Yet it is also the case that with other viruses the amount of antibodies in the blood peaks during an infection and drops after the infection has cleared, often within a few months: This is the fact that has some people worried about Covid-19, but it doesn’t mean what it might seem.

That antibodies decrease once an infection recedes isn’t a sign that they are failing: It’s a normal step in the usual course of an immune response.

Nor does a waning antibody count mean waning immunity: The memory B cells that first produced those antibodies are still around, and standing ready to churn out new batches of antibodies on demand.

And that is why we should be hopeful about the prospects of a vaccine for Covid-19.