antenna efficiency computation

I have the following problem: the radiation pattern of an antenna is measured in an anechoic chamber at a certain frequency and the gain is read by a VNA (path loss, cable loss ant receiving antenna gain are removed in the post-processing software). If I have the gain at different (theta,phi) angles, how can I compute the efficiency of that antenna.
Thank you!

you can integrate the received power values to find out total power radiated then you can find the efficiency by dividing by total power tx

I know the transmitting power (is 0 dBm), and I have s21 (in dB) - this is my antenna gain. If I integrate it (I did the average over all phi*theta angles) I obtain an avarage gain. This would mean that the efficiency if equal to the average gain of the antenna which I don't know if it is correct...

Average in linear numbers, not dB.
If H and V is measured in an anechoic environment is gain for a small antenna in each measured direction: √(H2+V2).
Integration of gain for full sphere gives total radiated power.
Abs. ant.eff. %= (Radiated power*100)/(Tx power-Reflected power)
Realized ant.eff. is less as there always are losses due to reflections.