“Access is a problem for all adult vaccines,” said L. J. Tan, chief strategy officer for the Immunization Action Coalition, a nonprofit group that works to increase vaccination rates, who was an early promoter of the term twindemic. “Adults may think, If I can get the flu shot easily, I might consider it.”

But as difficult as getting the flu shot to people safely will be, perhaps harder still will be persuading them to actually get it. In the 2018-2019 flu season in the United States, only 45.3 percent of adults over 18 got the vaccine, with rates for those ages 18 to 50 considerably lower.

Skepticism to this vaccine runs high, particularly in communities of color because of longstanding distrust and discrimination in public health. A 2017 study in the journal Vaccine noted that, compared with white people, “African Americans were more likely to report barriers to vaccination, were more hesitant about vaccines in general and the flu vaccine specifically, more likely to believe in conspiracy theories and use naturalism as an alternative to getting vaccinated.”

Across all demographic groups, perhaps the most striking reason given for avoiding the flu vaccine is that people do not see it as efficacious as, say, the measles vaccine.

Indeed, it is a good vaccine but not a great one. It must be repeated annually. Immunity takes up to two weeks to kick in. But its efficacy also depends on how accurately infectious disease centers worldwide forecast which strains are expected to circulate in the coming year. And then those strains can mutate.

Although the flu shot confers immunity at all ages over six months, it can be less complete in people over 65. Depending on many factors, the shot’s effectiveness in a given year can range from 40 to 60 percent.

“But a vaccine not given won’t protect anyone,” said Dr. Jane R. Zucker, assistant commissioner for the Bureau of Immunization at the New York City Health Department, which has been hosting webinars for providers about how to have conversations about the flu shot with hesitant patients.