Australia abandoned the development of a promising COVID-19 vaccine after several trial participants received false positive test results for HIV, meaning they didn’t actually have HIV.   

The Australian government had planned to purchase 51 million doses of the vaccine for about $750 million, The New York Times reports. That plan has now been scrapped. To compensate for the loss, the government will purchase more of the vaccines designed by AstraZeneca and Novavax.

How could a coronavirus vaccine interfere with HIV test results? The answer lies in the vaccine’s design. Researchers at the University of Queensland and biotech company CSL developed the vaccine using two fragments of HIV protein, according to the Times. The fragments, which can’t cause an HIV infection, helped create a molecular “clamp” that latches onto the spikes of the coronavirus. The clamp, in turn, caused the body to produce antibodies that are detected in HIV tests, thus leading to false HIV positives.

This may seem like a foreseeable problem. John P. Moore, an immunologist at Weill Cornell Medical College in New York City, described the situation to the Times as an “honest error” that has likely embarrassed the scientists. “It’s not great to be associated with a mistake like this,” he added. “But when you’re running at 90 miles an hour, sometimes you trip.”

Although it is possible to alter HIV tests so that they account for this, it would be time-consuming to do so. What’s more, officials worried that false positives might undermine the public’s faith in the vaccine.

“We can’t have any issues with confidence,” Prime Minister Scott Morrison told reporters, according to the Times, “and we are as a nation now, with a good portfolio of vaccines, able to make these decisions to best protect the Australian people.”

“I want to emphasize there are no adverse health reactions, and there’s no possibility the vaccine causes HIV infection,” CSL’s chief scientific officer Andrew Nash said in the Deutsche Welle. “This outcome highlights the risk of failure associated with early vaccine development and the rigorous assessment involved in making decisions as to what discoveries advance.”

As CNN reports, this isn’t the first potential vaccine to be abandoned or pushed back. For example, GlaxoSmithKline and Sanofi Pasteur, two pharmaceutical leaders in Britain and France, announced on Friday that their joint vaccine would not be released until late next year, if not later, because recent data showed “insufficient immune response” in the elderly.

In related news, a Food and Drug Administration expert advisory panel in the United States voted December 10 to recommend emergency use authorization of a vaccine. For more, see “Three Countries Authorize Pfizer COVID-19 Vaccine.”

To learn more about HIV testing—including the different kinds of tests and who should get tested—read these HIV Testing Basics.