A vacuum bulb is provided, including a sealed chamber; two electrical contacts, which move relative to one another, the chamber including a cylindrical body of a dielectric material and closed at ends thereof by two metal covers, each of the two metal covers being connected to one of the two electrical contacts: and a dielectric coating, which covers an outer surface of the chamber, and includes at least two layers, including an overmolding layer of a synthetic material and an intermediate layer of silicone, the intermediate layer being interposed between the outer surface and the overmolding layer, the intermediate layer being discontinuous and localized on metal portions of the chamber so as to cover at least partially an outer surface of the metal portions, and the silicone includes compressible hollow bodies having a skin of a thermoplastic material.