WHO's technical lead on COVID-19 said the delta variant first found in India has now been seen in over 60 countries and is more transmissible than the alpha variant found in Britain.

Iran PressEurope: Maria Van Kerkhove blamed "worrying trends of increased transmissibility, increasing social mixing, relaxing of public health and social measures, and eleven and inequitable vaccine distribution around the world."

Dr. Michael Ryan, WHO's emergencies chief, said over 80 percent of people would need to be vaccinated "where you could be significantly affecting the risk of imported cases potentially generating secondary cases or causing a cluster or an outbreak."

"So it does require quite high levels of vaccination," Ryan said. "Particularly in the context of more transmissible variants, to be on the safe side."

Many rich countries have been moving to vaccinate teenagers and children—who have a lower risk of more dangerous cases of COVID-19 than the elderly or people with comorbidities—even as those same countries face pressure to share vaccines with poorer ones that lack them.

Britain, which has vastly reduced case counts thanks to an aggressive vaccination campaign, has seen a recent uptick in cases attributed largely to the so-called delta variant that originally appeared in India—a former British colony.

Ryan acknowledged that the data wasn't fully clear about what percentage of vaccination coverage was necessary to fully have an impact on transmission.

WHO Director-General Tedros Adhanom Ghebreyesus, meanwhile, called on leaders of the developed Group of Seven countries to help the UN-backed vaccination program against COVID-19 to boost access to doses in the developing world.