EARLIER IN the coronavirus pandemic, it was thought that a threshold of natural and vaccine-acquired immunity, say 70 percent, would leave the virus little room to grow. President Biden set a goal of 70 percent of Americans getting at least one shot by July 4. The hope was that once the nation reached herd immunity, the virus would die out.

Thanks to the delta variant, it is time to reconsider. The country will fall short of Mr. Biden’s goal by Independence Day. But more importantly, the current level of vaccine immunity is insufficient to end the pandemic in the United States. Since it began, SARS-CoV-2 has evolved to become far more contagious. The goal posts must move when the virus moves. The higher the contagion, the larger share of the population must be immune; for example, measles, which is even more contagious than the coronavirus, requires about 94 percent to be immune to stop the virus from spreading. To really end the pandemic, a chunk of the unvaccinated in the United States must gain immunity, and the best way is through vaccines.

Since peaking in mid-April, the rate of vaccination in the United States has steadily declined. Significant pockets of unvaccinated people remain, especially in five states with fewer than 40 percent receiving one dose: Alabama, Mississippi, Louisiana, Wyoming and Idaho. Vaccine-hesitant people are spread throughout the country. Perhaps they feel they can safely remain among the 30 percent because everyone else will have gotten the shot. A study by the Centers for Disease Control and Prevention released Monday showed that young adults in the United States remain the least likely to be vaccinated and their weekly rates of vaccination are declining. They are making a big mistake for their own well-being and for that of the country.

Delta is truly more aggressive. A preliminary study of samples from around the country, posted Monday by Alexandre Bolze and others, showed that the alpha variant, first identified in Britain and more transmissible than the original virus, peaked in the United States on April 25 with 71.1 percent of the cases. Since then, the delta variant, first found in India, and one other, gamma, first detected in Brazil, have been aggressively crowding out alpha. The delta variant is significantly more transmissible than alpha — and the study showed it is growing fastest in U.S. counties where vaccination rates are lowest. Steve Edwards, chief executive of CoxHealth in Springfield, Mo., told CNN the health-care system has experienced a sixfold increase in hospitalizations. “I think it is the delta variant and there is a lot of kindling with low vaccination rates, so it’s spreading very rapidly,” he said. “Almost all of our cases are unvaccinated people that, in my opinion, have put themselves in harm’s way during this pandemic.” Abroad, too, delta spells trouble. It caused a massive surge in India and has triggered outbreaks in Britain, Israel, Portugal, Singapore, Russia and Indonesia, among other places.

Forget about herd immunity, 70 percent and done. The delta variant stalks the unvaccinated — and delays the day when the pandemic ends.

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