No, in the absence of special agreement between the two parties, the supplementary agreement does not necessarily need to be signed and sealed by both parties. Unless it is clearly stipulated in the contract that "it will take effect after being sealed and signed", it needs to be signed and sealed by both parties.

According to Article 490 of the General Principles of the Civil Law, if the parties conclude a contract in the form of a contract, the contract is established when the parties sign, seal or press their fingerprints. Before signing, sealing or fingerprinting, one party has fulfilled its main obligations, and the contract is established when the other party accepts it.