As public health officials look to fall and winter, the specter of a new surge of Covid-19 gives them chills. But there is a scenario they dread even more: a severe flu season, resulting in a “twindemic.”

Even a mild flu season could stagger hospitals already coping with Covid-19 cases. And though officials don’t know yet what degree of severity to anticipate this year, they are worried large numbers of people could forgo flu shots, increasing the risk of widespread outbreaks.

The concern about a twindemic is so great that officials around the world are pushing the flu shot even before it becomes available in clinics and doctors’ offices. Dr. Robert Redfield, director of the U.S. Centers for Disease Control and Prevention has been talking it up, urging corporate leaders to figure out ways to inoculate employees. The C.D.C. usually purchases 500,000 doses for uninsured adults but this year ordered an additional 9.3 million doses.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, has been imploring people to get the flu shot, “so that you could at least blunt the effect of one of those two potential respiratory infections.”

In Britain, Prime Minister Boris Johnson has been waging his own pro flu-shot campaign. Last month, he labeled people who oppose flu vaccines “nuts” and announced the country’s largest ever rollout of the shots. In April, one of the few reasons Australia allowed citizens to break the country’s strict lockdown was to venture out for their flu shots.

The flu vaccine is rarely mandated in the U.S. except by some health care facilities and nursery schools, but this month the statewide University of California system announced that because of the pandemic, it is requiring all 230,000 employees and 280,000 students to get the flu vaccine by November 1.

A life-threatening respiratory illness that crowds emergency rooms and intensive care units, flu shares symptoms with Covid-19: fever, headache, cough, sore throat, muscle aches and fatigue. Flu can leave patients vulnerable to a harsher attack of Covid-19, doctors believe, and that coming down with both viruses at once could be disastrous.

The 2019-20 flu season in the United States was mild, according to the C.D.C. But a mild flu season still takes a toll. In preliminary estimates, the C.D.C. says that cases ranged from 39 million to 56 million, resulting in up to 740,000 hospitalizations and from 24,000 to 62,000 flu-related deaths.

According to the C.D.C., flu season occurs in the fall and winter, peaking from December to February, and so was nearing its end as the pandemic began to flare in the United States in March.

But now, fighting flu proactively during the continuing pandemic presents significant challenges: not only how to administer the shot safely and readily, but also how to prompt people to get a shot that a majority of Americans have typically distrusted, dismissed and skipped.

With many places where the flu shot is administered en masse now inaccessible — including offices and plants that offered it free to employees on site and school health clinics — officials have been reaching out to local health departments, health care providers and corporations to arrange distribution. From now through Oct. 31, publicity campaigns will blast through social media, billboards, television and radio. Because the shot will be more difficult to access this year, people are being told to get it as soon as possible, although immunity does wane. There will be flu shot tents with heaters in parking lots and pop-up clinics in empty school buildings.

Because of the efforts, vaccine makers are projecting that a record 98 million flu shots will be given this year in the United States, about 15 percent more than doses ordered last year. The Kaiser Permanente health care system will be flooding more than 12 million of its members with flu shot reminders via postcard, email, text and phone calls.

Pharmacies and even supermarkets are expected to play a bigger role than they have in previous years. As of this week, Walgreens and CVS will have flu shots available. Walgreens will be hosting additional off-site flu vaccine clinics in community centers and churches. To reduce contact time, CVS is allowing patients to fill out paperwork digitally.

In New York City, which averages about 2,000 flu-related deaths a year, the health department has been reaching out to hundreds of independent pharmacies to administer the shots, because they are often located in outer-borough neighborhoods where the coronavirus has been rampaging. The health department has a detailed online flu vaccine locator.

“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 2018-19 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.

As health officials note, should a vaccinated person contract the flu, the severity will almost certainly be reduced, hospitalization rarely necessary. Especially with Covid-19 raging, public officials reason, those odds look pretty good.

Another reason people give for not getting the shot is they think it makes them sick.

“People who say ‘I’ll never get it because it gives me the flu’ have not had the flu and don’t know what it is,” said Patsy Stinchfield, senior director of infection prevention at Children’s Minnesota.

“What you’re feeling is your body’s immune response to the virus’s antigens,” said Ms. Stinchfield, a member of the C.D.C.’s influenza work group. “You may feel flu-ish. And that’s a good thing. It’s your body’s way of saying, ‘I am ready for the flu, and I won’t get as sick if I get the real one.’”

Public campaigns will describe the shot as a critical weapon during the pandemic. “Hopefully people will say, ‘There’s no Covid vaccine so I can’t control that, but I do have access to the flu vaccine and I can get that,’” Ms. Stinchfield said. “It gives you a little power to protect yourself.”

Other campaigns will emphasize familial and community responsibility.

Usually, flu vaccine compliance rates among people ages 18 to 49 are low. Vermont’s, for example, is only about 27 percent.

Christine Finley, the state’s immunization program manager, believes that rates will improve because of the pandemic’s stay-at-home households. “People are more aware that the risks they take can negatively impact others,” she said. “They’re often taking care of young children and older parents.”

If any example could prove instructive about protective behavior and flu vaccines during the coronavirus epidemic, it could well be Australia.

Australia’s flu vaccine rate tends to be modest, but this year demand was high. The government’s rollout of the shot began earlier than usual for the June-through-August winter because the coronavirus pandemic was exploding. Though the government had also issued strict no-entry limits among many states and territories and bans on international travel, the flu shot was one of the few reasons people could emerge from lockdown.

The prevalent strain circulating in the country is Type A, the most common and virulent form of flu, said Dr. Kelly L. Moore, a public health expert at the Vanderbilt University School of Medicine.

According to the C.D.C., Type A is the most likely to circulate globally. It mutates readily, particularly as it jumps between animals and humans.

“There are two strains of Type A influenza in the vaccine,” Dr. Moore said, “and so the very best way to protect yourself is to get the shot.”

Reported cases of flu in Australia have dropped 99 percent compared with 2019.

“Australia’s milder-than-usual flu season is likely the result of a number of factors — strong flu vaccination uptake, social distancing, but also severely decreased movement of people,” said Dr. Jonathan Anderson, a spokesman for Seqirus, a supplier of flu vaccine.

But though American public health authorities usually look to Australia’s flu season as a predictive, Australians say this year it’s not a reliable indicator.

“This situation is of no comfort as these measures do not apply to the United States where the populace has never been effectively physical distancing,” nor have the country’s entry restrictions been as onerous, said Dr. Paul Van Buynder, a public health professor at Griffith University in Queensland, Australia.

All that Americans can do is get vaccinated against flu, he added, because the circulation of the coronavirus remains high.

“It is likely they will have a significant influenza season this northern winter,” he said.