THE NEXT big challenge of the pandemic is called “delta.” On May 31, the World Health Organization created new names for the coronavirus variants. The one that was first detected in Britain was called alpha. The one first detected in India is now delta, and it already has a foothold in the United States and more than 60 other countries. Like other variants, this one looks more dangerous than the original virus, and may be more resistant to vaccines, too.

Delta provides another cautionary reminder that the pandemic will not end by flipping a switch. Evolution’s relentless pressure means that variants will emerge for a long time to come. Vaccines that work — and booster shots — may be humanity’s best chance to avoid severe illness and death, but in the United States and elsewhere, there are still unvaccinated and vulnerable populations. The threat of delta should prompt vaccine doubters to rethink.

Delta is taking off in Britain. According to a June 3 report from Public Health England, delta is “highly likely” to be “significantly more transmissible than Alpha,” which is more transmissible than the original virus. There is some evidence of increased risk of hospitalization; doctors in India say they have seen hearing impairment, severe gastric upsets and blood clots from delta. Moreover, reports from England and Scotland say vaccines are less efficacious against delta; the first dose is about 15 percent to 20 percent less effective, although two doses appear to work better. The reduction in effectiveness was seen in both patient data and laboratory tests.

What this means is that people who do not get vaccinated are leaving themselves open to this variant and its dangers, and those of variants to come. Not getting vaccinated is like jaywalking on a busy street. Why take the chance? Of the U.S. population aged 18 or older, only 53 percent have been fully vaccinated so far, or 136.7 million people, meaning that nearly half the country is still not, and the daily total of vaccinations is falling. Some Southern states have still not reached 40 percent of their populations. Racial disparities in vaccination are also worrisome. The Kaiser Family Foundation reports that, overall, Black and Hispanic people have received smaller shares of vaccinations compared with their shares of cases and with their part of the total population in most states. This is quite stark and disturbing in D.C., where Black people have received 40 percent of vaccinations, but make up 55 percent of cases, 70 percent of deaths and 46 percent of the total population.

All the blandishments, lotteries and rewards for vaccination are for a good cause — getting enough people vaccinated to save their lives and to bring closer a broad “herd immunity,” from vaccines and natural infection, so that the virus dies out. Mike Ryan, emergencies chief at the World Health Organization, estimated this week that the required threshold might be as high as 80 percent, given the waves of variants. The United States still has a ways to go. Do you want to meet delta in a dark alley? If not, get a shot.

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