Colds Are Back. What Does That Mean for Your Chance of Catching Covid-19?
It may seem scary that colds are spreading again. But it’s more complex than you think.
Last week, on my kids’ fourth day of in-person school, I came down with my first sore throat in six months. At first, I downplayed it — it was just allergies. Then I panicked and had to take a Xanax. Then I thought: How the heck could I have gotten sick? My kids and husband seemed perfectly healthy, and other than their school, which was being held entirely outside, the riskiest place I’d been over the past two weeks was the Walmart parking lot — once — to pick up curbside groceries.
My most pressing concern, of course, was whether I had Covid-19. Looking up the symptoms, I wasn’t reassured. On its website, the Centers for Disease Control and Prevention lists 11 possible coronavirus symptoms (including sore throat), then warns that this seemingly exhaustive list “does not include all possible symptoms.” A systematic review of 16 studies published in July concluded that no symptoms are common enough to distinguish Covid-19 from other infections, including the common cold.
Covid-19 “ranges from everything from just a cough to just a headache to just the sniffles to fever plus cough plus sniffles — the whole permutation and combination spectrum is there,” says Ian MacKay, PhD, a virologist at the University of Queensland in Australia. Without a Covid-19 test (which, like any test, also isn’t 100% accurate), I couldn’t distinguish whether my sore throat was an allergic reaction to ragweed or the infamous infection that has so far killed more than 200,000 Americans.
I’m better now — and much, much calmer after two negative Covid-19 tests. But I am not the only one baffled — and concerned — by the burgeoning spread of colds this autumn. This past spring and summer, colds and other respiratory viruses were down by a lot due to lockdowns, but things seem to be creeping back toward a more sniffly normal. The other day, I took to social media to ask if anyone else had recently been sick, and I couldn’t keep up with the worried replies. “All of us!” a mother from New York lamented. “Four colds in this house,” a mother from Boston joined in. “It only took one week of my preschooler being back in the classroom for us to get a cold,” a dad from Philadelphia said, even though, he added, everyone in his family was being cautious and wearing masks.
Are all the precautions we’re taking — social distancing, mask-wearing, frequent hand-washing — for naught?
These comments are just anecdotes, but regional and national reports, as well as recent data from Australia and Sweden, support the notion that colds are starting to spread again. Melissa Miller, PhD, director of the Clinical Microbiology and Molecular Microbiology Laboratories at the University of North Carolina School of Medicine, told me that approximately twice as many tests administered in the hospital came back positive for respiratory viruses other than Covid-19 in the month starting August 25 compared with the month starting July 24. Every week, too, some U.S. national labs share data with the CDC regarding the total number of tests they have run for respiratory adenoviruses — viruses that cause some common colds — and how many came back positive. Although adenoviruses are circulating much less than they were at this time last year, between September 12 and September 19, 2020, a much higher percentage of antigen and PCR tests for adenoviruses came back positive (although numbers have since dropped down a bit again).
The implications of this burgeoning viral spread are concerning, for sure. After all, if colds are spreading in schools and workplaces, couldn’t Covid-19, too? Are all the precautions we’re taking — social distancing, mask-wearing, frequent hand-washing — for naught? I dug into the science to learn more about what the spread of colds might mean for our collective health and our immune systems. I found some things to be concerned about but also a few possible silver linings.
Colds and Covid-19 spread similarly — and differently
Let’s start with the claim that if a cold can spread in a school or workplace, Covid-19 could too. It’s true that respiratory viruses tend to spread in similar ways. They enter our bodies through our noses, mouths, or eyes; they make more copies of themselves in our respiratory tract; and then they typically spread to other people through our respiratory mucus. People might breathe in air that contains our exhaled (or sneezed- or coughed-out) viral particles, or they might touch surfaces that our virus-laden hands contaminated and then transfer the virus to their respiratory tract when they touch their noses or mouths.
It stands to reason that “if there are opportunities for one of these viruses to spread, then that opportunity is going to be there for all of them, but possibly to different degrees,” says Sema Nickbakhsh, PhD, a research fellow at the University of Glasgow Centre for Virus Research.
“Rhinoviruses are really well adapted to live with humans and to exist in our common day-to-day life.”
Rhinoviruses are especially important because they cause the vast majority of colds, and each rhinovirus genotype looks different enough to our immune system that catching one doesn’t then protect us against the others. In a 2018 study, researchers periodically tested 225 kids in Washington state over the course of two years and found 41 different genotypes of rhinoviruses circulating among them, with some kids being infected by multiple rhinoviruses at once.
So the sheer ubiquity of rhinoviruses may help to explain why we are seeing them again. They’re everywhere, and we don’t have immunity to most of them. “Rhinoviruses are really well adapted to live with humans and to exist in our common day-to-day life,” MacKay, of the University of Queensland, said. Generally, viruses also survive and spread more easily when the air is cooler and less humid, so the fact that I started wearing a jacket at around the same time my sore throat appeared may not be a coincidence (although the belief that cold weather itself can give you colds is a myth).
Also, rhinoviruses (and adenoviruses) are hardy — hardier than coronaviruses — and able to survive for days on surfaces. Coronaviruses are wrapped in fragile outer envelopes that can easily be broken down over time and by different substances, but rhinoviruses and adenoviruses lack these. This is one of the reasons soap is so effective against Sars-CoV-2 — it both breaks down its outer envelope, destroying the virus, and also washes away the inactivated virus. (With hardier nonenveloped viruses, like rhinoviruses and adenoviruses, soap mainly just removes the viruses from our hands. It doesn’t always inactivate them.)
These different viral characteristics suggest that the steps we take to curb Covid-19 might not be as effective against colds. Sars-CoV-2 is believed to spread primarily through respiratory droplets and aerosols, which is why social distancing and mask-wearing are the primary recommendations for thwarting its spread. Although colds can spread via droplets and aerosols too, surfaces (sometimes called fomites) may be more important. In a 2007 study that makes me never want to stay in a hotel again, people with colds stayed overnight, individually, in hotel rooms. The next day, researchers tested various surfaces in their rooms — doorknobs, telephones, remote controls — and found that 35% of the surfaces still harbored cold virus RNA (although it’s unclear from the study whether this contamination would have been enough to sicken someone who later touched these surfaces).
Yet surface-based transmission routes aren’t always prioritized in Covid-19 prevention protocols because we’ve learned that surface transmission is rarely how the coronavirus spreads. Although most workplaces and schools disinfect surfaces during routine cleanings, it may not happen more than once a day — so cold viruses, which can remain viable on these surfaces for much longer than can Sars-CoV-2, may gain an upper hand.
Masks may also be less effective at preventing the spread of colds than they are at preventing the spread of Covid-19. In a study published in May 2020 in Nature Medicine, researchers tested how well surgical masks blocked different kinds of respiratory viruses from being exhaled into the air. They found that masks prevented the release of some influenza viruses and coronaviruses but not rhinoviruses, perhaps because rhinoviruses are so much smaller and can more easily pass through.
Based on these differences, we can’t assume that the spread of colds is a harbinger of Covid-19 doom. “It is not a perfect correlation to say that if colds spread easily in one environment, then Sars-CoV-2 will too,” says Samantha Jacobs, MD, an infectious disease physician at the Icahn School of Medicine at Mount Sinai in New York. That said, given that all respiratory viruses do spread somewhat similarly, if colds are spreading like wildfire in your school or workplace, it might be worth revisiting and perhaps ramping up your infection prevention protocols.
Viruses are competitive
The fact that Covid-19 is spreading less now than it was during the summer might also explain why colds are becoming commonplace. Respiratory viruses compete with each other: When one waxes, others wane. “People have seen, for years, these interesting patterns that look like epidemics are staggered,” says Ellen Foxman, MD, PhD, an immunobiologist and clinical pathologist at the Yale School of Medicine. In the fall of 2009, for instance, Europe braced itself for a surge in cases of the H1N1 swine flu when schools reopened, but that surge didn’t come, perhaps because, scientists have argued, the flurry of back-to-school colds held the pandemic flu in check.
Scientists are still working out exactly how this viral competition works, but it likely has something to do with our immune responses. When we get infected with one respiratory virus, our bodies mount a generalized immune reaction in the lining of our airways known as an interferon response, Foxman explains. This response might last for a week or two and makes us less susceptible to other respiratory infections. When these immune responses happen on a wide scale, they shape transmission dynamics and result in one virus seeming to be dominant over others.
It should be flu season in the Southern Hemisphere, yet the flu has been practically nonexistent, so colds may have swooped in to take its place.
In a 2019 study, Nickbakhsh and her colleagues analyzed nine years’ worth of diagnostic data in the U.K., finding that common colds are much less prevalent during flu seasons. And in a study published in the Lancet Microbe in September 2020, Foxman and her colleagues analyzed more than 8,000 respiratory samples collected at Yale-New Haven Hospital between 2016 and 2019, finding that fewer people tend to be infected with both influenza and rhinoviruses at the same time than would be expected based on chance. In a separate experiment, they infected human airway cells with rhinoviruses and found that these cold-infected cells were then resistant to infection with influenza three days later. Viral competition could also help to explain why colds are spreading so much in Australia right now, MacKay says. It should be flu season in the Southern Hemisphere, yet the flu has been practically nonexistent, so colds may have swooped in to take its place.
Alberto Paniz-Mondolfi, MD, PhD, a medical microbiologist at the Icahn School of Medicine at Mount Sinai, says that if rhinoviruses were a character in a movie, they would be King Julien in Madagascar — the “selfish egocentric,” he says. “Because rhinovirus is like King Julien, he usually grows fast and takes over the scene.” What we don’t know is whether similar dynamics might play out between colds and Covid-19 this fall — if colds become rampant, might they push out some Covid-19 that would otherwise have spread? Only time will tell.
Immunity, the double-edged sword
There’s another possible silver lining to the colds we’re getting now: Maybe, just maybe, some of them could help protect us from Covid-19.
In a small study published in June 2020 in Cell, U.S. researchers analyzed blood samples taken from people before the start of the Covid-19 pandemic. They found that half of the samples showed partial immunity to Sars-CoV-2 (in that they had immune cells called T-cells that specifically reacted to the virus), possibly because the individuals had been infected with coronavirus-based colds recently in the past. Other studies, including one that has not yet been peer-reviewed, report similar findings. Collectively, this research raises the question of whether, should we happen to catch coronavirus-based colds this fall or winter, we might develop some additional protective immunity to Covid-19.
Partial protective immunity like this plays an important role in warding off some infections, including malaria. When people live in malaria-endemic regions and are regularly exposed to the parasite, they often stay healthy. But when people leave these countries for a few years — as happens when a college student studies abroad — and then return, they often get really sick.
The problem is, no one yet knows whether the partial immunity that people develop to Covid-19 after being exposed to other coronaviruses actually staves off Covid-19 or makes it milder. “I’m not sure that there’s any conclusive evidence yet on that,” Nickbakhsh says. It’s even possible that this kind of protective immunity could make a Covid-19 infection worse (dengue, for instance, becomes more dangerous upon a second exposure).
Given everything I’ve read about how immunity shapes a response to infections, I also can’t help but wonder whether the global lockdown that started in March will make future colds more unpleasant. (The sore throat I had last week was no joke.) If we’ve had very little recent exposure to respiratory viruses, will our colds this year be more intense? Will each and every cold become the dreaded man-flu?
Experts say it’s possible. “If you haven’t had your immune system stimulated with a certain virus or pathogen that you’re used to seeing, maybe that natural immunity is a little less robust than it would have been in other years,” says Sallie Permar, MD, PhD, a pediatric immunologist at the Duke University School of Medicine. But since there are so many kinds of cold viruses, and our immune systems respond distinctly to most of them, we may not be in for anything worse than usual.
With all of the possible scenarios I’ve outlined here, there are far more questions than answers. Could the colds that are spreading be a sign that we’re failing in our attempts to control Covid-19? Maybe, maybe not. Could the colds that are spreading ward off Covid-19 through viral interference or protective immunity? Maybe, maybe not. The one thing we know for sure is that our immune systems are more complex than we usually give them credit for — and that viruses, tiny as they may be, are often just as inscrutable. This winter is going to be a wild, unpredictable ride.