The research showed that T-cells, critical in warding off severe symptoms, remain active among people who have been infected earlier or have been inoculated.
Preliminary reports from South Africa indicate a sharp rise in hospital admissions in Covid-positive children above the age of two, at a time when the Omicron variant is sweeping through the country.
U.S. health officials say that while the omicron variant of the coronavirus is rapidly spreading in the country, early indications suggest it may be less dangerous than delta, which continues to drive a surge of hospitalizations.
If Omicron proves more transmissible than the Delta variant, it could lead to a sharp spike in infections that could put pressure on hospitals, the director of South Africa's communicable disease institute said.