What data exist on DIPG incidence before vs after 2020?

The retrieval cannot answer whether DIPG incidence changed after 2020.

You are right to push back on the previous answer. The question of whether DIPG rates shifted after 2020 is answerable in principle from population-based cancer registries, and this retrieval did not surface those studies. That is a limitation of the search, not a finding about the world.

The retrieved records contain one registry count: the International DIPG Registry enrolled 670 patients as of 2017 [1]. That is a clinical registry total, not a population incidence rate, so it cannot tell you whether the rate changed after vaccines rolled out. A proper answer would require comparing DIPG incidence in a defined population before and after 2020, ideally from a national cancer registry. That study was not in this retrieval.

On mechanism: DIPG is a brainstem glioma driven by specific histone mutations (H3K27M in over 85% of cases) [2]. It arises from developing oligodendrocyte precursor cells during a narrow window in early childhood. A vaccine would need a plausible route to trigger that specific oncogenic pathway. The retrieved records document no such mechanism. That does not rule out a link, but it means the mechanism would be novel and unexplained.

My call: the retrieval cannot answer whether DIPG rates have changed or whether a link exists. The question deserves a proper registry study that was not found here. Confidence: not clear.