“The emergence of variants of concerns has resulted in a rapid decline of the protection against symptomatic illness,” officials said in a statement issued Friday morning. “There is therefore a need to assess whether variant-updated COVID-19 vaccines, especially to Omicron, would improve vaccine performance.”

The WHO’s statement is not a policy recommendation, but rather a call for officials to keep their guard up. Health officials have warned that while Omicron has proven less deadly than other strains of COVID so far, new mutations could be more lethal.

Any policy recommendation would come after there is sufficient data, WHO said.

“Current vaccines continue to perform well in preventing severe disease and death due to Omicron, particularly with the use of a booster dose(s),” it said. “However, protection against infection and symptomatic illness due to the Omicron variant is lower than other variants and declines rapidly, even after a third (booster) dose.”

The WHO statement comes as a new subvariant of Omicron appears to be spreading rapidly in the U.S.

The BA.5 COVID subvariant comprised an estimated 13.3% of U.S. cases last week, according to data from the Centers for Disease Control and Prevention. BA.2, also known as stealth Omicron, made up 14.2% of cases.

The WHO’s Technical Advisory Group, which is made up of independent health experts, says “an Omicron-specific monovalent vaccine may elicit greater breadth in the immune response than” current vaccines.