ISOFIX v seat belt
Child seats have been around for many years, and a persistent issue is that the vehicle seat belt is not used correctly when securing the child seat. If the seat belt is not used correctly, the child seat cannot do it's job correctly, if at all.
As a result of this issue, Britax and VW teamed up and developed ISOFIX, which was released in 1997. The idea of ISOFIX was to remove the use of the vehicle seat belt completely for children secured with a 5 point harness.
On the whole, the idea works very well, and when conducting child seat checks, ISOFIX seats have a significantly better rate of correct fitment, than belt fit seats.
That isn't to say that a belt fit child seat is unsafe - seats fitted with the vehicle seat belt are very safe, IF they are fitted correctly!
Unfortunately, that is a big 'IF'. Child seat checking data shows time and again over 70% of child seats being fitted incorrectly with the vehicle seat belt. Not only can there be issues with the seat belt, but also problems arising from the seat belt and buckle not being compatible with the child seat.
Incompatibility does of course apply to ISOFIX also, but it is much easier to find out if an ISOFIX seat fits safely, by simply checking the child seat manufacturers fitting list.
In some instances, belt fit is the only option - if one of the cars you will fit the seat into doesn't have ISOFIX, you can't find a compatible ISOFIX seat, or you opt for a 25kg extended rear facing seat, as a few examples.
An ISOFIX seat is generally easier to fit correctly, and check that it is compatible with your car - these benefits are not to be underestimated.
However, belt fit child seats are not dangerous when they are fitted correctly and offer very good protection to children.
With any child seat you choose, it's always important to find out that the seat is compatible with the child and vehicle, and preferably visit a retailer if you are able to - ensuring that they show you how to fit the child seat yourself.
You can also attend one of the many child seat checking events which take place throughout the UK year round, to ensure your child's seat is still safe (this applies to both belt fit and ISOFIX child seats)