Physical vs synthetic ETF replication: what difference does it make?
WPEA physically replicates the MSCI World while CW8 does so synthetically via a swap — yet both track the same index with a tiny gap. Understanding the difference lets you assess counterparty risk and explains why synthetic ETFs qualify for the French PEA.
Physical replication: buying the actual shares
A physically replicated ETF directly purchases the securities that make up its benchmark index. There are two variants:
- Full replication : the ETF buys every stock in the index at its exact weighting. Example: IWDA (iShares MSCI World) physically holds all 1,434 stocks in the MSCI World. Transparency is complete: you can check the full holdings list daily on the issuer's website.
- Optimised sampling : the ETF holds a representative subset of the index, selected to replicate the index's behaviour with fewer securities. VWCE (Vanguard FTSE All-World) uses this approach for less liquid emerging markets.
Examples of physically replicated ETFs
IWDA (iShares MSCI World, TER 0.20%), VWCE (Vanguard FTSE All-World, TER 0.22%), CSPX (iShares S&P 500, TER 0.07%). These ETFs are generally not eligible for the French PEA account since they hold predominantly non-European stocks.
Synthetic replication: the SWAP mechanism
A synthetically replicated ETF does not buy the stocks in the index. Instead, it holds a basket of securities (often European equities or bonds) and enters into a swap contract with a bank counterparty.
The mechanism works in two legs: the ETF pays the bank the performance of its physical basket, and the bank delivers the performance of the target index in return. The investor gets the desired exposure without the ETF holding a single stock from the index.
Simplified swap diagram
ETF (e.g. CW8)
Holds French stocks (TotalEnergies, ASML…)
Counterparty
Bank (e.g. Société Générale CIB)
The most well-known example in France is CW8 (Amundi MSCI World): it physically holds a basket of French and European equities and swaps their performance for the MSCI World return with Amundi's investment banking division.
Counterparty risk and collateral
The main risk of synthetic replication is counterparty risk: if the bank providing the swap defaults, the ETF no longer receives the index performance. This risk is controlled by two regulatory mechanisms:
- UCITS 10% cap : the European UCITS IV directive requires that the net swap exposure does not exceed 10% of the fund's net asset value. Beyond this threshold, the issuer must reset the swap.
- Collateral in practice : in reality, most issuers (Amundi, Lyxor, Xtrackers) reset the swap daily. Actual exposure rarely exceeds 1 to 2%. In addition, high-quality collateral (government bonds, large-cap equities) covers more than 90% of the swap value.
To put it in perspective
Counterparty risk in a UCITS synthetic ETF is very low in practice. It is negligible compared to market risk (the volatility of the index itself). Over the past 15 years, no investor in a UCITS synthetic ETF has suffered losses due to a swap counterparty default.
Impact on performance (tracking difference)
The tracking difference (TD) measures the performance gap between the ETF and its index over one year. Contrary to what one might expect, synthetic ETFs often display a better TD (closer to zero or even negative) than their physical equivalents.
Several factors explain this advantage: synthetic ETFs do not incur transaction costs from rebalancing individual stocks, they avoid withholding taxes on foreign dividends (the swap incorporates gross performance), and they generate income from the physical basket they hold.
| ETF | Replication | TER | Avg TD |
|---|---|---|---|
| CW8 | Synthetic | 0.38% | ~0.28% |
| WPEA | Physical | 0.19% | ~0.16% |
| IWDA | Physical | 0.20% | ~0.01% |
| CSPX | Physical | 0.07% | ~-0.02% |
| SP5C | Synthetic | 0.09% | ~+0.05% |
Indicative average TDs over 3 years (2022-2025). Source: issuer annual reports, JustETF. A negative TD means the ETF outperforms its index net of fees.
Why synthetic ETFs dominate within the French PEA
The French Plan d'Épargne en Actions (PEA) imposes a legal constraint: eligible funds must invest at least 75% of their assets in shares of companies headquartered in the EU or EEA. This rule mechanically excludes any physically replicated ETF tracking a global index (MSCI World, S&P 500…), since these indices are dominated by American companies.
Synthetic replication solves this elegantly: the ETF physically holds 75%+ European equities (satisfying the PEA rule) while swapping their performance for the target global index return. The investor gains global exposure with the PEA's favourable tax treatment (17.2% social contributions vs 30% flat tax in a standard brokerage account).
| ETF | Index | Replication | PEA eligible |
|---|---|---|---|
| CW8 | MSCI World | Synthetic | Yes |
| WPEA | MSCI World | Physical (sampling) | Yes |
| SP5C | S&P 500 | Synthetic | Yes |
| IWDA | MSCI World | Physical (full) | No |
| VWCE | FTSE All-World | Physical (sampling) | No |
| CSPX | S&P 500 | Physical (full) | No |
Note: WPEA (Invesco MSCI World) is a notable exception, physical replication that is still PEA-eligible. Invesco achieves this by building the portfolio from European equities selected for their correlation to the MSCI World, without using a swap.
Physical or synthetic: which should you choose?
The answer depends primarily on your tax situation and target index:
- Investing via a PEA targeting MSCI World or S&P 500: you have little choice, only synthetic ETFs (CW8, SP5C) or the Invesco exception (WPEA) are eligible. Choose based on TER and TD.
- Investing via a standard brokerage account: physical replication is often preferable for liquid indices (S&P 500, MSCI World). It offers maximum transparency, very low TERs (CSPX at 0.07%) and often a negative TD thanks to securities lending.
- Targeting illiquid markets (emerging markets, small caps, commodities): synthetic replication can be more efficient as it avoids transaction costs in difficult-to-access markets.
Our take
The physical vs synthetic debate is secondary to selecting the right index, TER, and tracking difference. A synthetic UCITS ETF with a TD of 0.05% is objectively better than a physical ETF with a TD of 0.20%, whatever your preference in principle. Always compare the real TD, not just the stated TER.
Frequently asked questions
Is a synthetic ETF riskier than a physical ETF?▾
In theory, yes: counterparty risk is added to market risk. In practice, this risk is tightly regulated (UCITS 10% cap, daily swap reset, high-quality collateral). Over the past 20 years, no UCITS investor has suffered losses from a swap counterparty default. Market risk (index volatility) remains by far the dominant risk.
What happens if the swap counterparty bank goes bankrupt?▾
The ETF retains its physical asset basket (the European equities it actually holds). It temporarily loses exposure to the target index, but not the entirety of its value. The issuer must then either find a new counterparty or liquidate the fund and reimburse investors at net asset value. The collateral posted by the defaulting counterparty is seized to cover losses.
IWDA and CW8 track the same index: why do they perform differently?▾
Both track the MSCI World but their tracking differences differ. IWDA (physical) has historically had a very low TD (~0.01%) thanks to an efficient securities lending programme. CW8 (synthetic) has a TD of ~0.28% despite a TER of 0.38%, because the swap incorporates gross dividends but incurs financing costs. The difference also stems from withholding taxes on US dividends: IWDA partially bears these, while CW8's swap structure can optimise them depending on the arrangement.
Can a synthetic ETF lose value when its index rises?▾
No. If the index rises, the swap counterparty must deliver that performance to the ETF, it is a binding contract. Counterparty risk does not mean you can lose money when the market goes up; it means that if the counterparty defaults, you might not receive the performance owed for the current period (at most 10% of NAV under the UCITS rule).
Summary
Physical replication buys the actual index securities, transparent and free from counterparty risk, but often ineligible for the PEA account when tracking non-European indices. Synthetic replication uses a swap to obtain index performance, enabling PEA eligibility while often delivering a better tracking difference through dividend tax optimisation.
For a French investor using a PEA, synthetic replication (CW8, SP5C) or Invesco's adapted physical approach (WPEA) are the only routes to access global indices with the favourable 17.2% tax rate. Counterparty risk remains marginal and is governed by UCITS regulation.
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