BA.2, a sublineage of Omicron causing case numbers to surge in many countries, is more severe in children than other variants, according to research from Hong Kong.
The new Lancet preprint study, which has not yet been reviewed by other scientists, found the strain was associated with higher odds of paediatric ICU admissions, mechanical ventilation and oxygen use.
Hong Kong's population had little natural immunity when Omicron hit the country in February.
Just 1 per cent of the population were estimated to have had Covid as of October 2021 because of strong restrictions implemented to stem the spread of the virus.
But once BA.2 gained a foothold, it ripped through the population, causing a huge surge in case numbers.
Researchers found 1,147 children who had not previously had the virus or been vaccinated against it were admitted to hospital during the wave in just over three weeks, from February 5 to 28.
The vast majority, 920, were less than five years old.
The numbers of children who were treated for severe disease were small, but suggested Omicron was not mild and could in fact cause serious disease, researchers said.
“The intrinsic severity of Omicron BA.2 in children who had no past Covid-19 or vaccination is not mild,” they wrote, "and in fact, they had higher odds of PICU admissions, mechanical ventilation and oxygen use.
"Omicron BA.2 is more neuropathogenic than previous Sars-CoV-2 variants, influenza and parainfluenza viruses, resulting in more seizures. It also targets the upper airways more than past variants and influenza."
They recommended that vaccination should be “rapidly implemented for children eligible, and in particular, for [those] under three years old, extension of use of current vaccines should be urgently explored”.
A total of 21 children, 1.83 per cent, had to be treated in the paediatric ICU during the BA.2 wave, compared with one child, 0.14 per cent, during the first four waves.
The rate was also higher than that of children with influenza, 0.79 per cent of whom were treated in PICU over a five-year period, or those with parainfluenza, 1.64 per cent of whom needed intensive treatment.
Eight children, 0.70 per cent, required mechanical ventilation, compared with none in previous waves.
In total, 171 received treatment for neurological complications, including febrile seizure (11.6 per cent), followed by seizure with fever (2.44 per cent).
Five (0.44 per cent) children had Covid-associated encephalitis/encephalopathy. During the first four waves, there were no admissions related to seizures.
There were four child deaths – an 11-month-old, a three-year-old, four-year-old and a nine-year-old.
“Three of them had good past health. The nine-year-old child had Duchenne muscular dystrophy. All were not vaccinated against Covid-19," researchers wrote.
“The cause of death for two cases was attributed to neurological causes: one with encephalopathy and the other with fulminant cerebral oedema, which recently became a recognised phenotype of encephalitis.”
The research chimes with a study released in February in the US, where it was found children were almost four times as likely to be admitted to hospital during the Omicron wave compared with the one caused by Delta.
Like Hong Kong, the risk was highest in the youngest children, 0-4, who registered the largest hospital admission rate increase among any group under 18 in the US, with 15.6 weekly admissions per 100,000 compared with 2.9 during the Delta wave.
There are currently very few vaccines available to young children.
Children aged three and over can receive a shot of Sinopharm in the UAE, where over-fives can be vaccinated with Pfizer-BioNTech.
But there is yet to be an mRNA vaccine approved for children under five anywhere.
Pfizer-BioNTech’s vaccine for the youngest children has suffered a series of setbacks and delays, with data expected on a three-shot low-dose course in April.
This week Moderna said its vaccine for young children was safe and generated strong immune responses in children from six months to five years old. But it demonstrated only modest efficacy of 43.7 per cent against symptomatic infections in children six months to two years, and 37.5 per cent in children between two and five.
Although seemingly low, the trial coincided with the Omicron wave, and the efficacy was comparable to that in adults during the period.