Evolving Pathogens and Airplanes as Disease Vectors

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“There are more viruses in a liter of seawater than there are human beings on the entire planet. If we could count up all the viruses on Earth, they would outnumber all forms of cell-based life combined, perhaps by a factor of 10.” — Carl Zimmer, “The Secret Life of a Coronavirus”

Today most educated people reject the theory of a flat Earth. They know the Earth is round. Human-made satellites revolve around it, helping iPhones guide us around unfamiliar cities.

Yet most of us still entertain a misconception about Nature just as fundamental as a flat Earth, but far more dangerous. We think there’s a “natural” hierarchy of living creatures, proceeding upward in importance based on rough size and brain power. Microbes and insects are at the bottom, and large mammals at the top. We humans, of course, are at the pinnacle, the supreme product of evolution or (for some of us) of “intelligent design” by a just God in his own image.

Unfortunately, our species’ real place in this world is nothing of the kind. We are part of, and immersed in, a global, primordial ooze, of the kind suggested by the headpiece quote. Scientists call it our Earth’s “biosphere.”

Our ooze is constantly changing and evolving, at a submicroscopic level, even inside our own bodies. Our so-called “biome”—the mass of microbes in the food we digest—contains more individual living organisms, let alone nonliving viruses, than there are cells in our entire bodies. Our individual bodies are just a small part of a huge, ever-changing, global biological soup, which permeates our biosphere and own bodies as well.

That’s why author H.G. Wells was so prescient in his early science-fiction novel, War of the Worlds. There he depicted microbes destroying advanced space-alien invaders after all our artificial human defenses had failed.

Just so, whenever something new evolves in our our vast, global biological soup, we can encounter a germ for which we have no intrinsic immunity. Then it’s a race between our immune systems and the germs.

Unlike us, germs reproduce in minutes. So they can overwhelm our bodies with sheer numbers. Apart from our own intrinsic deterioration with age, and from human-generated threats like nuclear proliferation and global warming, that’s the greatest biological threat to our species and to our societies, as well as to each of us individually. Our current pandemic demonstrates this truth.

None of this is mere theory. It has happened many times in human history. I’ll mention just three.

In the Middle Ages, the Black Plague wiped out an estimated one-third of Europe’s human population. It was also a factor in the downfall of the Mongol Empire, geographically the most extensive in human history. In American history, European diseases such as smallpox, to which native populations had no immunity, decimated native populations. Just over a century ago, the so-called “Spanish flu” killed over fifty million people worldwide. That’s as many as the most terrible war in human history, which didn’t begin for yet another two decades.

Once a new pathogen evolves, it seldom goes extinct. It hides in animal “reservoirs” or isolated human populations. The Black Plague is no exception. It’s responsible, I’m told, for killing off small “settlements” of prairie dogs near my home in northern New Mexico, and it’s endemic in the local rodent population.

Plague is no longer a big threat to humans because we have antibiotics to fight the germ that causes it, Yersinia pestis. We also know enough about it and its medieval vectors—from rats to fleas to us—to take precautions. But because our immune systems have never evolved to fight this germ, and because we have no vaccine, the Black Plague remains a potential threat to our species if, for example, we overuse our antibiotics and the germ evolves resistance. (Overuse is not an issue for viruses, for which we have no antibiotics, but viruses might similarly evolve resistance to monoclonal antibodies that we use to fight them.)

So our species is never immune to threats from evolving pathogens. We must constantly fight to preserve our individual lives and our social organization against germs that are continuously evolving into new and potentially more deadly forms.

This evolution cannot be stopped. It’s an automatic and continuous process that occurs everywhere pathogens exist. It goes on everywhere, even in the deep ocean. Phenomena as uncontrollable as cosmic rays cause random submicroscopic “errors” as pathogens transcribe their DNA and RNA while they reproduce in hosts. If an error-caused mutation makes a previously known pathogen more contagious or more deadly, it can become a bigger threat to us. Then it can cause a pandemic like Covid-19, or worse.

Every pandemic involves three races against time. The first is inside our own bodies. Germs reproduce in minutes, not hours, let alone the nine months that we humans take.

Everyone knows the legend of the chess board. The inventor of chess asked for a deceptively simple reward: a grain of rice on the first square, two grains on the second, four on the third, and so on, increasing by powers of two, up to all 64 squares. The surprised sultan readily agreed. But once he realized it would amount to more grain than his entire kingdom held, the sultan had the inventor beheaded.

When a microbe reproduces inside our bodies, it’s just like that chess board and the grains of rice. If germs’ division takes two minutes, for example, in just 128 minutes they will fill up the chess board. Then they will number over a million times more than all the cells in our bodies.

Of course in real life some things slow microbes down. They include our non-specific immune systems and the lack of enough host cells and nutrients. But these bare figures show how lopsided the internal struggle can be—and how short the race against time—when our bodies have no specific or “learned” immunity against a new and unfamiliar pathogen. That’s precisely what has just killed over 530,000 Americans, with a disease that’s not even very deadly as plagues go.

The second race against time also runs inside our bodies. It’s a race in which our immune system “learns” to recognize a new pathogen and produce antibodies against it, or enables our killer T-cells to recognize and eliminate a new pathogen specifically. This process usually takes a couple of weeks, just like the time required for the new Covid-19 vaccines to confer the peak of the immunity they provide. So if you aren’t vaccinated well before a pathogen invades you, your body will likely lose this second race. Then you’ll get sick and might die.

The third race against time is where airplanes come in. (You thought I’d never get to this, didn’t you? But context and conceptual accuracy are all.) It’s our species’ race to fight a new pathogen geographically, as it spreads among us human hosts on the way to becoming a regional epidemic or a global pandemic like Covid-19.

Any pathogen spreads more quickly the more we group together and the faster we travel among otherwise separate groups. In other words, the third race is the race for a means of prevention or cure against a plague’s geographical and societal spread by human-origin vectors like airplanes.

The history of great plagues shows how much advances in transportation have hurt us humans in this race. When the fastest transportation involved horses and sailing ships, it took over half a millennium for the Black Plague to spread from the Roman Empire under Justinian to the far-flung reaches of the Mongol Empire. As Europeans colonized and occupied North America, it took a century or so for the diseases they brought with them to decimate native populations.

By 1918, it took less than two years for a new “Spanish” influenza to circle the globe. That was before the dawn of intercontinental air travel. The very first commercial airplane flight had occurred in 1914, only four years before. So that plague spread mainly on troop ships going to and from the First World War, which took almost a week to cross the Atlantic.

In stark contrast, today’s Covid-19 pandemic established itself worldwide within three months of its first recognition as a separate disease in Wuhan, China. The earliest suspected case occurred on November 17, 2019; the disease had spread to Thailand by January 14, 2020; and the World Health Organization declared it a global health emergency on January 30 and a global pandemic on March 11. As compared to the Black Plague’s slow spread on horseback and sailing ships, and the “Spanish” flu’s spread on steamships, Covid-19 spread at “warp speed,” over a thousand times faster than the Black Plague.

Of course a pathogen’s race to spread disease depends on a number of factors besides its hosts’ speed of travel. Perhaps the most important is the incubation period—the time from infection to the first discernible symptoms of disease. If the travel time is shorter than the incubation period, there is no way, even in theory, to stop infected people from traveling and acting as inadvertent disease vectors. We can’t even quarantine them effectively just after they debark, before they disappear in the masses at their destination.

That’s precisely the case with air travel today. Airplanes can carry you halfway around the world in less than a day. But the shortest incubation period for a highly deadly disease like ebola is two days, with an average of 8 to 10 days. For Covid-19, the mean incubation period is 4-5 days, with outliers extending out beyond eleven days.

Thus there is no way, even in theory, to stop the spread of Covid-19 or similar airborne plagues by checking boarding passengers for symptoms like elevated temperature. Testing every boarding passenger with a nose swab would improve the odds, but it wouldn’t completely eliminate the risk. There is a shorter but still non-zero period between infection and the virus multiplying enough to turn a nose-swab test positive.

It gets worse. When you think about an airborne pathogen like SARS-CoV-2, the virus that causes Covid-19, it’s hard to imagine a better way of spreading it than a modern airplane. Airplane passengers sit cheek by jowl for five hours on a transcontinental flight and up to fourteen on an intercontinental flight. How many movies, plays, concerts, or religious services, let alone restaurant meals, last that long? And for obvious (and much lamented) reasons of engineering economy, airplane seats are closer together than any you will find in other mass venues. In long, flat church pews, for example, people space themselves out.

The single factor on which airplanes could beat other mass venues is mask visibility. With low lighting even during sleeping hours, flight attendants can, in theory, reprimand and correct mask scofflaws better than ushers at stationary mass events. But this advantage depends entirely on how strict airlines’ rules for mask wearing and enforcement are, and how diligently flight attendants enforce them.

Another aggravating factor that hasn’t gotten much press is modern airlines’ hub-and-spoke travel configuration. That configuration maximizes the risk that, for example, a New York City resident on a flight to San Francisco might be sitting next to someone who originated, less than a day ago, in Barcelona, Moscow, or Bangalore. While more point-to-point flying would not eliminate this risk (everyone flying has to come from somewhere), it would certainly reduce it. It would also make contract tracing and quarantining much easier. For example, passengers on a point-to-point flight from San Francisco to Hong Kong would be less likely to spread disease than if they got off their planes and roamed around airports at intermediate hubs like Seoul or Tokyo.

Let’s be clear and honest. There is no presently known way of eliminating the risk of airplane travel spreading Covid-19. Does this mean we should shut down air travel except for “essential” trips, whatever that means and whoever defines it? Probably not. With widespread vaccination under way, our species may be in the middle of a painfully slow process of relegating Covid-19 to a tolerable “background” risk like that of seasonal flu.

But Covid-19 is not the last airborne pandemic we will face. As the history above shows, the convenience of air travel has cut the spreading time for, and increased the pandemic risk of, diseases by at least three orders of magnitude since the Black Plague. Just since the turn of our new century, we have already experienced three airborne plagues spread in part by air travel: SARS, MERS and Covid-19.

The relatively low transmissibility of SARS and MERS were plain dumb luck. So is the relatively low death rate of Covid-19, reportedly 3.4% worldwide [search for third instance of “3.4%”], about four times the death rate of seasonal flu. Next time we might not be so lucky. The death rate of ebola, for example, ranged from 22% to 88% in various outbreaks. (Ebola is not an airborne disease; that fact alone saved us from a devastating global plague.)

As surely as we are part of a global biological stew, there will be other airborne plagues. The more they infect hundreds of millions of people, as has Covid-19, the greater will be their threat of mutating into something even more dangerous, as with the several new variants of Covid-19. And just as surely, a haphazard series of ad-hoc measures and too-little, too-late travels bans will do nothing significant to stop the spread of a new airborne plague, or even now-evolving more dangerous variants of Covid-19.

So it behooves global health authorities like the World Health Organization (WHO), governments, regulators, businesses, and those who manage airlines to start thinking about making airplanes less effective vectors of airborne disease. They should consider how planes are configured and constructed, how passengers are treated, how businesses and government use air travel, and how airlines run. They should prepare contingency plans to make planes less ready vectors for the next pandemic both before and as it occurs. Most of all, they should consider contingency plans for shutting down air travel to contain local outbreaks in hours, not days or months.

Here are a few thoughts, in decreasing order of efficacy, for those plans:

1. Preparing for less air travel generally. If Covid-19 has taught the air travel industry one clear lesson, it’s that a lot of air travel deemed “essential” turns out not to be so when a pandemic strikes. Modern audiovisual technology, coupled with the Internet, has made many kinds of in-person meetings obsolete.

Even transcontinental, let alone intercontinental, travel has a lot of not-so-hidden costs. In addition to direct expenses, there’s delay and downtime, jet lag, disruption of flyers’ circadian cycle, exposure to non-pandemic illnesses like colds and flus, and the inevitable unexpected delays, boredom and fatigue of travel. Zoom has proved it possible to eliminate these costs at little harm to most meetings’ effectiveness.

And what about pleasure travel, especially among our aging population? Will seniors continue to go on cruises, let alone land travel, dragging their crutches, wheelchairs and oxygen bottles with them? Or as online opportunities increase, will they turn to “virtual” travel from their dining rooms, easy chairs, or even their hospital beds? Will some seniors prefer to “travel” virtually with their kids and grandkids, experiencing their progeny’s fun and greater mobility virtually?

These points affect more than just the future of the airline industry. We as yet have no serious alternative to fossil fuels for air travel. So these facts also affect the stability of our climate and the health of our planet. Before making or authorizing the next big purchase or lease of planes, industry participants and government authorities had better think hard about these longer-term factors.

2. Preparing for contingencies. A key lesson of Covid-19 was that it caught the entire world off guard, including governments and businesses. We have to do better next time. Governments and airlines ought to have contingency plans for shutting off travel from outbreaks and hotspots on short notice. The quicker outbreaks and hotspots are isolated locally, the less drastic will be more debilitating limitations on regional, national and international traffic, as well as of stay-at-home orders.

Here the WHO’s rules and procedures require drastic revision, away from a bias against riling local political authorities. The WHO delayed unconscionably—almost two months—from the first confirmed foreign transmission in declaring Covid-19 a global pandemic. That delay was more than partly responsible for it becoming one.

That long a delay must never happen again. Individual governments and even airlines can and should lobby for revision of WHO rules that will curtail their losses and give them cover for effective voluntary containment measures that ultimately will save both businesses and customers’ lives, including their own.

3. Masking before boarding. Once an airborne pandemic starts, the single most cost-effective measure that airlines can take to keep their planes from becoming disease vectors is making sure that every passenger and every crew member wears a mask while on board. The best way to do that is to hand out the best masks, N-95s, at the door or in the boarding area.

At the height of the pandemic, I bought a box of American made (3M) N-95s at an inflated price of about $7 each. Pre-pandemic, I used to get the same masks (for garden and home work) at Lowe’s or Home Depot for about $1 each. Given the cost of air travel generally, there is no excuse for airlines not providing this essential protection, at least during pandemics, to all passengers and crew as a matter of course. The FAA, which is responsible for airplanes’ mechanical safety, ought to mandate that requirement and establish a stockpile of masks to accommodate air travel for at least a few months.

4. Better air control. Airplanes are ideal vectors for airborne diseases like Covid-19 because they are indoor venues where complete strangers from widely dispersed origins get packed closely together for long periods of time. If each passenger could be given a separate clean air supply, this source of contagion would virtually vanish. That’s probably too expensive a solution to be practical generally. But it does happen, even now, whenever a breach of cabin pressurization causes emergency oxygen masks to drop down from above.

Short of individual air supplies, there might be cheaper, more practical means of reducing airborne contagion. Central air purification—for example, with short-wave ultraviolet radiation—is a poor solution because nearby passengers can infect you directly without their exhalations passing through any central air system. A better solution might be a laminar-flow system in which air passes downward from above passengers’ heads to intakes in the cabin floor, to be filtered and/or purified and recirculated along with fresh pressurized air.

Anecdotally, such a personal “system” worked well for me for five or so years before the pandemic. Tired of catching colds and flus from traveling, I developed the habit of opening my personal air vent full blast and directing the flow down around my face and nose. (I also wore a heavy jacket to keep warm). Although I didn’t keep careful records, my impression was that this procedure substantially reduced the number of colds and flus I picked up while flying. A more carefully designed system, with multiple negative-pressure intakes at floor level, might work even better, along with mandatory masks. It might even simulate the outdoors.

5. Controlling fomites (virus-containing aerosols deposited on surfaces). One of the least effective precautions against Covid-19 taken by airlines has been obsessive cleaning and sterilizing of surfaces and touchpoints on seats and seat tables and in lavatories. These precautions seemed sensible at the outset of the pandemic, when fomites seemed a far greater risk. But subsequent experience showed that direct airborne transmission, not fomites, is by far the primary means of contagion for Covid-19.

Just so, fomites will probably be a less important means of contagion for all airborne diseases. Although internal, our lungs and airways contain our most vulnerable surfaces, for they must be permeable enough to pass oxygen into our bloodstreams. That’s a big reason why we evolved blood-borne immune systems. In contrast, our skin evolved to be impenetrable to most microbes, and the strong digestive acid in our stomachs deals with many, although not all, mouth-borne germs.

Not only is mask wearing essential to reducing airborne contagion. It also has a secondary effect: keeping passengers from touching their faces, thereby reducing the already low risk of contagion by fomites.

Airlines could reduce that rick even further by advising passengers to wash their hands carefully after using the toilet (and to open the door with a towel or tissue), and by giving passengers sterile wipes with which to clean their hands before eating or drinking anything onboard. (On long-haul flights that require food and drink to be served, airlines can also reduce the risk of airborne contagion by serving passengers alternately, so that no maskless eater or drinker is ever near another.)

* * *

This particular pandemic, Covid-19, appears to be nearing containment, at least in the US. Near-universal vaccination (or as universal as it gets) appears to be possible by fall.

Yet there are still three “wild card” threats not yet quantified by science: (1) how well the current vaccines protect against new variants of the virus; (2) whether and to what extent fully vaccinated individuals can carry the virus and infect others; and (3) how long current vaccines give the vaccinated good protection, whether booster shots will be necessary, and if so, at what intervals. Just the first of these—a more contagious or more deadly variant—could set off a new pandemic within the pandemic and set us back to square one. And as we know now, new variants are continually evolving.

So the present period of uncertainty is a good time for airlines and their regulators to practice their skills in planning and adaptation. Once the next pandemic hits, there will be little time for advance planning. Now is the time, with the present agony as motivation, for governments and airlines to put in place concrete plans for flexibility and resourcefulness in the short term, medium term and even long term. Their responses to this pandemic have so far been utterly inadequate.

Two things ought motivate everyone to undertake serious advance planning now. First, this pandemic is by no means over, and viral variants could make it far worse. Second, this pandemic will not be the last.

If the next pandemic is airborne, airplanes will be a principal, if not the primary, vector making the disease a pandemic and spreading it around the globe. A little prescient planning and anticipation—if only to the extent of accumulating a reserve stock of N-95 masks for air passengers and crews and setting up emergency procedures for isolating hotspots—will go a long way toward reducing the next pandemic’s severity and duration, as well as the disruption of air travel and global economies that it will cause.

In the final analysis, our mental picture of biology as a hierarchy of beings with us at the top is not just laughably childish. It’s suicidally dangerous. The first multicellular organisms evolved 600 million years ago, and perhaps as far back as 1.56 billion years. In contrast, early humans have been around for at most about 2 million years, an eyeblink in comparison. Relative to that, the mere five thousand years of recorded human civilization is but a nanosecond.

It took eons for us to evolve out of the primordial stew that is our biosphere. We could easily fall back into it and disappear, as have many other species. If we want to enjoy above-average longevity as a species, we had better attend to the greatest threat to our collective survival not of our own origin: continually evolving pathogens.

We are already attending to a less dangerous and more uncertain threat: the risk of asteroid-Earth collisions like the one that extinguished the dinosaurs. So the time for greater attention to the primary threat is now. If we fail, one of the crowning achievements of our civilization—global air travel—could become an instrument of unprecedented global misery, or even our species’ extinction.

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