There has been a lot of nonsense talked about the new mRNA vaccines, but one objection that seems well grounded is that they can be rather rough on people. Boosters, in particular, can make people quite sick for a couple of days and occasionally even put them in the hospital. This was apparent in the trial data when it first came out, and has only been confirmed since.

This effect is understandable if you look at the way they work.

The mRNA vaccines consist of “lipid nanoparticles” which have the mRNA enclosed in an outer “lipid” layer. That layer fuses with cell membranes (like two bubbles joining into one), introducing the mRNA into the cell, where it results in the production of proteins. This production is temporary: eventually the mRNA is scavenged (as all mRNA is), as are the proteins. But something about the process activates the immune system, making it learn that these new proteins are trouble.

It then reacts in two ways: by making antibodies against the protein, and by preparing to kill cells that express that protein. B cells multiply to do the former, and killer T cells multiply to do the latter.

Then suppose you get a booster shot. The antibodies you’ve made don’t see the mRNA particles: they recognize proteins, not the mRNA coding for them, and the mRNA is enclosed inside the lipid shell where antibodies can’t see it even if they did recognize it. But your cells still take up the mRNA and express the protein.

Your immune system then reacts by killing those cells. There can be a lot of those cells; they’re mainly at the injection site but there are also plenty in the liver and some in other places. Killing them produces substantial distress, as in what are called “flu-like symptoms”. This doesn’t happen in a first shot in someone immunologically naive to the virus, since the adaptive immune system takes at least a week to gear up, and by then the mRNA is long gone and cells are no longer displaying “kill me” signals. But it can happen in a first shot in someone who recently had the disease itself.

In contrast, a live-virus vaccine such as the measles vaccine, when injected into someone whose immune system is already all geared up to deal with it, gets glommed onto by antibodies and prevented from infecting cells. A killed-virus vaccine, of course, can’t even infect cells to begin with; likewise for a genetically-engineered-protein vaccine.

The remedy is simple: don’t boost with a mRNA vaccine too often, and don’t get vaccinated when you’ve recently had the disease. (And don’t make rules forcing others to do those things.) Vaccinate when you need it: when immunity is nonexistent or has waned considerably.

Or at least the remedy is simple in principle; what exactly “too often” is depends on age, with the elderly having weaker immune systems and thus being less likely to overreact. But there’s probably a reasonable compromise formula to be had; studies could be done to nail down the best compromise. More difficult would be determining whether you’ve recently had the disease: there are of course tests, but people often don’t bother with them, especially if their symptoms are mild.

This killing of our own cells happens each time we’re infected with a virus, so shouldn’t be regarded as dire or outrageous. But in large quantities it can be quite a nuisance, and it stresses the system, possibly provoking other disorders that were waiting to be provoked. It can’t, for instance, cause a heart attack, since heart attacks are the result of a disease process inside arterial walls which is years in the making. But it could provoke one in someone who was already on the brink of it. And just try telling the grieving family that it would have happened anyway! You might well be right, but that’ll hardly matter.