Rough-vacuum pump of dry type having: - at least two pumping stages (2a-2e) mounted in series between an intake and a discharge of the vacuum pump, - two rotors which extend into the pumping stages (2a-2e), the rotors being configured to rotate in a synchronized manner in opposite directions in order to entrain a gas that is to be pumped between the intake and the discharge, - an injection device configured to distribute a purge gas in at least one pumping stage (2a-2e), comprising: ∘ at least one injection member (12a-12e), and ∘ at least one injection valve (13; 16a-16e) which can be controlled to be on/off and is intended to be interposed between a purge-gas supply source and the at least one injection member (12a-12e), - a control device configured to control the opening and the closing of the at least one injection valve (13; 16a-16e) to inject a purge gas by successive pulses into at least one pumping stage (2a-2e), characterized in that the vacuum pump also has: - an additional injection device having: ∘ a distributor for distributing a purge gas in the pumping stages (2a-2e), ∘ an injection member (12a-12e) for each pumping stage (2a-2e) that is interposed between the distributor and a respective pumping stage (2a-2e), ∘ a controllable continuous-injection valve arranged on a branch of the distributor that is common to the pumping stages (2a-2e), the control device also being configured to control the opening of the at least one continuous-injection valve to inject a purge gas continuously into the pumping stages (2a-2e). - at least two pumping stages (2a-2e) mounted in series between an intake and a discharge of the vacuum pump, - two rotors which extend into the pumping stages (2a-2e), the rotors being configured to rotate in a synchronized manner in opposite directions in order to entrain a gas that is to be pumped between the intake and the discharge, - an injection device configured to distribute a purge gas in at least one pumping stage (2a-2e), comprising: ∘ at least one injection member (12a-12e), and ∘ at least one injection valve (13; 16a-16e) which can be controlled to be on/off and is intended to be interposed between a purge-gas supply source and the at least one injection member (12a-12e), ∘ at least one injection member (12a-12e), and ∘ at least one injection valve (13; 16a-16e) which can be controlled to be on/off and is intended to be interposed between a purge-gas supply source and the at least one injection member (12a-12e), - a control device configured to control the opening and the closing of the at least one injection valve (13; 16a-16e) to inject a purge gas by successive pulses into at least one pumping stage (2a-2e), - an additional injection device having: ∘ a distributor for distributing a purge gas in the pumping stages (2a-2e), ∘ an injection member (12a-12e) for each pumping stage (2a-2e) that is interposed between the distributor and a respective pumping stage (2a-2e), ∘ a controllable continuous-injection valve arranged on a branch of the distributor that is common to the pumping stages (2a-2e), the control device also being configured to control the opening of the at least one continuous-injection valve to inject a purge gas continuously into the pumping stages (2a-2e). ∘ a distributor for distributing a purge gas in the pumping stages (2a-2e), ∘ an injection member (12a-12e) for each pumping stage (2a-2e) that is interposed between the distributor and a respective pumping stage (2a-2e), ∘ a controllable continuous-injection valve arranged on a branch of the distributor that is common to the pumping stages (2a-2e), the control device also being configured to control the opening of the at least one continuous-injection valve to inject a purge gas continuously into the pumping stages (2a-2e).