Researchers speculated that the country’s youngest age group may have been hit the hardest due to lack of exposure to other seasonal coronaviruses over the past two years, in addition to a lack of vaccination.

In a commentary titled “Omicron severity: milder but not mild,” published in The Lancet in January, two World Health Organization-affiliated authors pointed out that a “generally young South African population” saw more than 20% of patients hospitalized with Omicron experience a “severe clinical outcome.” A population with lower levels of immunity from vaccines or prior infections could see a higher rate, they said.

“This report of typically milder disease following infection with the omicron versus delta variant in South Africa is encouraging, but we should not assume that omicron variant epidemics will have such a low health effect elsewhere,” they wrote.