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As doctors and public health officials work overtime to determine what drugs might treat the deadly coronavirus that's impacting the world, one avenue of possibility seems to have closed off.

Scientists had believed that antiviral drugs used to treat HIV could potentially also work against COVID-19. Both HIV and the coronavirus require an enzyme called a protease to make an infectious virus, Science News reported, and both of these drugs inhibit the action of the protease.

Researchers conducted a trial in Beijing of 199 people who were severely ill with pneumonia caused by COVID-19. They randomly assigned some to get the medications, known as lopinavir and ritonavir, plus standard care; others just received standard care alone.

Their findings, from 94 people who got the drugs and 100 patients who got standard care, showed no benefit to taking the drugs, as reported in Thursday's New England Journal of Medicine.

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The drugs shortened the time that it took to see clinical improvement by one day in the treatment group. However, that shorter time only happened for people who received the drugs within 12 days of symptoms first appearing.

According to researchers, earlier treatment of the infection may be a better option.

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Two other drugs, chloroquine and remdesivir, are also being eyed as possible treatments for COVID-19.

However, there remains no proven treatment for the novel coronavirus, from which most people recover. The virus has infected more than 230,000 worldwide and killed at least 9,300.