  We study the thermal transition of QCD with two degenerate light flavours by
lattice simulations using $O(a)$-improved Wilson quarks. Temperature scans are
performed at a fixed value of $N_t = (aT)^{-1}=16$, where $a$ is the lattice
spacing and $T$ the temperature, at three fixed zero-temperature pion masses
between 200 MeV and 540 MeV. In this range we find that the transition is
consistent with a broad crossover. As a probe of the restoration of chiral
symmetry, we study the static screening spectrum. We observe a degeneracy
between the transverse isovector vector and axial-vector channels starting from
the transition temperature. Particularly striking is the strong reduction of
the splitting between isovector scalar and pseudoscalar screening masses around
the chiral phase transition by at least a factor of three compared to its value
at zero temperature. In fact, the splitting is consistent with zero within our
uncertainties. This disfavours a chiral phase transition in the $O(4)$
universality class.
