  The NA62 experiment reports an investigation of the
$K^+\rightarrow\pi^+\nu\bar{\nu}$ mode from a sample of $K^+$ decays collected
in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity
of $(0.389\pm 0.024)\times10^{-10}$, corresponding to 2.2 events assuming the
Standard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. Two signal
candidates are observed with an expected background of 1.5 events. Combined
with the result of a similar analysis conducted by NA62 on a smaller data set
recorded in 2016, the collaboration now reports an upper limit of
$1.78\times10^{-10}$ for the $K^+\rightarrow\pi^+\nu\bar{\nu}$ branching ratio
at 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of
$(0.48^{+0.72 }_{-0.48})\times10^{-10}$, are currently the most precise results
worldwide, and are able to constrain some New Physics models that predict large
enhancements still allowed by previous measurements.
