This isn't really as simple as a linear decrease in immunity though - certain tools that your immune system uses like antibodies decrease over time but other to

This isn't really as simple as a linear decrease in immunity though - certain tools that your immune system uses like antibodies decrease over time but other tools like T-cell responses are long lasting. Thinking of it in terms of an acute response where you're very strongly protected and a persistent response where you have a slower but still effective protection is a better model.