The United States has a far higher share of seniors without full vaccine protection than many other wealthy countries, a key risk factor driving serious COVID-19 illness and death, a New York Times analysis shows.

As the delta variant has torn across the country, America’s pace of vaccinations has sped up after months of relative stagnation, and the full federal approval of the Pfizer vaccine could extend that momentum. Just over half of Americans are now fully vaccinated.

But national averages mask the high rate of older Americans who remain deeply vulnerable. Older people still account for most COVID-19 deaths, and in many counties, especially in the South and Mountain West, seniors without full vaccination make up more than 10% of the total population.

“We have swaths of populations in counties who are healthy Americans, over 60, who are not vaccinated,” said Claire Hannan, executive director of the Association of Immunization Managers. “These people are at extreme risk, and they don’t realize it.”

By contrast, unvaccinated seniors in Britain, Spain and Canada are relatively rare. And they are rare there no matter where they live: Those nations also have far fewer disparities between the most and least vaccinated areas, according to local immunization data.

Even London, which has higher rates of vaccine hesitancy among seniors than the rest of Britain, is far ahead of most of the United States in its vaccination rates.

That discrepancy might help explain why the delta wave has led to such a higher rate of death in the United States than in Britain, public health experts say. Although cases surged in both countries, those cases caused many fewer hospitalizations and deaths in Britain, suggesting that vaccines had weakened the link between infection and serious disease.

Although older Americans are more likely to be vaccinated than younger Americans, seniors without full vaccine protection are at much higher risk of dying from COVID-19.

The delta variant has hit many areas with clusters of vulnerable seniors particularly hard. Low senior vaccination rates in Arkansas, Florida, Idaho, Louisiana and Nevada have coincided with surging rates of hospitalization and death.

Comparisons between countries are inexact, and no single factor determines how many people get seriously sick or die during a COVID-19 outbreak. Overall vaccination coverage in a community, existing health conditions, access to medical care and individual behavior such as mask- wearing all play large roles.

Some areas at high risk in the United States have not yet experienced severe delta outbreaks, including parts of Colorado and North Dakota.

But vaccines are highly effective at preventing serious illness and death, even against the delta variant, and public health experts say the level of senior vaccination is a useful tool for understanding vulnerability during an outbreak.

Although younger, unvaccinated adults are making up an increasing share of new hospital visits, “the vast majority of people dying from COVID-19 are people who are older and unvaccinated,” said David Dowdy, an infectious disease epidemiologist at Johns Hopkins University.

As in the United States, France and Italy have struggled with vaccine hesitancy among many seniors. The rates of less protected older people in some areas, including Sicily in Italy and Marseille in France, rival those in the American South.

Vaccine hesitancy is not unique to the United States, but it is more widespread and forceful than in Europe, and it breaks down more clearly along political lines. State and local governments have also been less willing than European leaders to use mandates to pressure residents to be vaccinated.

The Food and Drug Administration’s decision to grant full approval to the Pfizer vaccine has led some people who have been hesitant to get vaccinated. And it has sped up vaccine mandates: The Pentagon requires shots for 1.4 million active-duty troops, and some hospitals, colleges, corporations and other organizations have followed suit.

But those mandates are still narrower than those in parts of Europe, where policymaking is more centralized. All French and Italian residents must show proof of vaccination or a negative coronavirus test to gain entry to most indoor venues, including restaurants and bars.

Signing up older Americans for their first shot remains a struggle, public health experts say, as people who really wanted a vaccine have already gotten it. While getting to a vaccine provider may still be an issue for some, especially in more rural areas, many more people are resistant to immunization because of their politics and personal beliefs, and those of their friends and family.

“We have really tried to get them vaccinated,” said Molly Howell, immunization director for the North Dakota Department of Health, where health officials have sent letters to every unvaccinated senior in the state. “There just seems to be a lack of trust in public health and government, and even a lack of trust of medical professionals.”

© 2021 The New York Times Company