Thursday, November 10, 2022

The Disappearance of Influenza - eugyppius: a plague chronicle

 The flu is gone. This is not an illusion. It's not down to the wilful or mistaken misdiagnosis of Corona or anything like that. Most countries have long-standing influenza surveillance programs, entire offices of people whose job it is to find and track the flu. These programs are still running, and influenza tests are still widely administered across the world. Despite all of this searching, nobody can find anything but a few outliers. As a seasonal phenomenon in the northern and southern hemispheres, influenza has disappeared.

There are many ways to confirm that flu is missing. To begin with, it has a very characteristic hospitalisation signature. It is dangerous to the olds, just like Corona; but it is also dangerous to infants and the very young, unlike Corona. Flu season has always caused substantial hospitalisations and deaths both in the elderly and in young children. We don't see the characteristic child mortality in our numbers, and we haven't since 2020. And then there's this: When the pandemic started, PCR tests for Corona were hard to come by, and so a lot of places tested manically for influenza to rule it out and establish SARS-2 as a probable diagnosis. Data from various countries thus provides a sharp picture of the atypically steep collapse in flu cases that occurred in March 2020, in perfect tandem with the surge in Corona infections.

Everything suggests that Corona has displaced influenza. Even if we can’t identify or even really conceive of the mechanism, Corona begins to resemble an invasive species that has disturbed the ecology of human-infecting respiratory viruses. Nobody will talk about this, because they are eager to claim any victory they can for mass containment. The absence of flu must be ascribed to lockdowns, even though flu is gone even in countries that have not locked down.

Respiratory viruses all have regional and seasonal features. That is to say, they rise and fall according to complex, intertwining, regionally varying temporal patterns. Throughout the northern hemisphere, mild yet robust rhinoviruses have a dual fall-spring seasonality. They surge before the winter viruses and after influenza, as if to avoid these heavier hitters. Most other commonly surveilled respiratory viruses, including human coronaviruses, peak around the winter solstice or just after. Influenza, finally, surges just as all these deep-winter viruses are receding, infecting the greatest number of people around March. These patterns shed curious light on the second and third waves of the Corona pandemic. The second, winter wave had typical human coronavirus timing, receding almost everywhere after December or early January. The third wave filled precisely the seasonal niche of influenza, with a March/April peak. It's almost as if Corona is playing two roles, its own and that of its defeated predecessor.

A lot of basic matters are poorly understood in the field of virology, and one of them is why waves of infection seem to spontaneously collapse, rather than continuing indefinitely until all susceptible have been taken ill. One reason seems to be that some viruses interfere with other viruses, such that the rise of one compels the decline of another. Plainly, not all pathogens are at odds with each other. Co-infections are common among the overlapping deep-winter viruses. Some viruses, however, definitely seem to exclude others, at least some of the time and in certain places. Influenza and Corona are two of these mutually exclusive viruses. Since it has killed the flu, Corona can operate both in the vacant flu season and in its own natively preferred dark winter months.

Some months ago, I suggested that Corona's victory over influenza could well represent a permanent change in the order of respiratory viruses – a revolution, perhaps a very rare one. The only conceivable historical precedent would be the Spanish Flu of 1918. While we have historical reports of influenza-like illness going back centuries, we don't have any sequenced viruses predating the second wave of this great 1918 pandemic. Before 1918, we can't be sure that seasonal flu-like illnesses were caused by influenza viruses at all. For all we know, coronaviruses were the dominant scourge prior to 1918, and their centuries-long reign was interrupted by the anomalous and highly destructive avian influenza that entered humans in that year. Perhaps the ensuing century of influenza was an unstable equilibrium, an anomaly, and Corona has restored a prior, more ordinary world.

All that's speculation, but we do know that the ensuing seasonal flus for decades afterwards were descended, directly or indirectly, from that first 1918 strain. What happened in 1918 was certainly a viral revolution, on the order of the upset Corona achieved in 2019. Much of virology, as a field, grew up in the shadow of 1918, as an attempt to understand the pandemic of that year and the obviously related seasonal infections to which it gave birth.

What happened in 1918 was unusual. The culprit was an avian H1N1 influneza A virus, but no good analogue has ever been found in birds, and no plausible animal source ever identified. The flu infected pigs just as easily as humans, and its descendents continued to afflict swine even after the reassorted H2N2 'Asian Flu' of 1957 displaced the direct progeny of the 1918 virus in human hosts.

A lot about the 1918 pandemic remains peculiar, indicative of greater things happening just beneath the surface of events. The virus hit in three waves, all of them somehow synchronised across the northern hemisphere. The first struck in spring and summer of 1918, with minimal mortality; the second in the fall and winter of 1918, with massive mortality; and the third in late winter 1919, also with considerable (though fewer) deaths. It is not certain that all of these waves were the same pathogen; infected tissue samples survive only from the second wave. Beyond this strange behaviour, the extreme pathogenicity of the influenza, with a case fatality rate of nearly 2.5%, is astounding. Later, closely related H1N1 viruses infected humans with far less carnage. This can only mean that humans everywhere were immunologically naive to this strain of flu. Other things, too, suggest that this was a wholly new moment in respiratory illnesses for humans. Historical records don't clearly attest to the regular occurrence of seasonal influenza before 1918. If my suggestion of a broader viral ecology is remotely tenable, it's possible that much of the seasonal ebb and flow of respiratory viruses is a characteristic of the world influenza A crafted for itself in the wake of 1918.

Corona has so far obeyed the patterns of that world, but maybe it won't always. Someday we will awaken from the Cult of Mass Containment with a greater openness to observing the world as it is. We will notice that reports of allergies and asthma have grown much less frequent, and that masks aren't really a plausible explanation for this. We'll notice that almost no children are dying of seasonal respiratory illnesses for a change, and we'll notice that seasonal Corona mortality has to be weighed against the missing deaths of seasonal flu. We’ll notice that, with every passing month, Corona grows less deadly, as resistance in the population grows, and we’ll begin to ask ourselves how the unstated policy goal of eradication is even remotely sensible. Destroying Corona would almost certainly mean bringing back influenza A, after much of our immunity has faded.