The World Health Organization (WHO) welcomed Moderna reporting on Monday that its experimental vaccine showed 94.5% efficacy but said that “many questions” remained and it was no time for complacency. Only very limited amounts of any vaccine will be available in the first half of 2021 for people other than priority health workers, WHO officials said. “While we continue to receive encouraging news about COVID-19 vaccines and remain cautiously optimistic about the potential for new tools to start to arrive in coming months, right now we are extremely concerned by the surge in cases we are seeing in some countries, particularly in Europe and the Americas,” WHO director-general Tedros Adhanom Ghebreyesus told a news briefing. It marked his return to the Geneva agency from quarantine after being exposed to coronavirus some 17 days ago. Tedros said he had no symptoms and had seen no need for a test. Moderna Inc’s experimental vaccine is 94.5% effective in preventing COVID-19 based on interim data from a late-stage trial, the company said on Monday, becoming the second US drugmaker to report results that far exceed expectations. Together with Pfizer Inc’s vaccine, which is also more than 90% effective, and pending more safety data and regulatory review, the United States could have two vaccines authorized for emergency use in December with as many as 60 million doses of vaccine available this year. Soumya Swaminathan, WHO’s chief scientist, said that the Moderna results were “quite encouraging”. Its final efficacy and safety profile would still be needed, as well as follow-up on trial participants for two months for any side effects. Pfizer and Moderna candidate vaccines both use mRNA technology and appear to achieve high efficacy, she added. “But there are many, many questions still remaining about the duration of protection, the impact on severe disease, the impact on different sub-populations especially the elderly, as well as the adverse events beyond a certain period of time,” Swaminathan said. Clinical trials must continue to collect more data, she said, adding that more results were expected in coming weeks from the other vaccine trials. “We are looking at at least the first half of the year as being a period with very very limited doses. Supplies are going to be limited, there are bilateral deals that many of the companies have done, so many of the doses have already been booked by some countries,” Swaminathan said. Moderna is a two-dose vaccine and its delivery means, as well as storage, were also important considerations, said Kate O’Brien, director of WHO’s immunisation department.