Flu season that quietly disappeared

Halfway through the northern winter, influenza activity in the northern hemisphere is running at less than a third of its pre-pandemic average, and the surprise is that the quiet is now the normal state. For four winters running, the flu seasons have been mild, and the epidemiologists who spent a decade explaining outbreaks are now spending their time explaining their absence. The explanations matter, because a mild flu season is not a gift; it is a result, and results have causes that can be borrowed or lost.

The most discussed cause is the vaccine match. This season’s vaccine was composed months ago, before the dominant strain had fully declared itself, and the match turned out to be unusually good, with the main circulating strain closely related to the vaccine’s components. A good match can cut the season’s severity by a third or more, and the surveillance data suggests that is roughly what happened. The word ‘roughly’ is doing real work here, because the flu vaccine has never been a precision instrument, and the difference between a good season and a bad one is often a single mutation that the vaccine cannot chase in time.

The second cause is demographic, and it is the one I find more interesting. The population structure of the northern hemisphere has been changing in ways that flu notices: the cohorts who grew up during the pandemic years, when influenza barely circulated, have arrived in schools with weaker immunity than any cohort in living memory, while the older cohorts, who carry most of the severe disease, have been steadily vaccinated for a decade. The result is a strange inversion, in which the virus finds plenty of susceptible children and very few susceptible adults, and the hospitals, which the virus would need to overwhelm, are protected by the immunity of the people who fill them.

The third cause is the one nobody wants to quantify, which is that the virus itself may be changing. The flu strains circulating this decade have shown a tendency toward lower severity, toward infecting the young and sparing the old, and some virologists believe the virus is drifting toward a milder equilibrium as the human population’s immunity profile shifts. That hypothesis is hard to prove and easy to overstate, and the epidemiologists are right to keep it at the back of the queue, but its existence is a reminder that influenza is a moving target, and that the quiet seasons of this decade may not be the template for the next one.

The quiet deserves to be enjoyed and used at the same time. A mild flu season frees hospital capacity, shortens waiting lists and gives health systems a year to breathe, and it should be used for the boring work, the cold-chain maintenance, the stockpiles, the campaigns that get postponed whenever a crisis arrives. The danger is the complacency that follows quiet, the assumption that the mild seasons will keep coming. They will not; the virus does not owe the world anything, and the next bad winter will arrive exactly when the health systems have decided they no longer need to prepare for it. The flu season that disappeared this year is a reprieve, and the measure of the field will be what it does with it.