U.S. health officials this week moved to expand access to flu shots and to tighten vaccination requirements for students, with the nation facing the prospect of flu season arriving amid the coronavirus pandemic and creating an unprecedented threat to health.

The two highly contagious respiratory illnesses are spread in similar ways, mainly through respiratory droplets. They have similar symptoms, including fever, chills and headaches. Each can cause life-threatening illness and death. Together, they could pose a double burden on the nation's already strained health-care systems, including labs that conduct flu and coronavirus tests.

"You've got this perfect storm in autumn. Everyone is back from vacation. Some are going back to work. In some parts of the country, schools are starting," said Lawrence Gostin, a global health professor at Georgetown University. "You have a mass migration, and with it people are going to be bringing both influenza and COVID-19. ... To me, there is no plausible reason why we wouldn't have both."

In anticipation of that double whammy of flu and COVID-19, the disease caused by the novel coronavirus, flu vaccine manufacturers have boosted production by about 15%, to record levels, with nearly 200 million doses for the United States. During the 2019-2020 flu season, more than the 175 million doses were churned out, industry officials said.

Massachusetts announced Wednesday it is requiring flu vaccinations for all youths 6 months or older who attend child care, preschool, K-12 classes or college in the state, becoming the first to require flu vaccination across all ages from child care to college. Six states require flu vaccination for day care, but no other state mandates it for K-12 students, Gostin said. The Massachusetts Department of Public Health said students will be expected to have a flu vaccine by Dec. 31 unless they provide a medical or religious exemption.

Childhood immunizations have dropped sharply during the pandemic. To expand access, federal health officials have issued a directive allowing pharmacists in all 50 states to give childhood vaccinations, including flu shots. The move was praised by some advocacy groups but drew sharp criticism from the American Academy of Pediatrics, which called the announcement "incredibly misguided." Saying that pediatricians' offices are open and safe, the physicians group said families should be encouraged to go to doctors they trust most in the middle of a pandemic.

The Centers for Disease Control and Prevention advises that everyone 6 months or older get a flu vaccine annually, with few exceptions. The vaccine is considered especially important for individuals at risk of severe disease, including children younger than 2, adults older than 65, people with a suppressed immune system, pregnant women and anyone with chronic lung, heart, kidney or liver disease.

The CDC recommends September and October as good times to get vaccinated. With increased availability of flu vaccine, and assuming flu viruses are circulating, vaccination should continue to January or later, the agency said. Experts have little data about the interaction between the coronavirus and the influenza virus, but it is possible for someone to be infected with both at the same time.

"If you are recovering from COVID and then you happen to get flu on top of that bad lung disease, that is not good," said Flor Munoz, a pediatric infectious-disease specialist at Baylor College of Medicine in Houston.

It takes about two weeks for the flu vaccine to provide full protection, so doctors recommend individuals be vaccinated at least two weeks before influenza is circulating widely.

Experts have monitored flu activity in Australia and elsewhere in the Southern Hemisphere as a possible predictor of the season in the United States. Many countries, including Argentina, Australia, Chile, New Zealand, South Africa and Zimbabwe, are reporting low levels of flu.

The global flu surveillance network found during the last two-week reporting cycle that only 37% of almost 300,000 test specimens came back positive for flu, a senior World Health Organization official said this week.

Experts cited public health measures put in place in countries heavily affected early in the coronavirus pandemic as helping to reduce flu circulation. They fear the picture in the United States is likely to be different. In Australia and elsewhere, "people were following government directives and staying put," Munoz said.

"Here in the United States, clearly people are relaxing more," she said. The consequence is that mass gatherings at parties, weddings and other social events have the potential to be "super-spreader" events.

More than ever, consumers should make flu vaccination a priority, health officials said. Pharmacies and doctor offices have put systems in place for social distancing. The CVS pharmacy chain said it is planning to supply twice the number of vaccines as last flu season. Starting in September, CVS is expanding digital options for making appointments, allowing people to make them via text for the first time.

"There is not going to be a vaccine shortage," said Arnold Monto, an epidemiology professor at the University of Michigan School of Public Health. "There are systems in place to make sure you do not wait in the crowd," he said, noting that people need to remember that the main way both diseases spread is through person-to-person contact. "This is not rocket science."

There is not yet a vaccine for the coronavirus, and if one is approved by the end of the year or early next year, only limited doses would be available initially. Reducing flu cases would free up space in hospitals, medical offices and laboratories to deal with COVID-19.

The influenza virus changes every flu season, and vaccine effectiveness varies between about 40% and 60%, depending on how well matched the vaccine strains are to the circulating virus. The vaccine's overall effectiveness last season was about 39%.

Each year in the United States, influenza is responsible for up to 45 million cases of illness, 810,000 hospitalizations and 61,000 deaths, according to the CDC. The disease caused by the novel coronavirus has infected more than 5 million people and killed at least 170,000 in the United States since February, according to Washington Post data.

Only about half of Americans get a flu vaccine each year. For Black, Hispanic and American Indian adults, immunization rates are even lower.

"These gaps in vaccination coverage are particularly concerning this season as COVID-19 reveals another facet of health inequity in the U.S.," according to CDC Director Robert R. Redfield, who co-authored an essay published Thursday in JAMA.

Those groups have also been disproportionately affected in the pandemic. An analysis of data from more than 3,000 U.S. counties through mid-April found that 20% of counties where Black residents made up more than 13% of the population accounted for more than half of covid-19 diagnoses and deaths, according to the CDC.

Getting a flu shot will be a critical intervention in reducing or preventing people from getting severe illness and dying, Redfield said.

"This fall and winter could be one of the most complicated public health times we have with the two coming at the same time," he said in a separate interview Thursday with JAMA. He said the CDC's goal is for 65% of Americans to get a flu vaccination this season.

The directive allowing pharmacists to give children vaccinations was issued by the Department of Health and Human Services to ensure immunization rates remain high during the pandemic. HHS Secretary Alex Azar used emergency powers granted during the coronavirus pandemic to issue the directive.

Currently, 28 states allow pharmacists to administer vaccinations to children, but 22 limit such vaccinations. The HHS directive would supersede those restrictions during the public health emergency. Pharmacists must complete a training program, according to an HHS statement.

Many of the symptoms of influenza and covid-19 are virtually indistinguishable. But there are key differences, experts say:

- The novel coronavirus is more contagious than influenza.

- Loss of taste or smell is a symptom for covid-19 but not flu.

- Nasal congestion and sore throat are among the most common symptoms for flu but not covid-19.