Elliptic Curve Diffie-Hellman (ECDH) with

Public parameters: Ep (a,b) and G = (x, y)
Private Keys: Na, Nb
Public Key: Pa = Na x G, Pb = Nb x G
Secret key: k = Na x Pb = Nb x Pa

Is the algorithm still secure if public key Pb is used more than once with different private keys Nb? Or are there any requirements to use use a public key more than once in a secure way?