What If Covid-19 and Flu Both Flare Up This Fall?

A photo of a woman wearing a mask looking out the window from her home.
Photo: Justin Paget/Getty Images

If the Covid-19 case curve continues to flatten and decline in the United States as hoped, the credit will go to physical distancing, improved hygiene awareness, and other preventive measures, experts say. But we should not presume the virus will be licked anytime soon. In fact, many infectious disease experts are worried about a comeback, a second wave, that could strike more quickly and harder than this initial wave of infections.

Worse, it could arrive just as flu season is ramping up, potentially exacerbating symptoms and causing fresh havoc for the health care system and the economy. “Once we start opening things back up, we’re going to have a lot of individual cases that are still brewing under the surface,” says Michael Mina, assistant professor of epidemiology at Harvard T.H. Chan School of Public Health, on an April 28 conference call. “If we don’t put out all the flames, then we’ll have this smoldering number of people that will all be able to ignite outbreaks at once.”

The math of a second wave could be much different, Mina explains. The initial outbreaks were isolated, starting mostly in a few cities, where one case might have led to four other infections, then 16. In a second wave, you could have, say, 100,000 cases at the start that spread all over the country and lead to 400,000 new infections, then 1.6 million, and so on.

The 1918-19 influenza pandemic offers a lesson on how viral outbreaks can come in waves, Mina says. The first U.S. outbreak was in March of 1918. The spread subsided that summer then resurged in the fall, creating a far deadlier second wave of infections. After a third wave during the winter and spring of 1919, some 675,000 Americans had died at a time when the total population was just a third what it is today.

Could a second wave come sooner than the fall?

While the 1918-19 pandemic was caused by the flu virus and therefore does not offer an apples-to-apples comparison to the coronavirus, Covid-19 is not the first coronavirus-caused disease to make waves. Exactly 17 years ago, “we were smack-dab between two waves of SARS infections in Toronto,” points out Mark Cameron, PhD, an immunologist and medical researcher in the School of Medicine at Case Western Reserve University in Ohio.

“We have not bent the curve enough to travel down the other side of this pandemic and release precautions in a rapid manner.”

On March 26, 2003, the government of Ontario, Canada, declared an emergency due to an outbreak of the coronavirus SARS-CoV. By late April, Toronto’s mayor said the city was safe, inviting tourists back. On May 17, Ontario lifted the emergency.

“Within two weeks, the second wave came roaring back, just as strongly as the first and for just as long,” says Cameron, who studied the disease while working at Toronto General Hospital at the time. He worries a second wave of Covid-19 infections could come well before fall if politicians and businesses rush too quickly to get back to normal.

“We have not bent the curve enough to travel down the other side of this pandemic and release precautions in a rapid manner,” Cameron tells Elemental. “I share some of the optimism in planning a return to some everyday activities, but to do this at the peak of an outbreak, no matter how flat the curve has become in some states, is inviting a second wave.”

And if the virus is unleashed in a second wave, whether now or this fall, Cameron says the ramp-up could be much quicker. “It would be crushing. We won’t have the slower city-by-city spread pattern we experienced this spring as the first wave of the pandemic ran its course. It will seem like it is hitting us from all directions. … The clock would be reset to zero, and the same precautions would need to be put back in place. It will feel like a bad game of Whac-A-Mole.”

Meanwhile, if we were worried about overwhelming the health care system back when businesses were closed and people were ordered to stay home, “then we should be more worried now,” says Marc Lipsitch, PhD, a professor of epidemiology and director of the Center for Communicable Disease Dynamics at Harvard T.H. Chan School of Public Health. “Though we’ve slowed down the increase, we’re in a worse position now than we were then” because there is more virus spreading now.

“Viruses don’t know where they were in the past,” Lipsitch says. “They only know where they are in the present.”

Lipsitch doesn’t think summer will have a tremendous effect on the coronavirus. He points to Australia and New Zealand, where the virus took hold during their summer, clearly showing it can spread in warmer weather.

“We can expect a little bit of reduction in transmission in the Northern Hemisphere” this summer, Lipsitch told a group of reporters April 30, “but not by itself enough to stop transmission.”

A big second wave is not inevitable

A serious resurgence of Covid-19 is far from a given, says virologist Andrea Amalfitano, DO, dean of the College of Osteopathic Medicine at Michigan State University.

While it’s almost a sure thing there will be at least some cases this fall, too little is known to predict the scope with any accuracy, Amalfitano argues. Some locations might have had enough cases to develop some level of herd immunity, he says, meaning a high percentage of people would be protected and therefore not catch or spread the disease, pushing down the rate at which one person infects others and squelching the spread.

Mina agrees that herd immunity may have been achieved in specific locations, like individual nursing homes or even densely populated neighborhoods. But he thinks any collective protection will be highly localized, not serving the population in general. Even if 10% of the U.S. population has been infected, which Mina thinks is on the high side and is 30 times the current roughly 1 million diagnosed cases, “it still isn’t enough to stop this virus from roaring back.”

“Depending how contagious an infection is, usually 70% to 90% of a population needs immunity to achieve herd immunity,” according to researchers at Johns Hopkins University. There are other wild cards. It’s not yet known how robust or long-lasting a person’s Covid-19 immunity will be. Experts expect some immunity, but they also know it tends to wane over time, and every virus prompts a different immunity response, which in turn varies by individual.

The pandemic is likely to last 18 to 24 months, according to an analysis led by Kristine Moore, MD, medical director of the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota. In the report, released April 30, Moore and her colleagues say Covid-19 “appears to spread more easily than the flu” and “likely won’t be halted until 60% to 70% of the population is immune.”

Meanwhile, there’s no guarantee that a useful Covid-19 vaccine can be discovered and mass-produced by this fall, though hopes are high. Researchers at Oxford University are fast-tracking a vaccine candidate and say it could be ready by September. Amalfitano, who has worked on efforts to create vaccines for the flu and HIV, tells me the Oxford group is “very well esteemed” and could deliver but notes that promising vaccine candidates sometimes flop completely during the clinical trials aimed at proving their effectiveness.

Somewhere between a third and a half of vaccine candidates aimed at other diseases in past efforts failed in the late stages of testing, Lipsitch points out. He is “hopeful but only cautiously hopeful” that a vaccine will be ready by this fall, putting the odds at somewhere between 1 in 3 and “maybe 1 in 2 if we’re lucky.”

Other vaccine experts predict that even having one ready in 18 months is highly optimistic.

Like many epidemiologists, Lipsitch is concerned states may open back up too quickly and too fully amid a lack of testing to reveal how many people actually have Covid-19 and without antibody tests that would begin to provide clues as to who might have some immunity to the disease.

“In the absence of much better testing capacity, it’s a dangerous thing to start lifting restrictions,” he says.

Flu and Covid-19: The potential double whammy

It’s not clear what would actually happen if influenza and Covid-19 reared their ugly heads together. Robert Redfield, director of the Centers for Disease Control and Prevention, said last week that “there’s a possibility that the assault of the virus on our nation next winter will actually be even more difficult than the one we just went through.”

That said, it’s possible that one viral infection could actually serve to protect a person from getting another viral infection at the same time. “If one virus elicits an innate immune response, that might help shield cells against viral entry,” Mina explains. Amalfitano doubts this would be the case, however, since the two diseases develop distinctly different antibodies, the immune-system proteins that fight the respective viruses.

Or people might catch both. “One virus distracts your immune system, and then the other virus comes in,” Mina says. Amalfitano agrees this is possible. And it could be disastrous for an individual, given the compounding of symptoms. In fact, a similar and common double infection of bacterial pneumonia is what actually kills many flu victims.

Dealing with two infectious diseases at the same time this fall — a real possibility — would also be a “double challenge” for the health care system, says Dr. Thomas Frieden, former director of the CDC.

There’s a potentially bright flip side to all this, however.

“If we take aggressive measures” against any possible reemergence of coronavirus this fall, “we could inadvertently prevent wide-scale transmission of influenza” and see a “remarkable” reduction in flu cases, Mina points out. Amalfitano agrees, and they’d both love to see some of the newly adopted health behaviors become permanent, from coughing into elbows and washing hands to actually staying home from work when sick.

“It’s a public health person’s dream to have people do a little bit of social distancing during flu season,” Mina says, adding, “Do we really need to shake hands during flu season?”

Independent health and science journalist, former editor-in-chief of LiveScience, writing about how we age and how to optimize your mind and body through time.

Get the Medium app