The South African Medical Association, which represents doctors, said there must must be a choice of Covid-19 booster vaccine after Johnson & Johnson won the right to run research on the half a million health workers who took the company’s shot in an initial study.

The Sisonke trial, which gave South African health workers a vaccine ahead of a general rollout of the shots, will be extended with health workers offered a second J&J inoculation, the government said last month.

SAMA, as the association is known, said it is concerned because of potential evidence that a booster in the form of a messenger ribonucleic acid, or mRNA, vaccine such as those produced by Pfizer Inc. is potentially more effective than a second dose of a more traditional vector-based shot, such as that produced by J&J.

“What is it that informs the decision to rollout the Sisonke phase 2 trial in isolation?” SAMA said in an open letter to South African Health Minister Joe Phaahla on Wednesday. “Will health care workers be given the right to choose which booster they choose to take?”

‘Vaccine Apartheid’

J&J’s dose uses a piece of a modified cold virus to stimulate an immune response, while mRNA vaccines use genetic material to instruct cells to defend themselves against the coronavirus.

The rollout of the booster trial will begin today and the addition of a choice would delay it by six weeks, said Nicholas Crisp, deputy director general of the South African health department. That would mean that health workers wouldn’t get more protection ahead of a possible a fourth wave of coronavirus infections starts, he said, adding that a Pfizer boost trial may be carried out later.

Because offering Pfizer as an alternative would set the process back “we elected not to go that route,” he said. “I actually am disappointed in SAMA raising this.”

SAMA’s intervention has also created a rift with the South African Medical Research Association, which is working with J&J on the trial.

“What they are asking us to do is to experiment further on health workers. They are saying disregard the evidence and conduct a further experiment,” said Glenda Gray, president of the SAMRC and co-lead of the J&J study in South Africa. “We have people who have never conducted a vaccine trial in their life telling the public that one vaccine is better than another and creating a vaccine apartheid.”