“It would not be particularly encouraging if we have a vaccine that’s capable of protecting 20-year-olds who probably have a pretty low risk anyway of getting sick, and doesn’t work at all for people over 65,” Francis Collins, director of the National Institutes of Health, said in an interview.

To find a vaccine that works safely in older adults, researchers at Pfizer PFE 0.54% Inc. and other companies are exploring possible options such as increasing the doses or adding a booster to the shot. At least one vaccine specifically for the elderly is in development. And Pfizer, the University of Oxford and others have started testing their coronavirus vaccine candidates in older adults.

Some of the experimental vaccines “may turn out to be better for older individuals, and that’ll be a big issue in terms of how we then end up deploying these,” Dr. Collins said.

Older adults have been especially hard hit by the new coronavirus. Between March and mid-June, Americans over the age of 65 had the highest rate of hospitalization among all age groups, about double the rate for people 50 to 64 years old and five times the rate for 18- to 49-year-olds, according to the Centers for Disease Control and Prevention.

Vaccines will be crucial, public-health experts say, to stopping the spread of the virus. More than 100 are in development and more than a dozen are in human testing, according to the World Health Organization.

The shots work by tricking the body into thinking it has been infected with a virus, prompting the immune system to make antibodies to fight the pathogen. Yet antibody production weakens over time, part of a process known as immunosenescence.

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