  Abundance ratios of the nuclear-spin isomers of H$_2$O and NH$_3$ have been
measured in about two dozen comets, with a mean value corresponding to a
nuclear-spin temperature of $\sim$ 30 K. The real meaning of these
unequilibrated nuclear-spin abundance ratios is still debated. However, an
equilibrated water ortho-to-para ratio (OPR) of 3 is also commonly observed.
The H channel of VIRTIS (VIRTIS-H) on board Rosetta provided high-resolution
2.5--2.9 $\mu$m spectra of H$_2$O vapour in the coma of comet
67P/Churyumov-Gerasimenko (67P), which are suitable for the determination of
the OPR of water in this comet. A large dataset of VIRTIS-H spectra obtained in
limb-sounding viewing geometry was analysed, covering heliocentric distances
from 1.24 to 2.73 au and altitudes from a few hundred metres to $>$ 100 km. The
OPR, together with the H$_2$O rotational temperature and column density, were
derived for each spectra. The weak lines of the $\nu_1$, $\nu_1+\nu_3-\nu_1$
and $\nu_2+\nu_3-\nu_2$ bands in the 2.774--2.910 $\mu$m range were used to
calculate by how much the strong $\nu_3$ band centred at 2.67 $\mu$m is
attenuated due to optical depth effects, expressed by the attenuation factor
$f_{\rm atten}$. Most OPR determinations are strongly affected by opacity
effects, as demonstrated by the observed anti-correlation between the OPR and
the column density, and the correlation between the OPR and attenuation factor
$f_{\rm atten}$. Based on both radiative transfer calculations and OPR values
obtained in low-opacity conditions, we derive an OPR of 2.94 $\pm$ 0.06 for
comet 67P. The water OPR measured in the coma of 67P is consistent with
laboratory experiments showing that water vapour that has thermally desorbed
from water ice has a statistical value of 3, regardless of the past formation
process of water ice.
