The World Health Organization forecasts that those most vulnerable to COVID-19, such as older people, will need to get an annual vaccine booster to be protected against variants, an internal document shows.

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

Vaccine-makers Moderna Inc. and Pfizer Inc., with its German partner BioNTech, have been vocal in their view that the world will soon need booster shots to maintain high levels of immunity, but the evidence for this is still unclear.