The World Health Organization (WHO) forecasts that people most vulnerable to Covid-19, such as the elderly, will need to get an annual vaccine booster to be protected against variants, an internal document seen by Reuters shows.

The estimate is included in a report, which is to be discussed on Thursday at a board meeting of Gavi, a vaccine alliance that co-leads the WHO's Covid-19 vaccine programme Covax. The forecast is subject to changes and is also paired with two other less likely scenarios...