A report from the European Centre for Disease Prevention and Control (ECDC) warns that variants BA.4 and BA.5 could lead to a surge of cases across the continent in the coming months.
The organisation has implored countries to “remain vigilant” for signs that the variants are present among infection cases.
It also advised countries to encourage over 60s and other vulnerable groups to receive another booster jab.
Both BA.4 and BA.5 were first detected in South Africa earlier this year and have since become the dominant strains in the country.
Despite widespread concerns over the transmissibility of the variants there is “no indication of any change in severity for BA4/BA5 compared to previous Omicron lineages”.
The ECDC said: “The presence of these variants could cause a significant overall increase in Covid-19 cases in the EU/EEA in the coming weeks and months.
“The overall proportion of BA.4 and BA.5 in the EU/EEA is currently low but the high growth advantages reported suggest that these variants will become dominant in the EU/EEA in the coming months.
“Based on the limited data currently available, no significant increase in infection severity compared to the circulating lineages BA.1 and BA.2 is expected.
“However, as in previous waves, if Covid-19 case numbers increase substantially, some level of increased hospital and ICU admissions is likely to follow.
“ECDC encourages countries to remain vigilant for signals of BA.4 and BA.5 emergence.’
It added: “For all age groups, it remains a priority to improve Covid-19 vaccine uptake of the primary course and first booster dose in populations who have yet to receive them.”
It comes as figures revealed that Covid-19 infections across the UK are now at their lowest level since before Christmas.
In England, infections have fallen for the fifth week running, with 1.2 million people likely to test positive for Covid-19, the equivalent of about one in 45 people.
This is down from 1.6 million, or one in 35, and is the lowest estimate since the week ending December 16.