Where their income is less than 15% of the social security ceiling, membership of social schemes is optional (de facto exemption from social contributions). On the other hand, where their income is above this threshold but below 25% of the same ceiling, membership is compulsory but the State pays 50% of sickness and maternity and old age insurance contributions.
To learn more:
Law No. 93-121 of 27 January 1993 (Article 16)