US President Donald Trump's administration withdrew from the body upon taking office in January, prompting the agency to scale back its work and cut its management team by half.
Washington is by far the UN health agency's biggest financial backer, contributing around 18 per cent of its overall funding.
The Geneva-based WHO projects that its workforce will shrink by 2371 posts by June 2026 from 9401 in January 2025 due to job cuts as well as retirements and departures, according to a presentation set to be shown to its member states on Wednesday.
It does not include the many temporary staff, or consultants, which UN sources say have been made redundant.
A WHO spokesperson confirmed the total number of staff leaving the organisation and said the workforce would shrink by up to 22 per cent, depending on how many vacant posts are filled.
While the global health agency said in August that hundreds of staff had departed, this is the first time it has given the full scale of the expected change to its global staff.
"This year has been one of the most difficult in WHO's history, as we have navigated a painful but necessary process of prioritisation and realignment that has resulted in a significant reduction in our global workforce," said Director-General Tedros Adhanom Ghebreyesus in a message to staff on Tuesday seen by Reuters, adding that the process was now nearing an end.
"We are now preparing to move forward with our reshaped and renewed Organisation," he added.
The slides also showed that the Geneva-based body has a $US1.06 billion ($A1.63 billion) hole in its 2026-2027 budget, or nearly a quarter of the total required, down from an estimated gap of $US1.7 billion ($A2.6 billion) in May.
That excludes some $US1.1 billion ($A1.7 billion) of expected funding that includes deals at various stages of negotiation, the slides showed, without giving details.
The WHO spokesperson said that the portion of the two-year budget currently unfunded was lower than in previous years, attributing that to a smaller budget; the launch of a fund-raising round; and an increase in member states' mandatory fees.
