Diseasonality - Why Diseases Peak in Winter - Article Recap

A recap of Scott Alexander's article investigating why diseases like flu and colds are seasonal, peaking in winter, and why no single theory fully explains this phenomenon.

  • Unclear mechanisms: Despite long-standing interest, it's still not perfectly understood why some diseases are seasonal with winter peaks.
  • Winter illness patterns: Most winter illnesses like flu, colds, viral diarrhea, and chickenpox peak in late winter—January/February in Northern Hemisphere, July/August in Southern.
  • Equatorial patterns: Near the equator where seasons are less defined, these diseases occur more consistently throughout the year, suggesting seasonality is real.
  • Pathogen preferences theory: Some pathogens thrive better in cold or low-humidity conditions, but this doesn't fully explain observed patterns.
  • Indoor crowding theory: People spend more time indoors and in closer proximity during winter, potentially increasing transmission rates.
  • Vitamin D deficiency theory: Lower sunlight leads to less vitamin D production, which might reduce immune function and increase susceptibility.
  • No single explanation: Scott notes none of these theories fully explain disease seasonality on their own—each has counterexamples.
  • Temperature counterexample: If temperature were the sole factor, you'd see more summer flu in cold places like Alaska, but both Alaska and Florida show similar winter peaks.
  • Humidity counterexample: Different places with varying humidity profiles still show similar winter disease patterns, suggesting humidity alone isn't the answer.
  • Crowding counterexample: Indoor crowding would predict reversed seasonality in hot places (Arizona, Saudi Arabia) where people avoid outdoors in summer, yet winter remains the peak.
  • Vitamin D skepticism: While heavily discussed, current research doesn't strongly support vitamin D as the primary cause of seasonal illness patterns.
  • Limited protection evidence: Studies don't show significant protection against colds/flu from vitamin D supplementation in most populations.
  • Chronic deficiency populations: Populations with chronic vitamin D deficiency don't universally get more colds, challenging this theory.
  • Multifactorial model: Scott speculates the answer may be a combination—perhaps some key metric blending dryness, coolness, and indoor activity peaks in winter almost everywhere.
  • Complex interaction: Winter illness seasonality is likely a complex interaction between environmental factors, behavior, and host immunity.
  • Environmental factors: Cold, humidity, and UV exposure all play roles but no single environmental factor dominates.
  • Behavioral factors: Indoor crowding and changed social patterns during winter contribute but don't fully explain seasonality.
  • Waning immunity: Host immunity wanes over time, and as immunity drops while winter environmental factors align, conditions become favorable for outbreaks.
  • Advanced models: Epidemiological models suggest seasonal factors (temperature, humidity, UV) multiply with population immunity levels to affect reproductive rate (R).
  • Epidemic cycling: As immunity wanes and winter hits, R spikes creating mini-epidemics; as infections rise (and thus immunity), R drops until cycle repeats.
  • COVID implications: COVID seems to follow similar seasonal patterns, though winter/summer spikes can be obscured by new variants and behavior changes.
  • Bottom line: No single variable fully explains winter respiratory disease spikes; best guess is multifactor blend where winter provides right conditions for all factors to align.

The full article is available here.