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Some dogs can have a vaccine reaction. Whether or not you believe this to be the case, you need to see the vet to have it checked out. Vaccine induced limpomas are common and are dangerous so you want to catch it early if you can.
Let me know if you still have questions.
Any lump can be dangerous because they can be cancerous. Lumps around the neck and back are even more dangerous because they are harder to remove. This is why many vets give vaccines in the leg now. If there is a lump caused by a vaccine that turns out to be cancerous, they can remove the leg. This cannot be done at the neck area.
You need to have this checked out.