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.
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.
Flu Season, Overall Child Vaccination Rate, Overall Adult Vaccination Rate 2010-11 51 40.5 2011-12 51.5 38.8 2012-13 56.6 41.5 2013-14 58.9 42.2 2014-15 59.3 43.6 2015-16 59.3 41.7 2016-17 59 43.3 2017-18 57.9 37.1 2018-19 62.6 45.3
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."
