WHO warns against people mixing and matching COVID-19 vaccines

Geneva: The World Health Organisation’s chief scientist has 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.” 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.

Higher rate of side effects

Meanwhile, a US health official said the United States is reviewing the need for a third COVID-19 booster shot among residents who have already been vaccinated but needs to see more data to know if additional shots could raise people's risk of serious side effects.

The official said the second dose for two-shot COVID-19 vaccine regimens was associated with higher rates of side effects, suggesting a third dose could potentially come with even greater risks.

"We're keenly interested in knowing whether or not a third dose may be associated with any higher risk of adverse reactions, particularly some of those more severe - although very rare - side effects," said Jay Butler, deputy director at the US Centers for Disease Control and Prevention, during a media briefing.

The US government has not made a decision on whether to administer booster shots but sees a greater potential need for them among the elderly and other groups at high risk for severe infection, Butler said.

Pfizer and partner BioNTech plan to ask US regulators within weeks to authorise a booster dose of its COVID-19 vaccine, based on evidence of greater risk of infection six months after inoculation and the spread of the highly contagious Delta variant of the virus.

Butler said he has not seen any evidence of waning immunity to COVID-19 among U.S residents who received shots in December or January.

He added that existing shots provide significant protection against the Delta variant of COVID-19, which was first found in India and has become the dominant strain in the United States.

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