  We consider a stochastic epidemic model with sideward contact tracing. We
assume that infection is driven by interactions within mixing events
(gatherings of two or more individuals). Once an infective is diagnosed, each
individual who was infected at the same event as the diagnosed individual is
contact traced with some given probability. Assuming few initial infectives in
a large population, the early phase of the epidemic is approximated by a
branching process with sibling dependencies. To address the challenges given by
the dependencies, we consider sibling groups (individuals who become infected
at the same event) as macro-individuals and define a macro-branching process.
This allows us to derive an expression for the effective macro-reproduction
number which corresponds to the effective individual reproduction number and
represents a threshold for the behaviour of the epidemic. Through numerical
illustrations, we show how the reproduction number varies with the mean size of
mixing events, the rate of diagnosis and the tracing probability.
