A Theory of Contagion from Vaccinated People

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Why can vaccinated people carry and transmit the delta variant of Covid-19 without getting dangerously sick or dying themselves? And why are measured viral loads from the delta variant as much as 1,000 times those produced by earlier variants? These results of recent research seem counterintuitive and even mutually contradictory, don’t they?

Perhaps answers can be found in human anatomy and where viral loads are measured.

The human nasopharanx and trachea—the so-called “upper respiratory system”— are basically passive tubes. Their primary function is just to transmit air to and from the lungs, without absorbing, using or modifying it.

Relative to the lungs and their alveoli—which actually pass oxygen into our blood and carbon dioxide out—these upper respiratory structures are simple and relatively poorly supplied with blood. They are therefore relatively devoid of microbiological defenses against viruses: blood-borne antibodies, B-cells and T-cells. Instead, their defenses are mostly mechanical: mucus to trap foreign matter mechanically, and moving cilia (nasal hair) to push it out.

When we measure viral load with nasal swabs, as we do most often, we probe only the upper respiratory system’s outer reaches. So we may be measuring viral load in vaccinated patients precisely where it is largest.

Here’s what’s probably going on. When a vaccinated patient is exposed to the delta variant, the virus first penetrates the lining of the nasopharynx, with its relatively weak blood supply and therefore relatively weak microbiological defenses. The delta variant penetrates the cells lining the nasopharynx and starts to replicate like gangbusters.

But as the rapidly replicating virus moves deeper into a vaccinated person’s respiratory system, it begins to encounter a greater blood supply, and therefore all the body’s microbiological defenses, fortified by vaccination. There the antibodies, B-cells and T-cells do their work, preventing deeper infection and the destruction of vital cells in the alveoli and the rest of the respiratory system, not to mention the circulatory system. So the vaccinated patient seldom gets really sick or dies, while nevertheless presenting huge viral loads in his/her nasal passages.

The situation is analogous to the high Himalayan border between China and India. Both countries are highly populated and have large armies. But for obvious reasons of supply, weather, and convenience, they don’t keep their big armies garrisoned at the high mountain border. Despite occasional minor skirmishes there, neither army ventures into the other’s lowlands, where it would encounter massive resistance.

In vaccinated people, our upper respiratory system may be like that high Himalayan border. It has sparse garrisons against infection. The lowlands are like the lower respiratory system, amply endowed with blood, lymph and microbiological defenses.

If this picture is accurate, what are its consequences? The large viral load in the upper respiratory system of vaccinated people is no less dangerous to bystanders for being in the infected person’s “Himalayan highlands.” But this picture may explain why the infected person often experiences no or few symptoms and rarely, if ever, gets hospitalized or dies, while remaining dangerously contagious to others.

This picture also suggests ways to reduce the vaccinated person’s contagiousness. Nasal sprays containing microbiological defenses against the delta variant could reduce the viral load in the upper respiratory tract and therefore the risk to uninfected people nearby. The nasal sprays could, for example, carry antibodies to the delta variant, whether from the vaccinated person of from others (like the antibody cocktail that may have helped cure the former president). They might even involve B-cells or T-cells taken from the vaccinated person himself. (Taking these cells from others would involve risks of an immune or allergic reaction.)

These sprays could be used to the reduce potential for contagion from vaccinated people to others, whenever a vaccinated person is close to strangers. They might be used in addition to masking and distancing, or perhaps even in lieu of masks. Where close contact is unavoidable or of long duration, as on airplane flights, using both masks and sprays (when available) would be prudent.

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