The Doctor’s Surgery

Why do some people never get COVID?

Humans, thank God, aren’t identical and suffer in different ways theodore dalrymple

In the 19th century, the chemist and hygienist Max von Pettenkofer conducted an unwise experiment.

He swallowed a concentrated culture of the cholera germ in front of others. He was trying to disprove Robert Koch’s theory that his newly discovered germ, Vibrio cholerae, was the cause of cholera.

At least Max von Pettenkofer proved the sincerity of his disbelief in Koch’s discovery. He did not subsequently suffer from cholera – at least not severely. From this he concluded, wrongly, that the germ was not the cause of the disease.

He strengthened his wrong conclusion by also drinking bicarbonate of soda with the cholera germs to neutralise the stomach acid Koch had suggested might protect against cholera.

There is a difference between a necessary and a sufficient condition in the causation of infectious diseases. The presence of Vibrio cholerae is a necessary but not sufficient condition for the development of the disease of cholera.

This is the same with the infective agent of most other infectious diseases. Factors such as the dose of the agent and the genetic susceptibility of both individuals and whole groups are of great importance.

The Spanish conquest of what became Latin America was facilitated, if not made possible, by the susceptibility of the Amerindian populations to imported diseases, such as smallpox and measles.

They had no previous experience of these diseases and therefore no relative immunity, conferred by the workings of natural selection.

It has been estimated that nine-tenths of the catastrophic population decline in the century after the arrival of the Spanish was caused by disease – and not by the cruelty so beloved of purveyors of the anti-Spanish Black Legend, la leyenda negra.

By contrast, Europeans were unable to penetrate the interior of Africa in any numbers until the cultivation of cinchona, the medicinal tree, on a large scale, because they had no natural immunity or resistance to malaria. Hence the old sailors’ jingle:

Beware, beware the Bight of Benin,

For few come out where many go in.

That is why European slave-traders had to use Africans as intermediaries.

Is it possible that the difference in mortality between East Asia on the one hand and Europe and America on the other is the result of differing exposures to like viruses in the past?

Individual variations in susceptibility to infectious disease, for example among people of different blood groups, have long been the subject of research.

If people exposed to similar doses of germs nevertheless differ in their clinical outcomes, some explanation of the difference must be possible, at least in theory, and might offer clues as to prevention or cure of the disease.

It was only to be expected that there should be differences in susceptibility and response to COVID-19 infections: in fact, anything else would have been surprising. Not everybody in a household in which there is a case of COVID-19 comes down with it. A doctor friend of mine who was ill with COVID for two weeks in the first wave, and remains to this day without a sense of smell, was nursed by his wife, who not only had no symptoms but had no virological evidence of infection in the first place.

Initially, only 15 per cent of the members of a household with a clinical case of COVID suffered themselves from the disease. Still, with the greatly increased transmissibility of the Omicron variant, this figure has risen enormously.

It was indeed fortunate, then – and not a foregone conclusion – that increased virulence did not accompany the increased transmissibility of the Omicron variant.

While differences in personal and group susceptibility may yield valuable clues, the problem of confounding variables is bound to be difficult to solve.

Humans are not bacteria in Petri dishes, even under the most stringent dictatorships, where conditions may be conveniently controlled so that everyone behaves in the same way. Let us hope this always remains the case.

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