Vanna and charm flows into OPEX: how they work
Vanna and charm flows into OPEX are the dealer hedging trades that come from implied volatility falling and time passing in the days before a large expiration, with no help from price. Under the common assumption that dealers are net short index puts, both forces push dealers to buy back short hedges, which is why the run-up to monthly expiration is often described as having a supportive tailwind. The story is mechanically sound but conditional, and it is worth knowing exactly what it depends on.
Two forces, one direction (under one assumption)
Start with the positioning assumption, because everything else follows from it. On broad indices, customers buy a lot of downside puts for protection, and the convention is that dealers sold them. A dealer short a put has positive delta exposure, since the put loses value as the market rises, and hedges it by holding a short position in futures or the underlying.
Charm is how delta changes with time. As expiration approaches with the market above those put strikes, the puts' deltas decay toward zero. The dealer's positive exposure shrinks, the short hedge is now too large, and the dealer buys some of it back. Vanna is how delta changes with implied volatility. If volatility drifts lower, the out-of-the-money puts' deltas also shrink, and the dealer again buys back hedge.
So in a calm market, with volatility easing and the clock running down, both effects produce dealer buying in the underlying. That is the full content of the claim. It is not a prediction that markets rise into expiration; it is a description of one source of flow that exists when those conditions hold.
Why the effect concentrates before expiry
Both charm and vanna get larger, per contract, as expiry approaches for options that are moderately out of the money. A put five percent below the market with two months left has a delta that barely changes from day to day. The same put with four days left can see its delta fall by a large fraction of its value in a single quiet session.
Monthly and quarterly expirations also carry the most open interest, so the per-contract effect is multiplied by the largest position sizes of the month. That combination of steep per-contract sensitivity and large open interest is what gives expiration week its reputation.
Once the expiry passes, the open interest is gone, and so is the source of flow. The remaining contracts are further from expiry and have smaller charm and vanna per contract, which is part of why commentary often contrasts the week before expiration with the week after.
When the flows reverse
The same mechanics run the other way when conditions change. If implied volatility rises, put deltas grow in magnitude, dealers need a larger short hedge, and they sell. If the market falls toward the put strikes, the puts move closer to the money, their deltas grow, and gamma adds to the selling. A calm tailwind can turn into a headwind in a single session when volatility jumps.
Calls work in the opposite direction throughout. If customers hold large out-of-the-money calls and dealers are short them, the dealers hold stock as a hedge, and falling volatility or passing time shrinks the call deltas so dealers sell that stock. On a single stock with heavy call open interest, the net vanna and charm flow into expiry can be selling rather than buying.
The positioning assumption can also be wrong at specific strikes. Large structured trades, such as a fund selling puts to finance call purchases, leave dealers long those puts rather than short. At those strikes, every sign in this article flips.
Gauging how large the flows are
The size of these flows depends on how much out-of-the-money open interest sits in the expiring contracts, how close they are to the money, and how much volatility is left to fall. A month where implied volatility is already low has little room for a vanna tailwind; a month following a volatility spike has much more.
It also depends on where the market is relative to the heavy strikes. Puts far out of the money have small deltas to begin with, so even a large proportional decay releases little hedge. Puts a few percent away carry the most delta that can decay.
A useful exercise is to compare the dealer delta exposure of the board with and without the expiring contracts. The difference is roughly the hedge that has to be unwound by expiration if nothing else changes. If that difference is small, there is not much flow to talk about, whatever the calendar says.
Common questions
Are vanna and charm flows bullish?
Only under specific conditions: dealers net short out-of-the-money puts, implied volatility steady or falling, and price not moving toward the put strikes. Change any of those and the flow can turn into selling.
Why does the week after OPEX feel different?
The expiring open interest, and the hedging flows tied to it, are gone. The remaining contracts have smaller time and volatility sensitivity, so those background flows are weaker.
Do vanna and charm affect single stocks?
Yes, but the sign depends on the stock's own book. Names with heavy call open interest can see the flows run the opposite way from the index.
Into a large expiration, decaying deltas and easing volatility force dealers short index puts to buy back hedges. The flow is real when those conditions and that positioning hold, reverses when volatility rises, and fades once the contracts expire.
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