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Coronavirus cases have been surging in the US, fueled by the highly transmissible Delta variant.
The more transmission, the more opportunity for the virus to replicate and gain dangerous mutations.
The coronavirus could be just a few mutations away from evading existing COVID-19 vaccines, according to the head of the Centers for Disease Control and Prevention.
"These vaccines operate really well in protecting us from severe disease and death, but the big concern is that the next variant that might emerge - just a few mutations potentially away - could potentially evade our vaccines," CDC Director Rochelle Walensky said in a Tuesday press briefing.
She added, though that "right now, fortunately we are not there," since current COVID-19 vaccines "operate really well in protecting us from severe disease and death."
Coronavirus cases, fueled by the highly transmissible Delta variant, have been surging in the US and across the globe - largely among those who are unvaccinated. In the last month, the US's seven-day average of new daily cases has quintupled: from 11,887 on June 26 to 56,635 on Monday.
The CDC announced Tuesday that people infected with Delta - vaccinated or unvaccinated - have higher viral loads (meaning they carry greater amounts of virus) compared to other versions of coronavirus. That means even vaccinated people could pass the virus along to others similarly to how unvaccinated people do.
More transmission means more mutations
All viruses change over time as they replicate inside an infected host. So countless versions of the coronavirus are circulating, each separated by a handful of tiny changes in its genetic code. Many of these have no real public-health impact, but the more people a virus infects, the more chances it has to mutate into a new, dangerous variant.
"The biggest concern at the moment is just the sheer number of people that have the virus and therefore the sheer number of variants that are being generated," Andrew Read, who studies the evolution of infectious diseases at Pennsylvania State University, previously told Insider. "Some of those might be the jackpot which are even fitter than Delta."
No coronavirus variant spotted so far is more concerning than Delta, which was first identified in India this winter. World Health Organization officials recently called Delta the "fittest" variant to date, since it spreads more easily and may lead to more severe cases and an increased risk of hospitalization than other variants like Alpha, the variant discovered in the UK.
What's more, emerging research indicates that a single vaccine dose doesn't hold up as well against Delta as it does against other coronavirus strains. Recent Public Health England analyses found that two doses of Pfizer's vaccine were 88% effective at preventing symptomatic COVID-19 from Delta, while a single shot was just 33% effective. That's compared to 95% efficacy against the original strain, with 52% after one shot.
Even though Delta is already the most transmissible variant, Read said it could still acquire combinations of mutations that make it even better at spreading - what he called a "Delta-plus" variant.
It's also possible that two separate variants - Delta and Alpha, for instance - could combine their mutations to produce an even more infectious strain.
The reason a future variant could evade vaccines is that the shots all target the coronavirus' spike protein, the sharp, crown-like bumps on the surface of the virus that help it invade our cells. If multiple, game-changing mutations alter characteristics of that protein, the antibodies induced by the vaccines might not be able to recognize or properly fight the new variant.
Virologists call variations of a virus that slip past the immune defenses we've built "escape mutants." If the virus continues to spread and mutate rapidly, such escape mutants could be around the corner, Ravindra Gupta, a microbiology professor at the University of Cambridge, previously told Insider.
"Full vaccine escape viruses, we're not necessarily that far away from them," he said.
Hilary Brueck contributed reporting.
Read the original article on Business Insider