  We derive light curves of the afterglow emission from highly collimated jets
if the power-law index ($p$) of the electron energy distribution is above 1 but
below 2. We find (1) below the characteristic synchrotron frequency, the light
curve index depends generally on $p$. (2) As long as the jet expansion is
spherical, the light curve index above the characteristic frequency increases
slowly as the spectral index of the emission increases. (3) Once the jet enters
the spreading phase, the high-frequency emission flux decays as $\propto
t^{-(p+6)/4}$ rather than $\propto t^{-p}$. All these results differ from those
in the case of $p>2$. We compare our analytical results with the observations
on the GRB 010222 afterglow, and conclude that the jet model may be unable to
explain the observed data. Thus, a more promising explanation for this
afterglow seems to be the expansion of a relativistic fireball or a mildly
collimated jet in a dense medium.
