Previous research has shown that training teaching assistants to deliver early phonic reading interventions can have measurable effects at immediate post-test. This study explored whether the effects of interventions delivered by classroom assistants (CAs) were still evident at the end of the first phase of schooling, 16 months after the early intervention finished. Children were divided into ‘treatment responder’ and ‘treatment non-responder’ groups based upon post-test decoding skills. The treatment responder group was significantly more likely to achieve average results in nationally administered tests (end of Key Stage 1 tests) and teacher ratings of attainment than the treatment non-responders. Treatment responders were indistinguishable from national averages on the mathematics test, writing test and reading task performance, but differed on reading comprehension test and on teacher-assessed attainment. Gains in reading delivered following early phonic reading interventions delivered by CAs are maintained for many children. Non-responders and treatment responders with only modest decoding skill may require additional support to achieve national targets in literacy.