  We present a systematic search for gamma-ray emission from supernovae (SNe)
in the Fermi Large Area Telescope (LAT) Pass 8 data. The sample of targets
consists of 55,880 candidates from the Open Supernova Catalog. We searched for
gamma rays from SNe by means of a variable-size sliding-time-window analysis.
Our results confirm the presence of transient gamma-ray emission from the
sources of non-AGN classes, including transitional pulsars, solar flares,
gamma-ray bursts, novae, and the Crab Nebula, which are projected near some of
these SN's positions, and also strengthen support to the variable signal in the
direction of SN iPTF14hls. The analysis is successful in finding both short
(e.g. solar flares) and long (e.g. transitional pulsars) high flux states. Our
search reveals two new gamma-ray transient signals occurred in 2019 in the
directions of optical transients that are SN candidates, AT2019bvr and
AT2018iwp, with their flux increases within 6 months after the dates of SN's
discoveries. These signals are bright and their variability is at a higher
statistical level than that of iPTF14hls. An exploration of archival
multi-wavelength observations towards their positions is necessary to establish
their association with SNe or other classes of sources. Our analysis, in
addition, shows a bright transient gamma-ray signal at low Galactic latitudes
in the direction of PSR J0205+6449. In addition, we report the results of an
all-sky search for gamma-ray transient sources. This provided two additional
candidates to gamma-ray transient sources.
