Antigenic Drift — Small Mutations Reshape the Flu Surface Every Season
Flu surface proteins slowly accumulate mutations each year.
- HA & NA: Surface proteins (Main antibody targets.)
- Continuous: Drift rate (Small changes every season.)
- Partial: Immune escape (Old antibodies bind less well.)
- RNA errors: Main driver (Polymerase lacks proofreading.)
Why the virus keeps changing shape
Placeholder: RNA polymerase errors accumulate, reshaping surface epitopes gradually.
Global Influenza Surveillance — Sampling the World Before Deciding
Labs worldwide send samples to WHO collaborating centers.
- GISRS: Network (WHO global surveillance system.)
- 100+: Countries (Contribute surveillance samples yearly.)
- Thousands: Samples analyzed (Sequenced and antigenically tested.)
- Genetic + antigenic: Data types (Sequence and antibody reaction data.)
Turning samples into a strain forecast
Placeholder: sequencing and ferret antisera testing map current variant spread.
The Strain Selection Meeting — Committing to a Guess Months Early
Committees meet each February and September to pick strains.
- Feb & Sept: Meeting dates (Northern and southern hemisphere.)
- 6–8 months: Lead time (Before vaccine reaches clinics.)
- 3–4: Strains chosen (Trivalent or quadrivalent formulas.)
- WHO panel: Decision body (Expert virologists vote on strains.)
Predicting future drift from current data
Placeholder: experts extrapolate trends to guess dominant strains months ahead.
From Selected Strain to Filled Vial — The Manufacturing Clock
Selected strains are grown in eggs or cells for months.
- ~6 months: Production time (Egg or cell-based growth.)
- Billions: Doses produced (Globally each flu season.)
- Multiple: Quality steps (Purity and potency testing.)
- Late summer: Distribution start (Ahead of flu season.)
Why manufacturing locks in the guess early
Placeholder: long production time forces early commitment to predicted strains.
Did the Guess Pay Off? Measuring Real-World Vaccine Match
Effectiveness depends on how much the virus drifted after selection.
- 40–60% VE: Good match years (When prediction holds well.)
- 10–30% VE: Mismatch years (When drift outpaces prediction.)
- Late drift: Mismatch cause (Change after strains are locked.)
- Ongoing: Monitoring (Mid-season effectiveness studies.)
Why some flu seasons see poor protection
Placeholder: extra drift after selection lowers real-world vaccine effectiveness.
Placeholder: closer surveillance and shorter lead times could improve match rates.