Comma May Be Omitted Between Short, Parallel Independent Clauses
The Comma
Rulebase Summary
When a coordinate conjunction is linking two independent clauses that are very short and very parallel, the comma before the conjunction may be omitted if you choose. This is a 'may omit' rule — the comma is not wrong when there is an independent subject and verb after a coordinate conjunction.
Two conditions must be met: the construction must be parallel, and both sentences must be short (roughly four words or fewer in each). It is not acceptable to omit the comma just because one sentence is short — both must be short.
Examples
A. He ran and she followed.
Both clauses are short and parallel — comma before 'and' may be omitted.
A. He ran and she immediately called for an ambulance and rushed to the scene.
A. He ran, and she immediately called for an ambulance and rushed to the scene.
Only the first clause is short — the second is long. The comma is required.
Style Guide References (1)
BGGP