What happened to SPX the last time Inflation Rate YoY Flash surprised the market?

The last significant CPI YoY Flash surprise to the downside occurred on 12 October 2023. Headline CPI came in at 3.7% versus 3.9% expected. SPX reacted with a strong upside gap and closed the day up 1.2%. Implied volatility collapsed, with VIX dropping from 19.2 to 17.8 that session.

From an iron condor perspective this was a classic positive gamma event. The move stayed well inside typical 16-delta short strike wings, allowing condors placed the prior day to capture nearly the entire credit in one session. ALVH readings spiked above 65 on the release, confirming the volatility contraction trade was favored.

The prior upside surprise (hot CPI) on 12 September 2023 saw SPX drop 1.4% and VIX spike from 13.5 to 15.8. That move tested the short put wing of most standard 45-50 point iron condors, forcing traders to either defend or roll the put side.

Key takeaway for current trading: when CPI surprises to the downside, expect rapid VIX compression and SPX grinding higher. Keep wings at least 45 points wide on both sides to absorb the initial pop. If ALVH is already above 60 heading into the print, reduce size or tighten the short strikes slightly to protect against the volatility crush.