The World Health Organization’s chief scientist has advised individuals against mixing and matching Covid-19 vaccines from different manufacturers, saying such decisions should be left to public health authorities. “It’s a little bit of a dangerous trend here,” Soumya Swaminathan told an online briefing on Monday after a question about booster shots. “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 had called mixing a “data-free zone” but later clarified her remarks in an overnight tweet. “Individuals should not decide for themselves, public health agencies can, based on available data,” she said in the tweet. “Data from mix and match studies of different vaccines are awaited – immunogenicity and safety both need to be evaluated.” Some studies are showing positive results from mixing vaccines, but these are in preprint stage and need further studies to support them. Mixing vaccines is seen as an option in some countries where supply is short of one particular vaccine. But WHO is concerned about a situation where individuals decide for themselves which vaccines to get and how far apart to space them without guidance from health authorities. The WHO’s Strategic Advisory Group of Experts on vaccines said in June the Pfizer Inc vaccine could be used as a second dose after an initial dose of AstraZeneca, if the latter is not available. A clinical trial led by the University of Oxford in the UK is ongoing to investigate mixing the regimen of AstraZeneca and Pfizer vaccines. The trial was recently expanded to include the Moderna Inc and Novovax Inc vaccines. The comments came as Vietnam announced it would offer the coronavirus vaccine jointly developed by Pfizer and BioNTech as a second dose option for people first inoculated with the AstraZeneca vaccine. Vietnam’s mass inoculation campaign is in its early stages, with fewer than 300,000 people fully vaccinated so far. It has so far used AstraZeneca’s viral vector vaccine and last week took delivery of 97,000 doses of the Pfizer/BioNTech mRNA shot. “Pfizer vaccines will be prioritised for people who were given first shot of AstraZeneca 8-12 weeks before,” the government said in a statement. Several countries, including Canada, Spain and South Korea, have already approved such dose-mixing mainly due to concerns about rare and potentially fatal blood clots linked to the AstraZeneca vaccine. A Spanish study found the Pfizer-AstraZeneca combination was highly safe and effective, according to preliminary results.