The World Health Organisation says people most vulnerable to Covid-19, such as the elderly, will need to have an annual vaccine booster to be protected against variants, an internal document has shown. The estimate is included in a report, seen by Reuters, which is to be discussed on Thursday at a board meeting of Gavi, the Vaccine Alliance, which co-leads the WHO's Covax programme. The forecast is subject to change and is paired with two other less likely scenarios. Vaccine makers Pfizer and BioNTech, and Moderna, have insisted that the world would soon need booster shots to maintain high levels of immunity but the evidence is still unclear. The document shows that the WHO considers annual boosters for high-risk people as its "indicative" baseline, and boosters every two years for the general population. It does not say how these conclusions were reached, but shows that under the scenario new variants would continue to emerge and vaccines would be regularly updated to protect against them. The UN agency declined to comment. A representative for Gavi said Covax was planning to consider a wide range of scenarios. The document, which is dated June 8 and is a "work in progress", also predicts under the base case that 12 billion Covid-19 vaccine doses will be produced globally next year. That would be slightly higher than the forecast of 11 billion for this year given by the International Federation of Pharmaceutical Manufacturers and Associations. The document predicts manufacturing problems, regulatory approval issues and "transition away from some technology platforms" as possible impediments to supplies next year. It does not indicate which types of vaccine could be phased out. But the EU, which has reserved the world's largest amount of Covid-19 vaccines, has invested in inoculations using messenger-RNA (mRNA) technology, such as those by Pfizer and Moderna. The bloc has forgone some purchases of viral vector vaccines from AstraZeneca and Johnson & Johnson. The scenarios will be used to define the WHO's global vaccination strategy and the forecasts may change as new data emerges on the role of boosters and the duration of vaccine protection, Gavi says in another document, also seen by Reuters. So far about 2.5 billion doses have been administered worldwide, mostly in rich countries where more than half of the population has received at least one dose. In many poorer countries, less than 1 per cent has been vaccinated, Gavi's estimates show. This gap could widen next year under the WHO's most pessimistic forecast, as the need for annual boosters could again push poorer nations to the back of the queue. In its worst-case scenario, the UN agency says production would be six billion doses next year, because of stringent regulation for new shots and manufacturing issues with existing ones. That could be compounded by the need for annual boosters for the entire world, and not just the most vulnerable, to combat variants and limited duration of protection. In the more optimistic situation, all vaccines being developed would be authorised and production capacity would increase to about 16 billion doses to meet demand. Vaccines would also be shared equitably across the world. There would be no need for boosters as vaccines would show strong efficacy against variants and long protection.