Lets say patients in a trial were followed for over 10 years, but there are no events in either treatment or control group after 5 years (the survival curves are flattened after about 5 years). When fitting a cox proportional hazards model, can we truncate the follow-up period to 5 years. I am asking this because I have an important variable that was documented in literature to affect the outcome. In fact, it did with a strong significant result when I restricted the time period to 5 years. However when I analyzed with the entire follow-up period, its significance was lost (no where near the significance level). So my question is will there be any loss of information or power if I truncated my follow-up period to 5 years?
A small clarification: Treatment is my main variable of interest. It is still significant with 5 or 10 period follow-up. I have a continuous covariate that is well know to relate to the outcome. When I restricted to 5 years, this covariate is significant but its sinificance was lost when I analyzed the full data set.