Soumya Swaminathan said several thousand patients had been enrolled in the lopinavir/ritonavir arm of the WHO-led Solidarity Trial and in a separate UK-led COVID-19 trial of the drug combination.
"This is already a huge number and should be enough to tell us whether this drug is actually having a mortality benefit or a benefit on disease severity," she told a media briefing.
Another arm of the WHO-led trial, looking at the potential effect of Gilead's antiviral drug remdesivir in COVID-19, might need more patients, she said.
"(We are) looking at ways to maximize the generation of knowledge about drugs which are promising," she said. "On the drug remdesivir, we still do not have a definitive answer on its impact on mortality - which is what we would like to see."
The Solidarity Trial started out with five arms looking at possible treatment approaches to COVID-19: standard care; remdesivir; the malaria drug hydroxychloroquine; the HIV drugs lopinavir/ritonavir; and lopanivir/ritonavir combined with interferon.
"We will come back very soon on further decisions on the Solidarity Trial. It's an adaptive design - you can change and adapt as you go on," Swaminathan said.
On Wednesday the WHO stopped the arm testing hydroxychloroquine as a treatment for COVID-19 patients, after studies indicated it showed no benefit in those who have the disease.Swaminathan said more studies are still needed to see whether hydroxchloroquine may be effective as a preventative medicine. US President Donald Trump has touted the benefits of hydroxychloroquine, and said he took it to prevent infection.