Will COVID-19 Coronavirus Decline This Spring And Summer? Here Is What It May Depend On

Wondering whether the Spring and Summer months will bring some kind of break from the COVID-19 coronavirus? Well, if transmission of this virus does go down after late May, it’s not just the heat, it’s the humidity that could make a difference. And that may include not only the humidity outdoors but also the humidity in indoor locations like the grocery store, the hospital, your home, and the teepee of toilet paper that you’ve built in your home.

This all assumes that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) will behave like a group of respiratory viruses dubbed the “Winter viruses” because they tend to be most active during the Winter months and least active between May and October in temperate regions of the world. This group includes the flu virus, the human respiratory syncytial virus (RSV), and other human coronaviruses. But the membership application of the SARS-Cov2 to this group is still pending. That’s because the SARS-CoV-2 is still so new that no one knows what it did last Summer, or any of the previous Summers for that matter. Only time will tell whether the SARS-CoV2 manages to join this icky club.

But if SARS-COv2 does end up behaving like the Winter viruses, transmission of the virus could end up decreasing in a few months because Summertime can be a bit like a trashy romance novel: hot and steamy. As Miyu Moriyama and Akiko Iwasaki from the Yale University School of Medicine and Walter J. Hugentobler from the University of Zurich described in a review article recently published in the Annual Review of Virology, humidity seems to be a big reason for the observed seasonality of Winter viruses.

How can the amount of water vapor in the air affect the spread of viruses? Well, first of all, let’s see how it affects you. You tend to like it wet. Well, at least your respiratory tract does. Animal studies have shown that inhaling dry air may damage the cells lining your respiratory tract and inhibit their repair, making it easier for respiratory viruses to invade the cells in this lining. Additionally, dry air may dehydrate the protective mucus that is supposed to coat your respiratory tract and catch particles that you may inhale.

Dry air may also impair the functioning of tiny little hairs called cilia that line your lower respiratory tract. Normally, these cilia are supposed to beat rhythmically like a bunch of arms in a concert pushing along some crowd-surfers. Instead of crowd-surfers though, these cilia push the mucus and particle goo up and out of your lower respiratory tract into your pharynx where you can then either swallow or cough out the goo.

Then there’s that thing in the middle of your face that holds up your sunglasses, you know, your nose. Breathing in cold, dry air could prompt the blood vessels inside your nose to constrict in an effort to conserve heat. However, your blood vessels also serve as little interstate highways for white blood cells and other soldiers of your immune system. Closing up these highways can make it more difficult for immune cells to reach your nose and fight any invading viruses.

There are other things that the Winter months may do to your immune system, but it’s not be all about you. Changes in your body’s defenses don’t seem to be enough to fully account for the observed seasonality of Winter viruses. The main drivers may not be how prevailing environmental conditions affect humans but rather how they affect the virus itself and the respiratory droplets that carry these viruses.

Respiratory virus transmission typically occurs through two general means. One is by contact that’s either direct or indirect. In this case, someone who is infected and actively shedding the virus contaminates an object like his or her hands, smartphone, writing implements, fanny pack, or collection of life-sized One Direction mannequins. Whoever then touches these contaminated objects and then his or her nose or mouth can then become infected, assuming that he or she is not yet immune.

The other means is through the air. In this case, an infectious person coughs, sneezes, or in some other way exhales or spits out respiratory droplets that range from micrometers to millimeters in size. These respiratory droplets are not made out of sugar and spice and everything nice. Instead, they consist of saliva, mucus, viruses, and other gunk. If you don’t believe that you regularly spew out droplets from your mouth, try hanging a tissue in front of your mouth the next time that you are telling someone else how awesome you are and see how wet the tissue gets.

How long these droplets may hang in the air depends in part on their size. The bigger droplets tend to fall to the ground more quickly because that’s what gravity does. By contrast, smaller droplets, especially those that are considered droplet nuclei (i.e., smaller than five micrometers in size) are more likely to hang in the air longer. These droplet nuclei are also more likely to make it all the way down to your lower airways if you happen to inhale them.

So what can affect the size of these droplets? How about humidity? More water vapor in the air can mean larger droplets on average. Conversely, drier air can mean drier and smaller respiratory droplets that hang in the air longer and can get deeper inside you.

Larger droplets can get the drop on viruses in other ways as well. As a study published in the journal Applied and Environmental Microbiology suggested, more fluid in the droplet may end up affecting the droplet’s pH and salt concentration, potentially even to the point of inactivating the viruses. So, all in all, Winter viruses are kind of unsexy. They don’t like it wet but instead want it dry.

So if all of this applies to the COVID-19 coronavirus, will the Earth turning on its gigantic natural humidifier later this Spring then decrease the transmission of SARS-CoV2? Perhaps. In their review article, Moriyama, Hugentobler, and Iwasaki did point out that people are spending lots of time indoors, especially with social distancing occurring, and suggested that indoor air conditions could play a role in the continuing spread of the COVID-19 coronavirus. In other words, even when the outdoors gets more humid, the lack of humidity in indoor locations could keep the virus going. One solution could be running humidifiers indoors. A study published in the journal BMC Infectious Diseases found that running humidifiers in Minnesota grade schools during Winter months may have reduced the one-hour virus survival of the influenza virus by 30%. Such findings prompted Moriyama, Hugentobler, and Iwasaki to recommend in their review article “humidification of indoor air to maintain humidity to 40–60% relative humidity at room temperature. They also suggested wearing a face mask “to keep the nose warm and moist,” because who wouldn’t want a warm and moist nose.

Again, this all presumes that the SARS-CoV2 will behave like a Winter virus, which isn’t a foregone conclusion. Not all respiratory viruses behave like Winter viruses. For example, adenoviruses, the human bocavirus, the human metapneumovirus (hMPV), and rhinoviruses typically continue to transmit all year around, although rhinoviruses tend to cause worse infections in the Winter. Then there are the enteroviruses that seem to be most active during the Summer. The SARS-CoV2 remains a mystery, sort of like the Tinder date that won’t leave and had only “infectious personality” and “wants to have lots of kids,” on its profile.

Moreover, the seasonality of Winter viruses only really applies to temperate regions of the world. In tropical regions that stay hot and humid, Winter virus activity usually remains more constant throughout the year. In these tropical regions, the constant humidity may be suppressing airborne transmission, leaving direct and indirect contact as the primary mode of transmission. And there is no evidence that the number of times that you touch yourself (and other things) varies from season to season.

Even if transmission of the COVID-19 coronavirus were to drop in June, don’t expect things to go completely back to normal. If you are expecting to return to doing tequila shots out of people’s belly buttons and yelling “woo” at Happy Hours while raining spit onto others’ faces, don’t hold your breath. Social distancing measures shouldn’t be completely relaxed until there is clear evidence that the virus is not longer actively circulating in the community. And that kind of evidence would have to come from widespread testing, much more widespread than what is currently occurring. What the Summer may bring instead is a respite for the many health care professionals who are currently overwhelmed and possibly, just possibly, some relaxation of social distancing measures.