Chinese health authorities told the World Health Organization that the source of an outbreak of respiratory diseases among children are known pathogens, after reports about a surge in illness sparked concerns that a novel virus may have been the cause.

The Chinese Center for Disease Control and Prevention and Beijing Children’s Hospital told the WHO on Thursday that there’s been increases in outpatient visits and hospitalizations due to the spread of mycoplasma pneumoniae, respiratory syncytial virus, adenovirus and influenza. The uptick has come earlier than what’s been experienced historically, but isn’t unexpected given the lifting of Covid curbs, the WHO said in a statement following a teleconference between health officials.

Read More: What We Know About the Child Pneumonia Outbreak Sweeping China

The response came after WHO this week requested more information from China on the situation after reports warned of undiagnosed pneumonia in children’s hospitals in Beijing, Liaoning and other places.

Local Chinese media have reported a steady rise in infections from mycoplasma among kindergarten and primary school children. While the germ tends to cause only mild colds in older kids and adults with robust immune systems, younger children are prone to develop pneumonia — with symptoms lasting for weeks.

Media reports have shown crowded hospital waiting rooms, with anxious parents sometimes waiting up to seven hours for their children to see a doctor. The National Health Commission is boosting local clinics’ capability to treat infections and identify critical illness in a bid to ease pressure on some top hospitals, state news agency Xinhua reported on Thursday, citing an NHC official it didn’t identify.

Read More: The Germ Plaguing China’s First Post-Lockdown Winter

On Friday, Chinese health authorities warned that the country could see a number of respiratory pathogens converging into a major outbreak from winter to spring. Flu and mycoplasma infections have been on the rise since October, while a rebound in Covid cases is also possible, state broadcaster CCTV reported.

The rampant spread of mycoplasma has caused particular concern as many kids appear to not respond to azithromycin, which is commonly used to treat the infection. Among Chinese children, resistance to the antibiotic and others in the same class is more than 80%, the highest in the world, Yin Yudong, an infectious disease doctor at Chaoyang, told Beijing News earlier this month.

(Updates to add health authorities’ warning in sixth paragraph.)