The U.S. has vaccinated half of adults. The problem is the second half.

By Editorial Board,

NOW THAT more than half of the adult population of the United States has been vaccinated with at least one dose, and eligibility is open nationwide to all over age 16, the next monumental task is to vaccinate as much of the remaining half as possible. The goal is to reach some kind of herd immunity, when enough people are immune through either vaccines or previous infection that the coronavirus can’t spread fast or far.

That threshold might be 70 or 80 percent, a moving target as new variants take hold. To get there, vaccine hesitancy remains an enormous hurdle, born of fear about side effects, denial about the disease, historic neglect by health-care systems, feelings of invincibility or fatalism, and the pernicious impact of disinformation and conspiracy theories.

It cannot be said strongly enough: The covid-19 vaccines are safe and remarkably effective at preventing serious disease.

[Full coverage of the coronavirus pandemic]

Still, pockets of resistance persist, such as in the military. Some 48,000 Marines have declined to be vaccinated among 123,000 offered shots, according to the website Military.com. Because the vaccines are being used under an emergency use authorization by the Food and Drug Administration, commanders cannot order troops to accept a covid vaccine; it is a personal decision. This should not be hard: Would those troops face combat without protective gear? No. The shot is just as important as the helmet.

The good news from a recent poll by the nonprofit Kaiser Family Foundation is that, overall, the country is growing more enthusiastic about getting vaccinated. The most positive change in March was among Black Americans, who have traditionally been wary. In the poll, about 55 percent of Black adults said they had been vaccinated or planned to be soon, up 14 percentage points from February and a rate that, as the foundation put it, “now approaches that of Hispanics, at 61 percent, and whites at 64 percent.” But there are still hurdles. Republicans and White evangelical Christians were the most likely to say they will not get vaccinated, with almost 30 percent of each group saying they will “definitely not” be vaccinated.

[Read the transcript of Dr. Leana S. Wen’s coronavirus live chat]

This is a time for leadership. A January survey showed evangelical leaders overwhelmingly planned to get vaccinated; they should step up to persuade their congregations. Former president Donald Trump should speak more loudly to his base. Role models are also valuable. Elvis Presley got the polio vaccine backstage on “The Ed Sullivan Show” in 1956, then publicized it on radio, and vulnerable teenagers flocked to follow. Today’s stars should, too. Another tool: incentives. President Biden on Wednesday called on employers to give paid time off for vaccination; how about other inducements, such as bonuses?

Also, it is important for public health and government leaders to squarely address the safety concerns that are driving reluctance to get a shot. The cases of a blood clot condition are extremely rare.

Ultimately, vaccines are essential to put out the wildfire of infection around the globe and help prevent variants from spreading. They will bring us closer to pre-covid normality — a gift not to be hesitant about.

Read more: Leana S. Wen: The covid-19 vaccines are an extraordinary success story. The media should tell it that way. Michael Gerson: If you are healthy and refuse to take the vaccine, you are a free-rider The Post’s View: No shot, no shoes, no service. We must try to make vaccine passports work. Cornel West and Jeremy Tate: Howard University’s removal of classics is a spiritual catastrophe Eugene Robinson: Derek Chauvin’s conviction shouldn’t feel like a victory. But it does.