2020–21
Flu Season

Historically anomalous — masking, distancing, and reduced travel nearly eliminated flu.

CDC FluView ILINet · Delphi CMU Epidata

Season Peak

Peak Period

Weeks Active

Dominant Strain

Week by week

Season Arc

ILI% nationally — percentage of outpatient visits for influenza-like illness, CDC FluView ILINet, Weeks 40–20.

Very High (>8%)

High (5–8%)

Moderate (3–5%)

Low (2–3%)

Very Low (<2%)

Context

How it compares

Peak ILI% nationally for recent seasons — shows where 2020–21 sits in the broader historical picture. Bar length = peak severity.

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Context

About this season

Context: This season is an outlier. The near-zero flu activity was caused by COVID-19 behavioral interventions, not by any property of the flu virus or the vaccine. It should not be interpreted as typical flu season behavior.

Virology

Flu A vs. Flu B

Share of positive flu tests by type, aggregated across the full season from CDC clinical laboratory data (national).

Geography

Regional peak activity

Peak ILI activity by HHS region across the 2020–21 season — regions sorted by severity.