It's the highly-infectious variant that changed the course of the Covid pandemic, but the Delta variant may have defeated itself in one country, researchers say.
Japan was hit by its fifth and worst wave yet in July, with infections fuelled by the Delta variant.
And while daily cases peaked at more than 26,000 in August, infections have dropped to just several hundred each day.
But what triggered such a dramatic reduction in cases?
Professor Ituro Inoue of Japan’s National Institute of Genetics says social distancing and mask wearing paired with vaccine coverage, which stands at 76 per cent for two doses in Japan, is not enough to drive cases down in the manner that they have.
He believes the reason is the Delta variant reached a point of "natural extinction".
“If the virus were alive and well, cases for sure would increase as masking and vaccination do not prevent breakthrough infections in some cases,” Prof Inoue told the Japan Times.
He said Delta had blocked other variants from taking off but as it continued to mutate, his team believes it became "faulty" and was unable to make copies of itself.
"We think that at some point during such mutations it headed straight toward its natural extinction," Prof Inoue said.
He says such a phenomenon could occur in other countries.
New Covid variants may still emerge
Australia has defied some experts' warnings that a vast easing of restrictions would trigger a surge in Covid cases across the country.
Yet other countries have not been as lucky, with several nations with similar vaccination rates such as South Korea reporting high levels of infection daily.
Japan, which ended its strict Covid restrictions last month, is still at risk from existing and new variants, Prof Inoue warns.
"Because there’s nothing now to keep them at bay, there’s room for new ones to enter as the vaccines alone would not solve the problem."
He said it meant quarantine measures for overseas arrivals should still play a role in the pandemic moving forward.
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