By Nellie Peyton

JOHANNESBURG (Reuters) - South African lab technician Nozipho Mlotshwa was waiting for the test results for a potential HIV vaccine, which has eluded scientists for decades, when the order came from USAID to stop work.

The first round of vaccines she and her colleagues made in Johannesburg had produced an immune response in rabbits, which was promising but not conclusive - so they tweaked the formula and sent off four new versions for pre-clinical tests.

"This was very exciting. We were getting quite good results," Mlotshwa, 32, told Reuters in the lab in the Antiviral Gene Therapy Research Unit at the city's University of the Witwatersrand.

Now the animal blood samples containing their results are sitting untouched in a freezer.

A trial of an earlier, separate vaccine candidate, which was about to be tested on humans in South Africa as well as Kenya and Uganda, is also on ice.  

Both trials are among the casualties of U.S. President Donald Trump's decision to dismantle the United States Agency for International Development (USAID).

They are part of a wider South African-led HIV vaccine development scheme known as BRILLIANT and funded entirely by a $45 million grant from USAID. It is unclear if or when the project could resume. The U.S. State Department did not immediately respond to a request for comment.

"It feels like you're building something and you could really make a huge difference," Nigel Garrett, Chief Scientific Officer at the Desmond Tutu Health Foundation, a partner in the project, said.

"And then it's wiped away."

The project is one of many research efforts worldwide to be hit by Trump's actions since taking office last month. Others include halting efforts to protect food crops from pests and diseases and blocking publication of a paper on the mpox outbreak.

'HOLY GRAIL'

HIV's ability to mutate quickly has confounded efforts to create a vaccine ever since it was first identified in 1983. The researchers in Johannesburg are using the mRNA technology that created some COVID-19 vaccines.

Several other mRNA-based HIV vaccine candidates worldwide have reached clinical trials. BRILLIANT is unique in being Africa-led, aiming to develop capacity for producing vaccines in Africa.

For the past year the Johannesburg team had been working with genetic sequences from two South African patients who have HIV but whose bodies produce a rare type of antibody that neutralizes the virus. They are trying to simulate that immune response.

"We were gaining momentum," said Patrick Arbuthnot, director of the research unit, adding: "an HIV vaccine is the holy grail of the field". 

Trump in January ordered a 90-day pause in all foreign development assistance pending assessment of its consistency with his "America First" foreign policy. 

Separately, he has targeted South Africa with an executive order to cut all funding to the country, citing disapproval of its land reform policy and its genocide case against U.S. ally Israel.

The U.S. foreign aid freeze has affected programmes across the globe, stranding shipments of life-saving medical supplies, including HIV drugs, and leaving disaster response teams unable to deploy. Waivers for "life-saving humanitarian assistance" have been hampered.

'GOOD FOR THE WORLD'

Because South Africa has the world's largest population of people living with HIV, at more than 8 million, it is a hub for research on the virus.

"Most of the landmark and groundbreaking studies have been conducted in this country. But these have been good for the whole world," said Ntobeko Ntusi, CEO of the South African Medical Research Council, which is spearheading the HIV vaccine search. 

Ntusi said he did not expect funding for projects like BRILLIANT to resume, given the executive order on aid to South Africa. The council gets about a third of its funding from U.S. federal sources, for research that is mostly on HIV and tuberculosis but covers other areas including maternal and infant mortality and antimicrobial resistance, he said.

Garrett said the shot that was ready for testing on humans was a mix of two vaccine substances developed in the United States and the Netherlands which have shown promise but never been tested together.

They are now sitting in storage.

"We had a huge opportunity, good funding. It's difficult for other funders to fill that gap," he said.  

(Additional reporting by Catherine Schenck and Shafiek Tassiem; Editing by Tim Cocks and Aidan Lewis)