Aspen Pharmacare will switch about half of its Covid-19 vaccine production capacity onto other products if demand doesn't pick up within six weeks, its CEO warned, as South Africa's president and health officials urged more Africans to take up the shots.

Aspen completed a deal in March to package, sell and distribute Johnson & Johnson's Covid-19 vaccine in what was considered a game-changing moment for an under-vaccinated continent frustrated by sluggish Western handouts.

However, while only a sixth of adults in Africa are fully vaccinated, according to the latest World Health Organization figures, expectations of high demand for the South African firm's own-brand version of the shot have proved misplaced.

Aspen said on Saturday it had not received a single order for its Aspenovax vaccine.

"What is the point of keeping the capacity? It is like an empty World Cup rugby stadium," CEO Stephen Saad told Reuters in an interview on Wednesday.

Aspen currently has the capacity to produce around one million vaccine doses per day. Roughly half of that is being used to fulfil a supply agreement with J&J.

The remaining capacity, which had been expected to produce Aspenovax shots destined for the African market, is currently sitting idle, Saad said.

"For four to six weeks, we will sit and wait to hear where these multilateral organisations, where the region and the world bodies stand in terms of the actual support for Aspen and African production," he said.

"At the end of the day, we've got to go ahead and look to repurpose what we're doing."

South Africa's President Cyril Ramaphosa told State broadcaster SABC on Wednesday he had reached out to the governments of Zambia, Uganda, Malawi and Kenya in an attempt to drum up orders for Aspen.

"It's cause for concern that vaccines are just laying fallow and not being procured," he said. "We will also be engaging with the rest of the world to procure vaccines made in Africa."

The Africa Centre for Disease Control and Prevention (CDC), Africa's top public health body, said on Thursday it was doing everything it could to prevent a situation where Aspen closes its facility due to a lack of orders.

"We have a market in Africa," Africa CDC Deputy Director Ahmed Ogwell Ouma told journalists.

"All those who are purchasing for Africa must change the way in which they purchase these vaccines so that our African producers are first on the line to have their products being bought."