A beginner's guide to duration matching and immunization

Insurance companies and pension funds must pay out large sums years from now. Changing interest rates can make that harder. Duration matching, also called immunization, is a way to protect against this risk. This page explains how it works.

Quick answer

Duration matching, also called immunization, is a bond strategy where the duration of a portfolio is set equal to the time until a future payment must be made. When interest rates change, the fall in bond prices and the rise in reinvestment income roughly cancel each other out. This helps the portfolio reach the required amount on the target date.

Key takeaways

• Duration matching links bond duration to a future payment date.
• It is also called immunization.
• Price risk and reinvestment risk offset each other.
• Insurers and pension funds use it for long-term promises.
• It needs regular rebalancing as rates and time change.

What is duration matching?

Duration is a measure of how sensitive a bond's price is to interest rate changes. It is also the weighted average time until you get the bond's cash flows back. Matching means setting this number equal to the date when you need the money.

The idea was described by British actuary Frank Redington in 1952. He called it immunization, because it aims to make a portfolio 'immune' to small changes in interest rates.

Why does it work?

When interest rates rise, bond prices fall. That is price risk. But the coupons you receive can now be reinvested at higher rates. That is the other side, called reinvestment risk.

When rates fall, the opposite happens: bond prices rise but reinvestment income falls. If the portfolio's duration equals the time horizon, these two effects roughly balance, so the final value stays close to the target.

A simple example

An insurer must pay Rs 100 crore in seven years. It can build a portfolio of government bonds with a combined duration of seven years and a value that, at current yields, grows to Rs 100 crore by then.

If rates move a little, the price change and reinvestment change largely cancel out. The insurer is more likely to have the money it needs, without having to predict where rates will go.

What are the limits?

The protection is not perfect. Duration changes as time passes and as rates move, so the portfolio must be rebalanced regularly. Large rate moves also bring in convexity, which simple duration does not capture.

The method also assumes the yield curve moves in parallel. If short-term and long-term rates move differently, the match can slip. Credit risk, where a bond issuer fails to pay, is a separate risk that matching does not cover.

Why it matters for investors

You may not run an insurance company, but the lesson is useful. If you need money on a known date, choosing bonds or debt funds with a duration close to that date can reduce interest rate risk. Target maturity funds work on a similar idea.

In short, the method links the timing of assets to the timing of needs, which is a sound idea for any goal-based plan.

Frequently Asked Questions