Note: This story was last reviewed in September 2021 and still reflects current medical thinking.
As the flu season approaches in the United States, health experts are warning that the addition of another respiratory illness on top of the ongoing COVID-19 pandemic could overburden the health care system, strain testing capacity, and increase the risk of catching both diseases at once.
In a bad flu season, which peaks from December to February, 40 million to 50 million Americans may catch the flu, with some 800,000 requiring hospitalization, according to Charles Chiu, MD, PhD, an infectious disease expert at UC San Francisco.
“So the worry is that with the onset of the flu season, you’re going to get peaks of flu and COVID-19 cases at the same time,” he said. “Even with a mild flu season, the convergence with a COVID surge could very rapidly overwhelm our hospital system.”
Unlike COVID-19, however, the flu is a familiar foe, and a safe and effective vaccine is available every year.
Although the infection fatality rate of flu is less than 0.1 percent, roughly 10 times less than that of COVID-19 (current estimates range from 0.5 percent to 1 percent), it still kills 30,000 to 60,000 Americans every year. “Here we have a disease, influenza, a scourge of humankind going back centuries,” said epidemiologist George Rutherford, MD. “We have a vaccine that’s pretty effective. There’s no reason for us to be filling our ICUs with people with influenza who didn’t get vaccinated.”
For both the flu and COVID-19, the elderly and those with underlying conditions are more susceptible, but the flu also hits children particularly hard and spreads readily in schools. The more people in all age groups who get a flu vaccine, the more protected these vulnerable populations will be, experts said.
And the time to get a vaccination is early in the flu season. “It is best to have your flu shot in late September or early October in order to ensure that you have time – usually two weeks – to develop an immune response before there is widespread transmission of the flu,” said Josh Adler, MD, chief clinical officer of UCSF Health. “A flu vaccine in this time frame generally provides protection throughout the entire flu season.”
Risk of Overwhelming Hospitals
The effectiveness of the flu vaccine can range from about 20 percent to 60 percent depending on how accurately scientists have predicted the circulating flu strains that season. Effectiveness can also vary from person to person depending on their age and health. Even if the vaccine doesn’t prevent you from getting the flu, it may make the illness less serious should you get infected, said Chiu.
And this year, any reduction in flu cases and their severity could make a difference for hospital capacity, said Rutherford.
The worst-case scenario is that a surge in COVID-19 cases this winter fills ICUs so that even a mild flu season would overwhelm hospitals – not to mention leaving less capacity for patients needing intensive care from traffic accident injuries or heart attacks.
“We already know that with COVID-19 peaks, we were maxing out on our ICU capacity in places like New York City in late March of this year. So it wouldn’t take too much to put us over the edge again,” said Chiu.
Testing Could Be Strained
Another concern is testing capacity and shortages in the substances, called reagents, needed to run the tests.
“It turns out that the definitive tests for flu use essentially the same approach that we use for COVID-19 tests – it’s PCR,” said Chiu.
The same reagents are needed to extract DNA or RNA from samples in both tests. With flu and COVID-19 sharing similar symptoms, the demand for both tests may increase and lead to reagent shortages, said Chiu.
His lab is currently developing a single rapid test that would look for flu and COVID-19 at the same time, but such multiplex testing is not yet widely available for high-throughput screening.
You Don’t Want to Have Both At the Same Time
It’s also possible to be infected with the flu and COVID-19 at the same time, although it’s too soon to tell how common such co-infections will be.
“It’s not yet been established that dual or consecutive infection causes more severe disease, but I would speculate that that’s likely the case because the same site of infection is involved,” said Chiu.
What is clear is that the flu and COVID-19 both target the lung, said Rutherford.
“And to the extent that the lungs are damaged by one disease and the other one comes along and damages it more, you can get into trouble faster,” he said.
He added that it’s also known that influenza can make you more susceptible to infection by other pathogens. “In fact, a lot of the deaths of the 1918 flu pandemic were probably from staphylococcal pneumonia as a superinfection on top of influenza pneumonia,” he said.
In the 2018-19 flu season, about 49 percent of Americans received a flu vaccine, averting an estimated 4.4 million illnesses, 58,000 hospitalizations, and 3,500 deaths, according to the CDC. Experts hope more Americans will choose to get a flu shot this year – for themselves and for others.
Vaccination Rates Between Children and Adults in the U.S.
And while the Southern hemisphere, which experiences the flu earlier than the Northern hemisphere, has been experiencing a relatively mild season, that is at least partially thanks to COVID-19 mitigation measures such as social distancing and mask wearing. The trends in places like Australia suggest a milder season for the United States as well, but there are no guarantees, experts said. Colder winters in the north mean that people will be forced indoors where the risk of respiratory infections from close contacts is higher, said Chiu.
“If you’re walking around with the flu, you’re going to be a big transmitter,” said Rutherford. “So a vaccine is a chance for you not to transmit. In a season of COVID, you don’t want to have both at the same time, believe me.”
Still Not Sure About Getting a Flu Vaccine This Year?
In a typical flu season, less than half of Americans get a flu shot, putting themselves and their loved ones at risk of a preventable disease.
Some people may have doubts about vaccine efficacy. Others think the flu is no big deal. And during COVID-19, some worry about the risks of going to a doctor’s office.
We asked the experts about these common concerns.
I’m not sure about getting a flu vaccine because…
1. Going to the doctor’s office seems risky during COVID-19.
“The risk at a doctor’s office is very likely to be much less than the risk of going out to a supermarket or a crowded location,” said Chiu. That’s because health care workers are vigilant about taking the appropriate precautions. Most documented COVID-19 outbreaks have occurred in gatherings where people were not adhering to standard precautions like mask wearing and social distancing. “This is exactly the opposite of a controlled situation such as the doctor’s office,” said Chiu.
Experts say local pharmacies are also safe settings to get a flu shot.
Like any activity during COVID-19, it’s about balancing the risks and benefits. “The benefits of getting a vaccine far outweigh the very, very low risk of one visit to a doctor’s office while wearing a mask and social distancing,” said Chiu.
2. I’m already social distancing and wearing a mask, so I won’t catch the flu.
It’s true that your risk of catching the flu is much less if you wear a mask and social distance one hundred percent of the time, said Chiu, but “we know that people get tired of these measures” and may not always be rigorous.
As colder weather sets in, more activities will move indoors where transmission is more likely. At the same time, more states may be opening up and lifting restrictions, more people will be going back to work and kids going to back to school. “Although the risks might be low now that people are still behaving, the worry is that it’s going to change,” said Chiu. “It only takes one episode where you forget to wear your mask, or choose not to, that can put you and others at risk.”
3. I heard the flu season is going to be mild this year
It’s true that Australia, where the flu season peaks from June to August, has so far seen a much milder flu season, but that’s likely due to social distancing, travel restrictions, and a higher than usual vaccination rate.
Those factors may not translate to the colder temperatures and variable mitigation measures across the U.S. The risk will also fall disproportionately on essential workers who don’t have the luxury of sheltering at home, said Chiu.
“My worry isn’t that we’re going to have an overwhelming flu season,” said Chiu, “My worry is that even a mild flu season may be enough to tip our health care system over the edge.”
4. The flu vaccine isn’t very effective anyway.
The effectiveness of the flu vaccine varies from year to year depending on how closely we’ve guessed the strains, but we won’t know the effectiveness of this year’s vaccine until the flu season is over next spring, said Chiu. It’s too early even to glean from the Southern hemisphere, which is still in its flu season.
“What’s valuable about the vaccine is that even if it is, say, only 50 percent protective one year, it has been shown that getting the vaccine may decrease the severity of illness and your risk of being hospitalized for influenza,” said Chiu.
Besides the personal benefit, getting a flu vaccine also contributes to the wider public health effort. “I think it’s important to realize that taking the vaccine is not just for you, it’s also to prevent spreading the virus to others,” he said. “So on a community level, the more people that are vaccinated, the fewer overall cases we’re going to get.”
5. I’m young and healthy, so the flu isn’t dangerous for me.
“People die of influenza at all age groups,” said Rutherford. “It causes a lot of morbidity and mortality.”
“Even in someone who is young and healthy, influenza can put them out of work for one to two weeks because of the fever and overwhelming fatigue,” said Chiu. “It’s not a disease that you want to get.”
Another reason for young people to get the flu shot, said Chiu, is to protect those who are more vulnerable. You can spread the virus to others even before you are overtly symptomatic. “If you get influenza, you may be putting your loved ones – the very young, the very old, such as your children, parents, and grandparents – at risk.”
6. The vaccine contains harmful chemicals and can cause disease.
“The components of a flu vaccine have been extensively studied and vetted by not only the CDC, but also the FDA, and the data show overwhelmingly that these vaccines are safe and effective,” said Chiu.
“Just like any vaccine, there’s a possibility of some soreness or redness at the site of the shot, or low-grade fever and aches,” said Chiu. “But flu vaccines do not cause flu and they do not cause conditions like autism.”