The rapid global spread of the so-called Delta variant—a highly contagious strain of Covid-19 now present in nearly 100 countries—has public health experts and advocates demanding immediate international action to distribute vaccines to unprotected populations amid growing concerns that the deadly virus is mutating more quickly than nations can administer shots.

“Vaccine equity is urgent,” Madhukar Pai, an epidemiologist at McGill University in Montreal, said Monday, pointing to data showing that coronavirus cases are surging in Asian nations such as Bangladesh, Thailand, and Indonesia—countries that have fully vaccinated less than 6% of their populations.

Experts are attributing the ongoing infection spike in Asia, Africa, South America, and other regions to the Delta variant, which is believed to be 60% more transmissible than the previously dominant Alpha mutation. First detected in India in December, the Delta strain has since been found in 98 countries and is running roughshod through unvaccinated communities, including pockets of the United States.

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