Thread by @ZdenekVrozina

Why does COVID-19 cause multiple waves per year while flu peaks only once?
A new Harvard preprint says: the key drivers are short-lived immunity and climate!
Not variants. Not human behavior.🧵

https://twitter.com/atranscendedman/status/1939035397537091610

The authors analyzed US COVID data (2020–2023) using wavelet analysis and an epidemic model with gradually waning immunity.
Goal: explain why COVID produces 2–3 waves per year, sometimes in summer, sometimes in winter.

They found two dominant rhythms across US states:
One annual (winter wave)
One sub-annual (waves every 3–6 months)
The strongest predictor? Minimum winter temperature.

Cold states (eg Minnesota) had strong annual winter waves
Warm states (eg Florida, Texas) showed regular sub-annual waves year-round
Annual periodicity negatively correlated with winter temps (r2= 0.23)

Indoor behavior doesn’t explain the rhythm.
The regularity of waves (eg every 4-6 months) can’t be caused by seasonal habits alone.

The study shows it stems from the dynamics of immunity.

The authors modeled gradual waning immunity using a flexible Weibull function.
What led to sustained sub-annual waves in simulations?

Short protection (half-life < 200 days)
Low long-term immunity (ω< 0.5)
Sharp drop-off (sigmoid waning)

Seasonal forcing (eg winter holidays) amplified annual waves but could not generate sub-annual periodicity on its own.
Rapidly waning immunity was necessary to create COVID’s “every 3–6 month” cycle.

What about variants?
Variants increase wave size, but not their timing.
Even the first Omicron wave didn’t disrupt the underlying rhythm.

Variants do contribute indirectly:
Antigenic drift accelerates “effective” loss of immunity
Immunity mismatch = functional susceptibility

That mimics the effect of short-lived immunity in the model.

The study acknowledges: immunity is not static.
It evolves via:
repeated infection
vaccination
immune imprinting or class-switching (eg to IgG4)

But even without variant turnover, short-lived immunity alone reproduced sub-annual waves

Summary:
COVID’s persistent multi-wave pattern is driven by:
biologically short immunity
mild seasonal forcing (climate + behavior)
not by variants or human mobility patterns

That’s why COVID doesn’t follow the “flu model” with a single winter wave.

Unless population-level immunity changes substantially, frequent waves are likely to continue, especially in warm climates.

Is this pattern stabilizing?
Not yet.
The authors warn: no clear transition to annual seasonality has occurred.
COVID still shows sub-annual cycles across many US regions - even in 2023.

And as long as immunity remains short-lived and the virus keeps evolving,
periodicity may never fully stabilize.

Predicting when waves happen requires understanding both immunology and climate dynamics - not just tracking variants.

Rubin et al. (2025), Seasonal forcing and waning immunity drive the sub-annual periodicity of the COVID-19 epidemic
medrxiv.org/content/10.110…