Director-General of the World Trade Organization (WTO) Ngozi Okonjo-Iweala attends a press conference on the annual global WTO trade forecast at the WTO's headquarters in Geneva, Switzerland, on March 31, 2021. (SALVATORE DI NOLFI / POOL / KEYSTONE / AFP)

GENEVA - The COVAX vaccine-sharing program has struggled to meet vaccine delivery targets because some countries are able to offer more for scant supplies, the head of the World Trade Organization said on Tuesday.

"The supply scarcity is driving behavior," director-general Ngozi Okonjo-Iweala said in an online interview with the Atlantic Council, without naming any countries.

The World Health Organization's chief scientist on Monday advised against people mixing and matching COVID-19 vaccines from different manufacturers, calling it a "dangerous trend"

"Many of them (countries) supported COVAX but many of them bid away vaccines from COVAX and that is why COVAX has been struggling to deliver what it should."

Meanwhile, the World Health Organization's chief scientist on Monday advised against people mixing and matching COVID-19 vaccines from different manufacturers, calling it a "dangerous trend" since more data is needed about the health impact.

"It's a little bit of a dangerous trend here," Soumya Swaminathan told an online briefing. "It will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, a third and a fourth dose."

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Swaminathan called mixing a "data-free zone" on Monday but the WHO clarified on Tuesday that some data was available and more was expected.

Its Strategic Advisory Group of Experts on vaccines in June said the Pfizer vaccine could be used as a second dose after an initial dose of AstraZeneca, if the latter is not available.

The results of a further clinical trial led by the University of Oxford that will look at mixing AstraZeneca and Pfizer as well as Moderna and Novovax vaccines is underway.

"Data from mix and match studies of different vaccines are awaited - immunogenicity and safety both need to be evaluated," the WHO said in emailed comments.

It should be public health agencies who make decisions, based on available data, and not individuals, the WHO added.

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