  We consider theories of weakly interacting higher spin particles in flat
spacetime. We focus on the four-point scattering amplitude at high energies and
imaginary scattering angles. The leading asymptotic of the amplitude in this
regime is universal and equal to the corresponding limit of the Veneziano
amplitude. In this paper, we find that the first sub-leading correction to this
asymptotic is universal as well. We compute the correction using a model of
relativistic strings with massive endpoints. We argue that it is unique using
holography, effective theory of long strings and bootstrap techniques.
