- 'Stealth Omicron' - While some have blamed governments for relaxing restrictions too quickly, epidemiologists also pointed the finger at the BA.2 sub-lineage of the Omicron variant which has become dominant in many countries.
Sometimes called "stealth Omicron" because it is more difficult to detect, BA.2 is estimated to be about 30 percent more contagious than its predecessor BA.1.
Lawrence Young, a virologist at Britain's University of Warwick, said the rising cases in Europe were due to a combination -- "a perfect storm" -- of three factors: the lifting of restrictions, waning immunity after vaccination and BA.2. "Removing restrictions has fuelled the spread of BA.2 and could also lead to the generation of other variants," he told AFP.
Antoine Flahault, director of the Institute of Global Health at the University of Geneva said there were "a couple of hypotheses on the table, which are not mutually exclusive".