The Ebola virus outbreak in West Africa created a rare opportunity: New vaccines could be tested, and if they worked, serve as a firewall in future epidemics.

It now appears this chance is slipping away amid public health officials’ squabbles over the right way to test vaccines. As a consequence, there may never be a definitive answer about the vaccines’ effectiveness.

The study generally regarded as the most scientifically solid, which is run by the U.S. National Institutes of Health, began in Liberia but is struggling as new Ebola cases have subsided. The other studies, in Guinea and Sierra Leone, fall short of the scientific gold standard—a randomized, placebo-controlled study—partly because some medical officials have opposed giving a placebo to anyone at risk for the deadly disease. As a result, this outbreak could end without the vaccines’ being rigorously tested.

“I don’t see how it’s going to happen unless our trial gets expanded,” said Dr. H. Clifford Lane, deputy director of the NIH’s National Institute of Allergy and Infectious Diseases (NIAID), who is helping to lead the trial in Liberia. He said expanding the study to Guinea, or perhaps Sierra Leone, “is the right thing to do.”

Just this week, following inquiries by The Wall Street Journal, the World Health Organization said it and the government of Guinea will allow the NIH study to expand there. However, a senior Guinean health official said in an interview that no such decision has been made. Some other doctors in the Guinea trial oppose NIH study expansion into Guinea because it might harm their own research.

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