The chief executive of the US drugmaker Moderna has predicted that existing vaccines will be less effective against Omicron than they have been against the Delta coronavirus variant, a comment that sharply lowered global stock markets. Stéphane Bancel said it would take two weeks to get data on how the existing vaccines performed against the new Covid variant and whether it caused severe disease – but it would take several months to tweak vaccines to tackle it. “There is no world, I think, where [the effectiveness] is at the same level … we had with Delta,” he told the Financial Times. Other leading voices, however, moved to try to calm fears. The maker of the Pfizer vaccine and the University of Oxford predicted existing jabs would continue to prevent severe disease. “We think it’s likely that people will have substantial protection against severe disease caused by Omicron,” said Uğur Şahin, the chief executive and co-founder of Pfizer’s German partner BioNTech, who defined severe disease as cases requiring hospital treatment. “To my mind there’s no reason to be particularly worried. The only thing that worries me at the moment is the fact that there are people that have not been vaccinated at all.” The two companies said on Friday that they could produce and ship an updated version of their vaccine within 100 days if the new Covid variant detected in southern Africa was found to evade the current one. Oxford University, which makes the AstraZeneca vaccine, said in a statement: “We will carefully evaluate the implications of the emergence of [Omicron] for vaccine immunity. “Despite the appearance of new variants over the past year, vaccines have continued to provide very high levels of protection against severe disease and there is no evidence so far that Omicron is any different.” Israel’s health minister expressed cautious optimism. Nitzan Horowitz was speaking on Tuesday after another two cases of Omicron were identified in the country, bringing its total to four. Without citing any data or reason for his optimism, he told reporters: “In the coming days we will have more accurate information about the efficacy of the vaccine against Omicron, but there is already room for optimism and there are initial indications that those who are vaccinated with a vaccine still valid or with a booster, will also be protected from this variant.” The remarks come amid conflicting information about how effective various vaccines and therapies are likely to be against the new variant. The German pharmaceutical Merck said it expected its Covid drug to be effective. Bancel suggested pharmaceutical companies would struggle between targeting Omicron and the existing Covid variants, and said it would be risky to shift Moderna’s entire production capacity to an Omicron-specific jab. He said there might be a case for giving more potent boosters to elderly people or those with compromised immune systems. He said the high number of mutations on Omicron’s spike protein, which the virus uses to infect human cells, and the rapid spread of the variant in South Africa, suggested there could be a “material drop” in effectiveness. The Moderna comments fuelled further falls in share prices around the world, adding to Friday’s sell-off. The Hong Kong stock index closed at its lowest level in more than a year, falling 1.6%. The FTSE 100 index in London fell by 1.5% to its lowest level in seven weeks, before closing down 50 points or 0.7% at 7059 points after the BioNTech comments. The FTSE 100 recorded its worst month in more than a year, trading down about 2.5% in November, the biggest drop since October 2020, when it lost nearly 5%, just before successful vaccine trials prompted a global rally in November 2020. Mohit Kumar, the managing director of Jefferies, said Bancel’s comments were concerning. “The comments probably reflect the reality of the current situation and the uncertainty surrounding the Omicron impact. We should get more clarity in a couple of weeks, but the market would remain subject to headline risk till then.”