Could 48 hours by theoretically plausible? How?

The short version: 48 hours is biologically plausible for some mechanisms, but the retrieved evidence does not document a single case with that onset.

The strongest documented case in these records is the IFNAR2-deficient child who died of encephalitis after MMR, with measles, mumps, rubella, and HHV6 all detected [1]. That proves live vaccine virus can reach the brain and kill in an immune-compromised host, but the study reports no day-by-day onset. For a healthy child, the classic teaching is that live-virus encephalitis follows the viral replication cycle, which typically takes 5 to 15 days after MMR [2]. A 48-hour onset would be unusually fast for that mechanism.

But there are other mechanisms that could act within 48 hours. An immediate hypersensitivity or autoimmune reaction to vaccine components, including the aluminum adjuvant, does not need viral replication. It can begin within hours to a couple of days. The same applies to Hep A, which is an inactivated vaccine and cannot replicate at all; any encephalitis after it would have to be immune-mediated, and that could plausibly start fast. The records do not contain a controlled study mapping onset times for these reactions, and the passive surveillance systems that would catch such cases detect almost nothing by design [3].

The honest answer: 48 hours is biologically plausible for an immune-mediated reaction, but not for the classic live-virus replication timeline, and no retrieved study documents a case at that onset. The evidence base is too thin to rule it in or out.

My call: 48 hours is theoretically plausible but unproven, and the evidence does not establish it as a documented timeline. Confidence: low.