What it means
The most common financial use is portfolio immunisation. An investor who knows a payment is due on a certain date, such as a pension fund paying benefits, can build a bond portfolio whose value moves the same way as the liability when interest rates change.
If rates rise, the bonds lose value, but the money reinvested earns more, and the two effects cancel out. The technique relies on duration, which measures the average time it takes to receive a bond's cash flows, weighted by their present value.
If the duration of the assets equals the duration of the liability, the portfolio is protected against small, parallel moves in interest rates. It must be rebalanced over time because duration changes as the date approaches.
The second use is legal. Sovereign immunity can protect a government from being sued in certain courts, and safe harbour rules can shield directors or companies from liability if they follow certain procedures.
For a business, it matters when dealing with state-owned bodies or international contracts, where the ability to enforce a claim may be limited. A third use appears in tax and insurance.
Immunity from certain taxes or duties can apply to diplomatic missions or some international bodies, while certain insurance structures aim to protect the insured from specific losses. In each case, immunity is narrower than it sounds, and it applies only to the risks named in the rules.
The main nuance is that no protection is complete. An immunised portfolio still faces credit risk, which is the chance that a bond issuer fails to pay, and large or uneven interest rate moves can break the match.
Legal immunity usually has exceptions, so advice from a lawyer is essential before relying on it. In practice, managers use the word loosely to mean any protection, so it pays to ask which risk is covered and which is not.
A clear answer to that question prevents false comfort.
In practice
Real-world examples.
Example
A pension fund must pay out $20,000,000 in benefits in eight years. The manager builds a bond portfolio with an average duration of eight years. When interest rates move, the gain or loss on the bonds is offset by the change in the value of the liability.
Example
A construction company signs a contract with a foreign government body. Its lawyers warn that the counterparty may claim sovereign immunity if a dispute arises. The company asks for a clause in which the body agrees to waive that protection.
Example
An insurance company wants to protect its bond holdings from interest rate risk. It uses duration matching on its liabilities and reviews it each quarter, adjusting the bonds as durations drift.
Formula
Calculation
Immunisation condition: Duration of assets = Duration of liabilities, and Present value of assets = Present value of liabilities
Suppose a company must pay $1,000,000 in exactly 5 years, so the liability has a duration of 5. It can invest in two zero coupon bonds, one with a duration of 3 years and one with a duration of 7 years.
Let w be the weight in the 3 year bond. Then 3w + 7(1 - w) = 5, which gives 7 - 4w = 5, so w = 0.5. Half of the money goes into each bond, and the portfolio has a duration of 0.5 x 3 + 0.5 x 7 = 5 years, matching the liability.Case study
Seen in the real world.
Oakridge Mutual is a fictional insurer that expects to pay $50,000,000 in claims in six years. Its investment team worried that falling bond prices could leave it short of cash.
The team bought a mix of two and ten year bonds weighted so that the average duration was six years: 2w + 10(1 - w) = 6 gives w = 0.5. Over the next year, interest rates rose by 1%, so the bond prices dropped, but reinvested coupons earned more.
In this illustrative case the value of the portfolio stayed close to the present value of the claims, so the insurer remained fully funded. The team kept rebalancing each quarter as the durations shifted, and it reported the funding position to the board every six months.
Watch out
Common mistakes.
- Believing an immunised portfolio is free of all risk, when credit risk and large interest rate shifts can still hurt it.
- Setting up the match once and forgetting it, since duration changes as time passes and the portfolio needs rebalancing.
- Assuming legal immunity covers every claim, when most protections have exceptions and conditions.
Questions
People also ask.
What is the difference between immunisation and hedging?
Immunisation is a specific method that matches duration to protect against interest rate moves, while hedging is a broader term for reducing any risk.
What kind of risk does immunisation address?
It mainly addresses interest rate risk for a known future payment.
Is sovereign immunity absolute?
No. Many countries limit it, especially for commercial dealings, and contracts often include waivers.
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