LONDON: Existing Covid-19 inoculations will struggle against the fast-spreading Omicron variant, the head of vaccine manufacturer Moderna warned Tuesday, as countries ramp up vaccination programmes and impose further restrictions in an effort to curb growing concern. First reported to the World Health Organization in South Africa less than a week ago, the new strain has rapidly spread from Africa to the Pacific, and from Europe to North America as dozens of countries have announced travel restrictions. While no deaths have yet been reported from Omicron, and it could take weeks to know how inf…