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Unraveling the mysterious mutations that make delta the most transmissible COVID virus yet

Liz Szabo, Kaiser Health News on

Published in News & Features

Upon first inspection, the mutations in the highly contagious delta COVID-19 variant don’t look that worrisome.

For starters, delta has fewer genetic changes than earlier versions of the coronavirus.

“When people saw that the epidemic in India was driven by delta, they did not suspect it would be so bad or overtake other variants,” said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Research Center.

But those expectations were wrong.

Delta has kept some of the most successful mutations found in earlier variants, but also contains new genetic changes that enable it to spread twice as fast.

Delta is more dangerous in many ways. It has an incubation period of four days, rather than six, making people contagious sooner. When the pandemic began, people spread the original coronavirus to an average of two or three people. Today, people infected with delta infect six people, on average.


As of this week, the delta variant had caused at least 92% of the new infections in the United States, according to, a research firm in Bern, Switzerland.

Although delta isn’t necessarily any more lethal than other variants, it can kill huge numbers of people simply because it infects so many more, said Dr. Eric Topol, founder and director of the Scripps Research Translational Institute.

Scientists have sequenced delta’s mutations but are still trying to understand their significance, said Angela Rasmussen, a virologist at the University of Saskatchewan’s Vaccine and Infectious Disease Organization. “When we see the same mutations appearing repeatedly and independently, that suggests they’re important,” Rasmussen said.

Scientists have the best understanding of mutations on the so-called spike protein — which sticks out from the surface of the virus like a club — and which have been studied the most intensely because of its serious ramifications, Rasmussen said. The coronavirus uses the spike protein to enter human cells, and changes in the spike can help the virus evade antibodies.


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